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  • Mapping Matters - 2011
    each new realization the IERS publishes revised positions and velocities for a worldwide network of several hundred stations For any geocentric reference system the most important parameter for determining its accuracy is the distance from the center of its reference ellipsoid to the Earth s center of mass Scientific evidence shows that the IERS determined the Earth s actual center of mass to 10 cm level and therefore the ITRF coincides with the Earth s center of mass to Earth within that distance making the ITRS and therefore the ITRF the most accurate geocentric reference system ever established In the early stages 1990s the U S Department of Defense DoD realized the advantages of the ITRF as a global accurate and reliable reference frame So they decided to align the World Geodetic System WGS84 which originally was equivalent to NAD83 86 to follow the realizations of the ITRS or the different ITRF This alignment caused the WGS84 to coincide with the ITRF to within a few centimeters so ITRF based geodetic datum such as CORS and WGS84 are essentially the same Several realizations of the WGS84 have evolved since the modernization program started in the 1990s Among these are WGS84 G730 WGS84 G870 and the latest WGS84 G1150 which are equivalent to ITRF2000 The Gxxxx used in these expressions denotes the GPS week that the DoD adopted the version Adopting an ITRF based reference system was a smart move by the DoD since it establishes an evolving reference system that is accurate and is updated in accordance with the dynamic nature of the Earth s crust The main reasons behind the high fidelity of the ITRS are as follows 1 The use of various observing techniques such as a Very Long Baseline Interferometry VLBI b Lunar Doppler Ranging LLR c Satellite Laser Ranging SLR d Global Positioning System GPS e Doppler Orbitography and Radiopositioning Integrated by Satellite DORIS 2 The IERS establishes different solutions each of which are independently computed by well respected scientific organizations around the world These solutions are either combined or used as checks against each other in order to finely adjust the reference frame Adopting an ITRS based geodetic datum offers the following advantages 1 Supports accurate 3D positioning worldwide 2 Taps into the responsible nature of the IERS for maintaining the ITRS and the ITRF The IERS continuously revises positions and velocities for several hundred base stations around the globe resulting in ever evolving reference frames ITRF88 ITRF89 ITRF2000 ITRF2005 ITRF2008 3 Provides a single standard for collecting exchanging and storing geospatial data 4 Ensures compatibility between surveys systems and users of the data on local state federal and global levels 5 Defines and facilitates the transition to an international standard for datum definition 6 Provides compatibility with GNSS and GPS which are based on ITRF tools and applications 7 Simplifies users understanding of datum and datum transformation and 8 Prevents waste through reduced replication of data data multiutilization and unnecessary data transformation Several countries

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  • Mapping Matters - 2010
    of RMSEX RMSEY 10 cm or better and vertical accuracy of RMSEv 10 cm 2 Engineering class II grade maps that require a horizontal accuracy of RMSEX RMSEY 20 cm or better and vertical accuracy of RMSEv 20 cm 3 Planning class I grade maps that require a horizontal accuracy of RMSEX RMSEY 30 cm or better and vertical accuracy of RMSEv 30 cm 4 Planning class II grade maps that require a horizontal accuracy of RMSEX RMSEY 50 cm or better and vertical accuracy of RMSEv 50 cm 5 General purpose grade maps that require a horizontal accuracy of RMSEX RMSEY 75 cm or better and vertical accuracy of RMSEv 75 cm 6 User defined grade maps that do not fit into any of the previous five categories This concept provides more flexibility for data providers in designing and executing the project However it may be problematic for users who are not well educated in relating map classes to product spatial resolution GSD Keep in mind that due to the fact that digital sensors are manufactured with different lenses and CCD array sizes different scenarios for image resolution and post spacing may result in the same final product accuracies and therefore it is important that users clearly define their required GSD or work with the vendor to determine the optimal GSD for their needs 4 The new standard should address aerial triangulation sensor position and orientation accuracies Currently there is no national standard that addresses the accuracy of sensor position and orientation As a result the subject has been left open to interpretation by users and data providers The accuracy of direct or indirect sensor positioning and orientation whether derived from aerial triangulation IMU or even lidar bore sighting parameters is a good measure to consider in determining the final accuracy of the derived products Furthermore issues can be detected and mitigated prior to product delivery if the standard defines and helps govern sensor performance In the past we adopted the rule that says aerial triangulation accuracy must be equal to RMSE 1 10 000 of the flying altitude for Easting and Northing and 1 9 000 of the flying altitude for height Obviously the preceding criteria were based on the then popular large format film cameras that were equipped with 150 mm focal length lenses Today s digital sensors come with different lenses and are flown from different altitudes to achieve the same ground sampling distance GSD so relying only on the flying altitude to determine accuracy is no longer scientific or practical and new criteria needs to be developed When examining the 1 9 000 and 1 10 000 criteria the following accuracy figures apply for 1 7 200 scale imagery that is flown using a large format film metric camera such as Leica RC 30 or Zeiss RMK to produce a 1 1 200 scale map RMSEX RMSEY 1 10 000 H 1 10 000 1 100 0 11 m RMSEZ 1 9 000 H 1 9 000 1 100 0 12 m When using the current ASPRS class 1 standard the following accuracy figures would be expected for a map derived from the same imagery RMSEX RMSEY 0 30 m RMSEZ 0 20 m assuming 0 60 m 2 ft contours were generated from the imagery The previous accuracy figures call for aerial triangulation results that are 270 more accurate than the final map accuracy Old photogrammetric processes and technologies required stringent accuracy requirements for aerial triangulation in order to guarantee the final map accuracy and past map production methods have transitioned through many different manual operations that ultimately resulted in the loss of accuracy Today s map making techniques have been replaced with all digital processes that minimize the loss of accuracy throughout the entire map production cycle In my opinion the new standard should support accuracy measurements for aerial triangulation based on the resulting GSD Considering all of the advances we are witnessing in today s map making processes aerial triangulation horizontal and vertical accuracy of 200 of the final map accuracy should be sufficient to meet the proposed map accuracy Accordingly the aerial triangulation accuracy required to produce a map product with a final GSD of 0 15 m regardless of the flying height is shown below RMSEX RMSEY RMSEZ 0 625 GSD 0 625 0 15 0 09 m if the final map accuracy is based on RMSEX RMSEY RMSEZ 1 25 GSD 0 1875 m Similar calculations can determine the required accuracy for direct orientation no aerial triangulation required using systems such as IMUs To derive the required accuracy for raw pitch heading and position the previous aerial triangulation error budget of 0 09 m can be used to mathematically derive the acceptable errors in the IMUderived sensor position and orientation Lastly I feel that a new approach should be developed to calculate lidar orientation and bore sighting accuracies Since the sensor s geopositioning and not the laser ranging is the main contributor to the geometrical accuracy of lidar data this calculation should link lidar final accuracy to sensor orientation and positioning accuracies In the forthcoming issue of PE RS I will introduce the final part Part III of my answer which focuses on the importance for the new standard to deal with data derived from non conventional modern mapping sensors such as lidar IFSAR and under water topographic survey using acoustic devices such as active SONAR SOund Navigation And Ranging In addition Part III will provide recommendations on the statistical methodology and confidence level to be used in the standard August 2010 Download a PDF 183Kb Question I noticed that according to both ASPRS and NSSDA standards the vertical accuracy is more stringent than the horizontal accuracy For example if I produce orthophoto products from 15 cm 6 in digital imagery the stated ASPRS standard for horizontal accuracy is 30 cm 1 ft while the expected vertical accuracy is 20 cm 0 67 ft We always believed that the

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  • Mapping Matters - 2009
    G730 represents the GPS week number when it was implemented In the late 1980s the International Earth Rotation Service IERS introduced the International Reference System ITRS to support those civilian scientific activities that require highly accurate positional coordinates Furthermore the ITRS is considered to be the first major international reference system to directly address plate tectonics and other forms of crustal motion by publishing velocities and positions for its world wide network of several hundreds stations The IERS with the help of several international institutions derived these positions and velocities using highly precise geodetic techniques such as GPS Very Long Base Line Interferometery VLBI Satellite Laser Ranging SLR Lunar Laser Ranging LLR and Doppler Orbitography and Radiopositioning Integrated by Satellite DORIS Every year or so since introducing ITRF88 the IERS developed a new ITRS realization such as ITRF89 ITRF90 ITRF97 ITRF00 etc Since the tectonic plates continue to move subsequent realization of WGS84 were published such as WGS84 G873 and WGS84 G1150 One of the newest realization is equal to ITRF 2000 2001 0 i e ITRF 2000 at 1 1 2001 As time goes on the NAD83 datum drifts further away from ITRF realization unless a new adjustment is conducted The later HARN adjustments for example are closer in values to the NGS coordinated network of Continuously Operating Reference Stations CORS system than the earlier ones CORS provides GPS carrier phase and code range measurements in support of three dimensional positioning activities throughout the United States and its territories Surveyors can apply CORS data to the data from their own receivers to position points The CORS coordinates in the U S are computed using ITRF coordinates and then transformed to NAD83 The problem with using ITRF for this purpose lies in the fact that the coordinates are constantly changing with the recorded movement of the North American tectonic plate In the latest national adjustment of NAD83 conducted in 2007 only the CORS positions were held fixed while adjusting all other positions This resulted in ITRF coordinates for all NGS positions used in the adjustment as opposed to only CORS published ITRF positions Projection System Transformation Projected coordinates conversion such as converting geographic coordinates latitude and longitude of a point to the Universal Transverse Mercator UTM or a State Plane Coordinates System represents another confusing matter among novice users State plane coordinate systems for example may include multiple zones e g south north central etc for the same state and unless the task is clear the user may assign a certain coordinates set to the wrong zone during conversion The vertical datum conversion poses a similar risk as here in the U S maps were originally compiled in reference to the old North America Geodetic Vertical Datum of 1929 NGVD29 and conversion is necessary to relate data back and forth between the NGVD29 and the new more accurate vertical datum of 1988 NAVD88 Similar problems arose since most surveying practices are conducted using GPS observations Satellite observations are all referenced to the ellipsoid of WGS84 and the user has to convert the resulting elevation to geoid based orthometric heights using a published geoid model As for NAD83 updates the geoid model also went through many re adjustments and different geoid models were published over the years such as geoid93 geoid99 geoid03 and the most recent geoid06 which only covers Alaska so far Without having details about the data at hand a user may easily assign the wrong geoid model during conversion resulting in sizable bias in elevation for a small project When a new geoid model is published a new grid of geoid heights the separation between ellipsoid and geoid is provided and most conversion packages utilize these tabulated values to interpolate the elevation for non nodal positions As for the vertical datum conversion between NGVD29 and NAVD88 a program similar to NADCON called VERTCON is used throughout the industry to convert data from the old to the new vertical datum Judgment Calls As for the question of whether every dataset is a candidate to be re projected the answer is simply NO To transform positional coordinates between ITRF96 and NAD83 CORS96 U S and Canadian officials jointly adopted a Helmert transformation for this purpose Helmert Transformation which is also called the Seven Parameter Transformation is a mathematical transformation method within a three dimensional space used to define the spatial relationship between two different geodetic datums The IERS also utilized a Helmert transformation to convert ITRF96 and other ITRS realization The NGS has included all of these transformations in a software package called Horizontal Time Dependent Positioning HDTP which a user can down load from the NGS site http www ngs noaa gov TOOLS Htdp Htdp html While the Helmert transformations are appropriate for transforming positions between any two ITRS realization or between any ITRS realization and NAD83 CORS96 more complicated transformations are required for conversions involving NAD27 NAD83 86 and NAD83 HARN as the inherited regional distortion can not reliably be modeled by simple Helmert transformation Even with the best Helmert transformation employed in converting positions from NAD27 to NAD83 CORS96 the converted positions may still be in error by as much as 10 meters In a similar manner NAD83 86 will contain distortion in the 1 meter level while NAD83 HARN will contain a distortion in the 0 10 meter level In summary on the conversion possibilities and tools HTDP may be used for converting between members of set I of reference frames NAD83 CORS96 ITRF88 ITRF89 and ITRF97 while NADCON can be used for conversion between members of set II of reference frames NAD27 NAD83 86 and NAD83 HARN No reliable transformation tool is available to convert between members of set I and set II of reference frames in addition no conversion is available for transforming positions in NAD83 CORS93 and or NAD83 CORS94 to any other reference frames As for WGS84 conversions it is generally assumed that WGS84 original is identical to NAD83 86 WGS84 G730 is identical to ITRF92 and that WGS84 G873 is identical to ITRF96 Other transformations between different realizations of WGS84 and ITRF are also possible Based on the above discussions data conversion between certain NAD83 and WGS84 is not always possible or reliable As I mentioned earlier existing data in NAD83 may not be accurately converted to certain WGS84 realizations as NGS did not publish all reference points in WGS84 and most WGS84 reference points are limited to military personnel Unless a new survey is conducted in WGS84 it is always problematic to convert older versions of NAD83 based data from and to the newer WGS84 realizations Conversion packages that make such tasks possible assume the term WGS84 to be equal to the first realization of WGS84 which was intended to be equal to NAD83 86 Free Conversion Tools GEOTRANS The US Army Corps of Engineers provides a coordinate transformation package called GEOTRANS free to any US citizen In a single step user can utilize GEOTRANS to convert between any of the following coordinate systems and between any of over 100 datums Geodetic Latitude Longitude Geocentric 3D Cartesian Mercator Projection Transverse Mercator Projection Polar Stereographic Projection Lambert Conformal Conic Projection UTM UPS MGRS The GEOTRANS is also distributed with user manual and Dynamic Link Library DLL which users can use it in their software CorpsCon Another good free package called CorpsCon is distributed by US Army Topographic Engineering Center TEC and solely for coordinates conversion for territory located within the United States of America Effect of Datum Conversion on Contours When existing sets of contours are converted from one vertical datum to another the resulting contours do not comply with the rules set governing contour modeling Contours are usually collected or modeled with exact multiples of the contour interval e g for 5 ft contours it is 300 305 310 etc Applying a datum shift to these contours could result in the addition or subtraction of sub foot values depending on the datum difference therefore the contours will no longer represent exact multiples of the contour interval for the previous 5 ft contour example the new contours may carry the following values 300 35 305 35 310 35 etc assuming that the vertical datum shift is about 0 35 ft Consequently after conversion a new surface should be modeled and a new set of contours that are an exact multiple of the contour interval should be generated Similar measures should be taken for the spot elevations as they represent a highest or lowest elevation or a region between two contours without exceeding the contour interval When the new contours are generated the new contours are no longer in the same locations as the previous set of contours The existing spot elevations may no longer satisfy the condition for spot elevations and new spot elevations may need to be compiled Vertical shift based on one shift value is not recommended for large projects as the geoid height may change from one end of the project to another The published gridded geoid heights data should be consulted when converting the vertical datum for large projects that span a county or a state Small projects may have one offset value and therefore applying one shift value that is derived from the suitable geoid model tables for the project area may be permissible Conversion Errors and Accuracy Requirements As a final note the previous discussions on the effect of conversion accuracy on the final mapping product may not pose a problem if the accuracy requirement is lenient and the discrepancy between the correct and assumed coordinates values fall within the accuracy budget To clarify this point the difference between NAD83 86 and NAD83 HARN in parts of Indiana is about 0 23 meter Therefore if you provide mapping products such as an ortho photo with 0 60 meter resolution or GSD scale of 1 4800 and whose accuracy is specified according to the ASPRS accuracy standard to be an RMSE of 1 2 meter the 0 23 meter errors inherited in the produced ortho photo due to the wrong coordinates conversion may go by undetected as opposed to providing ortho photos with 0 15 meter resolution scale of 1 1 200 with an accuracy requirement of 0 30 meter where the error in the data consumes most of the accuracy budget for the product However errors should be detected and removed from the product no matter how large or small they are Best Practice In conclusion I would like to provide the following advice when it comes to datum and coordinate conversion 1 When it comes to coordinate conversion DO NOT assign the task to unqualified individuals The term unqualified is subjective and it varies from one organization to another Large organizations that employ staff surveyors and highly educated individuals in the field may not trust the conversions made by staff from smaller organizations that can not afford to hire specialists No matter what the size of your organization practice caution when it comes to assigning coordinate and datum conversion tasks Play it safe 2 Seek reliable and professional services when it comes to surveying the ground control points for the project Reliable surveying work should be performed or supervised and signed on by a professional license surveyor Peer reviews within the surveying company of the accomplished work represents professional and healthy practices that may save time and money down the road 3 GIS data users need to remember that verifying the product accuracy throughout the entire project area is a daunting task if it is all possible Therefore it is necessary to perform field verification for the smallest statistically valid sample of the data and rely on the quality of the provided services and the integrity of the firm or individuals provided such services for all areas fall outside the verified sample That is why selecting professional and reputable services are crucial to the success of your project 4 When contracting surveyors to survey ground control points for the project ask them to provide all surveyed coordinates in all possible datums and projections that you may use for the data in the future Surveyors are the most qualified by training to understand and manipulate datums and projections and it does not cost them much to do the conversion for you It is recommended that in your request for proposal you ask the surveying agency to provide the data in the following systems Horizontal Datum NAD27 if necessary WGS84 NAD83 86 if necessary NAD83 latest HARN NAD83 CORS NAD83 2007 Coordinates System projected Geographic latitude longitude UTM correct zone Sate Plane Coordinate System Vertical Datum WGS84 ellipsoidal heights NGVD29 if necessary NAVD88 latest geoid model 5 When you are asked to provide data for a client always make sure that you have the right information concerning the datum and projection It is common to find that people ask for NAD83 without reference to the version of NAD83 If this is the case ask them specify whether it is NAD83 86 NAD83 HARN certain year NAD83 CORS or NAD83 2007 6 If you are handed control data from a client or historical data to support their project verify the exact datum and projection for that data 7 If a military client asks you to deliver the data in WGS84 verify whether they mean the first WGS84 where the NAD83 was nominally set equal to WGS84 in the mid 80s Most of their maps are labeled WGS84 referring to the original WGS84 Otherwise provide them with NAD83 CORS or ITRF at a certain epoch suitable for the realization they requested unless they give you access to the WGS84 monument located in or near their facility The most accurate approach for obtaining WGS84 coordinates is to acquire satellite tracking data at the site of interest However it is unrealistic to presume that non military users have access to this technique 8 Pay attention to details People are frequently confused about the vertical datum of the data Arm yourself with simple yet valuable knowledge about vertical datums If the project is located along the U S coastal areas the ellipsoidal height should always be negative as the orthometric height i e NAVD88 is close to mean sea level or zero value and the geoid height is negative Therefore if you are handed data with an incorrectly labeled vertical datum look at the sign of the elevations given for the project A negative sign for elevation data on U S coastal projects is an indication that the data is in ellipsoidal heights and not orthometric heights such as NAVD88 9 Equip your organization with the best coordinate conversion tools available on the market Look for a package that contains details of datum and projection in its library Here apply the concept of the more the better 10 Cross check conversion from at least two different sources It is a good practice to make available at least two credited conversion packages to compare and verify conversion results 11 If you are not sure about your conversion or the origin of the data that you were handed always look for supplementary historical or existing ground control data to verify your position Take advantage of resources available on the Internet especially the NGS site Many local and state governments also publish GIS data for public use on their web sites Even Google Earth may come in handy for an occasional sanity check May 2009 Download a PDF 519Kb Question What is the correlation between pixel size of the current mapping cameras in use and the mapping accuracy achievable for a given pixel size e g for data collected at a 30 cm GSD what would be the best mapping horizontal accuracy achievable Dr Abdullah Unlike f lm based imagery digital imagery produced by the new aerial sensors is not referred to by its scale as the scale of digital imagery is diff cult to characterize and is not standardized Digital sensors with different lenses and sizes of the Charge Coupled Device CCD can produce imagery from different altitudes with different image scales but with the same ground pixel resolution In addition the small size of the CCD array of the digital sensors results in very small scale as compared to the f lm of the f lm based cameras This latter fact has made it diff cult to relate the image scale to map scale through a reasonable enlargement ratio as is the case with flm based photography As an example the physical dimension of the individual CCD on the ADS40 push broom sensor is 6 5 um therefore for imagery collected with a Ground Sampling Distance GSD of 0 30 m the image scale is equal to 6 5 0 30x1000000 or 1 46 154 Such small scale can not be compared to the scale of the equivalent f lm imagery or 1 14 400 which is suitable to produce maps with a scale of 1 2 400 or 1 200 Here the conventional wisdom in relating the negative scale to map scale which has been practiced for the last few decades is lost perhaps forever Traditionally in aerial mapping the f lm is enlarged 6 times to produce the suitable map or ortho photo products This enlargement ratio is too small to be used with the imagery of the new digital sensors if we equate the CCD array to the f lm of the f lm based aerial camera Imagery from the ADS40 sensor as it is used today has an enlargement ratio of 19 Traditionally aerial f lm is scanned at 21 um resolution and Table 1 lists the different f lm scales the resulting GSD and the supported map scale based on an enlargement ratio of 6 Table 1 Film Scale Scanning Resolution Resulting GSD m Supported Map Scale Supported Contour Interval m 1 7 200 21 um 0 15 1 1 200 0 60 1 14 400 21

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  • Mapping Matters - 2008
    matter further previously when we used only three principal pass points per photo the entire frame during orientation space resection was controlled by an average of nine pass tie and perhaps a few control points In this case the control had a higher weight in the least squares adjustment and using adjusted coordinates versus original surveyed coordinates for ground control points could have a drastic impact on the photo orientation during mapping This is not the case with the auto correlated collection of tie pass points Most softcopy aerial triangulation packages perform either space resection or bulk orientation after all the pass tie points are adjusted and densifi ed into control points Therefore having one surveyed control point if any between hundreds of pass tie turned into control points has minimized the effect of the original ground control on the fi nal exterior orientation computation for that individual frame The individual control point or two present between hundreds of photo controls will have minimal weight and it will be overweighed by the presence of the dense network of densifi ed pass tie points in the fi nal exterior orientation computation Based on the above my recommendation is that if you are performing aerial triangulation today with hundreds of adjusted pass tie points and you are re computing the exterior orientation parameters again after the fi nal bundle block adjustment was fi nalized and accepted it does not really matter whether you overwrite or not However if the aerial triangulation was performed 20 years ago then it will be a different story Finally as for the question on whether one should on a routine basis overwrite the given ground control values with the adjusted coordinates or keep the original surveyed coordinates as provided by the land surveyor I believe that the adjusted coordinates should be used for all subsequent computations or orientation This is due to the fact that the mathematical and statistical models have found the best fi t for that ground control within the different elements of the block Introducing a different set of coordinates in this case the one provided by the land surveyor will offset that balance or fi t assuming that all of the measurements and values used in the aerial triangulation were of high quality To provide an example for this argument assume that there is one control point that the mathematical model found to be erroneous by about 40 cm The new adjusted value which is off by 40 cm from the surveyed value desirably fi ts the entire network of the block Introducing the original value erroneous according to the math model in any subsequent computations of the network or part of it will cause misfi t between that control point and the adjacent points September 2008 download a PDF 594Kb Q When shopping for lidar data how do I know what point density I need for my project and whether I need breaklines to support the terrain modeling Dr Abdullah In my last article I answered this question in terms of lidar data acquisition requirements for different terrain modeling applications In this issue I will address the question as it pertains to requirements for 3D modeling applications 3D Urban Modeling Applications The high density of lidar point clouds meets wide acceptance in different user communities who need high defi nition elevation data for applications other than terrain modeling These applications include but are not limited to line of sight 3D city modeling 3D urban fl y throughs and simulation and security and emergency planning Homeland security agencies for instance have shown a strong interest in the use of dense lidar datasets for intercity combat planning and high profi le security planning In addition the emerging capabilities of oblique imaging and modeling have added a greater emphasis on high quality and high defi nition elevation data requirements that would be cost prohibitive without lidar technology In most of the urban modeling applications users are more concerned about the defi nitions and details of the lidar dataset than with the centimeter level accuracy Most 3D city modeling can be achieved with a lidar point density of 5 to 10 points per square meter We are however witnessing an emerging new market for dense to ultra dense lidar data and many lidar providers are equipping their operations with the sensors designed to meet such demand Figures 1 and 2 illustrate the quality of the scene as represented by lidar intensity with post spacing of about 20 points per square meter It is amazing how fi ne the details are that such data provides Bio mass and Forest Modeling Lidar points clouds are also proven to be very effective in studying and modeling forest fl oor and canopy Lidar derived spatial data ultimately can be used to achieve the following resource management goals accurate inventory and composition of forested land harvest planning habitat monitoring watershed protection and fuel management for fire management Furthermore the Mapping Matters article published in the November 2007 issue of PE RS provides more details on this very same subject In that article I suggested a lidar point density of 0 1 to 10 points per square meter depending on the nature of the study August 2008 download a PDF 522Kb Q When shopping for lidar data how do I know what point density I need for my project and whether I need breaklines to support the terrain modeling Dr Abdullah The subject of point density in lidar datasets and the resulting accuracy of derived products are of great importance both to users and providers of lidar data Unfortunately there are no set rules to govern this topic leaving many users to establish their own guidelines when requesting lidar data acquisitions This fact becomes very obvious when studying the point density requirements specified by different requests for proposals RFPs At a loss in this ever confusing topic many users request lidar data with sub meter post spacing to achieve an accuracy that is easily obtainable with less dense lidar datasets Unless the task calls for 3D modeling and above ground manmade or natural features asking for highly dense lidar data may harm the budget with very little accuracy benefits especially when the collection of breaklines is requested During the Second National Lidar Meeting held recently at the USGS headquarters in Reston Virginia speakers presented a variety of views and levels of understanding as to what constitutes a reasonable and practical lidar dataset The most misleading approach is the one calling for a lidar database to fit the broad needs of all users and here I mean all users including those whose applications require 10 points or more per square meter An advocacy call like this not only wastes valuable taxpayer money but also makes for an impossible task as there is very little capital available for such an expensive undertaking unless you live in the UAE that is With the above phrases I have made my political statement clear so now let us get to the technical heart of the matter Lidar data specifications should be tagged with user specific needs and specifications In order to address the issues adequately my response will span the next few issues of the column due to the limited space allocated for each article The following sections represent different user communities requirements and the recommended data specifications 1 Terrain Modeling Applications Terrain modeling is a requirement of nearly all lidar projects spanning a wide range of uses and specifications The most common terrain modeling applications requested by lidar users follow a Contours generation The dwindling use of paper hardcopy maps combined with advancements in 3D terrain modeling software capabilities have driven down the need for traditional contour generation The demand for contours and contour specifications in RFPs involving lidar data collection continues however despite availability of new terrain modeling and viewing methods such as 3D rendering and shaded relief maps To create lidar based contours that meet cartographic and geometric qualities lidar data with modest post spacing of around 2 to 4 meters can be augmented with breaklines derived from image based photogrammetry If imagery is not available for breakline production then a lidargrammetry approach is possible In this method very dense lidar datasets with post spacing of around 1 meter are used to create detailed stereomates by draping the lidar intensity image over the lidar DEM these stereomates are then used to generate breaklines using any stereo softcopy system Once the breaklines are collected either through photogrammetry or lidargrammetry the lidar points can be thinned to a great degree depending on the complexity of the terrain The thinned dataset is then merged with the breaklines to create a digital terrain model DTM required for modeling the contours In addition all lidar points within a buffered distance around the breaklines should be removed to achieve acceptable contours without sacrificing accuracy This process makes sense as that is how we have always modeled contours from a DTM The issue of utilizing breaklines in modeling contours from lidar data often gets confused however as service providers attempt to mix very dense and accurate lidar data with manually collected and possibly less accurate breaklines Without buffering or thinning the lidar points close to the breaklines the contours will appear problematic whenever a lidar point appears right next to a breakline The last statement is true even with a photogrammetrically collected and modeled DTM In constructing lidar derived DTMs we should consider all the best practices previously developed and utilized for modeling photogrammetric DTM during the past decades A good quality DTM is achieved by having accurately modeled breaklines and minimum mass points outside the breaklines when necessary Lidar indiscriminately collects dense mass points throughout the project area including on and around the later collected breaklines Unless lidar data is thinned and breaklines are buffered and cleared from the lidar points around it it will be very difficult if not impossible to achieve cartographically acceptable contours b 3D Terrain Modeling Most modern lidar data users and providers are equipped with 3D modeling software that allows them to model lidar data for different applications such as flood and environmental hazard watershed management etc Depending on the required vertical accuracy of the model many applications can utilize lidar data without the need for breaklines However for hydro enforced terrain modeling where the user expects a downhill uniform flow of water breaklines or manual 3D modeling is required around such water features to assure the effect For most applications lidar data with post spacing of 1 to 2 meters is adequate Hydro enforcement of lidar derived terrain model is still cumbersome and costly and logical automation is strongly needed in this field To be continued in the next issue of PE RS July 2008 download a PDF 160Kb Q What is meant by color or colorized lidar and what is it used for Answer The literal meaning of the terms color lidar or colorized lidar could imply two different things Colorized lidar The latest topographical lidar data processing techniques utilize imagery to aid in the interpretation and fi ltering of lidar data Many vendors are now acquiring digital imagery concurrent with the lidar data mission Having an integrated lidar digital camera solution provides many advantages for data providers and users alike On the data providers level the digital imagery whether natural color RGB or color infrared CIR can be used for Generating simply georeferenced or accurately orthorectified imagery to aid in terrain analysis and interpretation when attempting to convert the lidar data to a bare earth elevation model The orthorectified imagery can also be provided to the end user as a useful by product with minimum cost Assigning the spectral color of the digital imagery to the corresponding lidar returns points that fall within the same geographic location of the digital pixel of the imagery This more sophisticated technique results in pseudo lidar intensity or elevation data that resembles the color digital imagery Such products can greatly benefit the interpretation and examination of the lidar surface since the human brain functions more effi ciently in interpreting colorized terrain data as opposed to black and white data sets Applying supervised or non supervised digital image classification an advanced concept widely used in remote sensing applications to spectrally classify imagery and then assign these spectral classes to the lidar data in a fashion similar to the technique described above Accomplished by using specialized processing software the spectral classification of the digital imagery delineates with great success the different terrain cover categories such as water bodies vegetation types and impervious surfaces that are diffi cult to achieve from lidar data alone Once the results of the spectral classifi cation are attributed to the lidar points the filtering software utilizes this new attribute information combined with the spatial property of the lidar surface elevation and slope to come up with the most accurate and automated way of classifying and filtering the lidar surface A technique like this not only enhances the quality of the bare earth elevation model but also reduces costs by minimizing or eliminating many of the manual editing and filtering efforts Color lidar The term green laser is widely used to describe the bathymetry lidar used for three dimensional high precision surveys of seabeds and objects in the water column Using light energy to penetrate seawater in much the same way as a multi beam echo sounder bathymetry lidar systems usually comprise a twin laser generator red infrared and bluegreen portions of the electromagnetic spectrum providing an effective depth sounding frequency The basic laser sounding principle is similar to acoustic methods A pulse of laser light is transmitted from the system toward the water surface in a predefi ned pattern The red infrared laser light is reflected at the water surface whereas the blue green laser light penetrates into the water column and refl ects from the objects or particles along the laser path or the seabed if it makes it all the way there The water depth is equal to the time elapsed between the two echo pulses multiplied by the speed of light in water Typical water depth penetration is in the range 20 40m but in good conditions depths as great as 70m are possible June 2008 download a PDF 604Kb Q I am looking for a brief but encompassing overview of the map accuracy standard s used in the United States of America to evaluate geospatial data accuracies and whether it applies internationally This answer is available in PDF form only Click the link above Thank you April 2008 download a PDF 66Kb Q How effective are lidar datasets in mapping land features such as roads and buildings Answer Constructing two dimensional and three dimensional building models and other land features requires accurate delineation of sharp building edges and this traditionally has been accomplished using photogrammetric stereo compilation All past attempts to automate the extraction of ground features from dense lidar datasets with a post spacing of one to three meters for the purpose of planimetric mapping have failed for one reason the rounded and jagged edges of delineated buildings and other manmade features Despite some software efforts to employ smart algorithms that correct the geometrical shape of objects this type of modeling remains less appealing to service providers and customers alike as it does not meet horizontal map accuracy requirements for large scale mapping The ASPRS standard requires buildings be placed within one foot of their true ground position when compiled from a map with a scale of 1 100 Recent advancements in lidar systems enable the collection of ultra dense lidar datasets with point density of fi ve to eight points per square meter ppsm which makes the data more suitable for use in the aforementioned modeling algorithms The downside of such demanding software requirements is the high cost associated with the aerial acquisition due to the additional fl ight lines required to collect the ultra dense lidar data Traditionally a lidar dataset used for terrain modeling is collected with a nominal post spacing ranging between one meter and three meters or a point density ranging between one ppsm and 0 11 ppsm The ratio between the data densities of the normally collected dataset and the ultra dense dataset ranges from 20 1 to 32 1 This is true if we assume the normal density dataset is collected with two meter post spacing or 0 25 ppsm This does not necessarily translate to the same ratio in cost increase but it could come very close In many cases the high cost of acquisition coupled with the massive amount of resulting lidar data prohibits the use of ultra dense lidar data for accurate building modeling and may encourage providers and customers to consider other means such as traditional photogrammetric modeling for this purpose Finally while ultra dense lidar datasets may not currently be cost effective for largescale modeling the technology is impressive in the sense of delineating details A recent acquisition of an ultra dense lidar dataset with about six ppsm over the city of Baltimore Maryland reveals great details of the baseball game underway in the Camden Yards baseball stadium as shown in Figures 1 through 3 Baseball fans can easily observe which base was occupied at that moment March 2008 download a PDF 66Kb Q What does oblique imagery mean and how effective it is in GIS and mapping activities Anonymous Answer Rather than collecting imagery directly beneath the aircraft with the camera pointing at nadir oblique imagery is acquired from an angled position Oblique imagery has been used for decades by military intelligence during aerial reconnaissance missions In recent years however its use has spread to the commercial market with an expanded range of applications The modern approach combines oblique and vertical imagery to produce accurate 3D digital maps that can be interfaced with any modern GIS database or used for fl y through analyses and

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  • Mapping Matters - 2007
    Information ASPRS Films Other News NCEES Materials Unmanned Aircraft Systems Report Card on the U S National Spatial Data Infrastructure Publications PE RS Journal PE RS Submissions PE RS Advertising Conference Proceedings Manual of Photogrammetry Errata Publications Catalog PDF Online Bookstore Skip to content Home PE RS Journals Mapping Matters Mapping Matters 2007 PE RS Journals Mapping Matters 2007 2007 Archive December 2007 download a PDF 505Kb November 2007 download a PDF 69Kb October 2007 download a PDF 331Kb August 2007 download a PDF 161Kb July 2007 download a PDF 120Kb June 2007 download a PDF 194Kb May 2007 download a PDF 79Kb April 2007 download a PDF 108Kb February 2007 download a PDF 111Kb Click Here to Report a Problem on this Page Report a Problem Please complete all required fields Your Name Invalid Input Email Address Invalid Input What Problem are you reporting Broken Image Broken Link Spelling Other Issue Invalid Input Additional Comments Invalid Input Please enter these 4 NUMBERS in the box Refresh the Code Invalid Input For overall comments on the site in general please submit comments in our comment form ASPRS Online Home Certification ASPRS Foundation Students Membership Information Join Now Search Certified Professionals

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  • ASPRS
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  • ASPRS Examination Copies
    email or fax your request including the name of your course the estimated class size and the adoption decision date on school letterhead to the ASPRS Distribution Center at asprspubs brightkey net or Fax 301 206 9789 An invoice will accompany your examination copy If you decide to adopt the book a minimum order of 5 copies of the book is required keep the examination copy and return the original invoice with a copy of your request to the ASPRS Distribution Center If you do not adopt the book you may either pay the invoiced amount and keep the book for your personal library or return it unmarked and in salable condition books must not have a broken spine or bent covers to the Distribution Center To ensure proper credit please enclose the original invoice Schools that do not resolve invoices within the 45 day examination period will be required to prepay future orders Click Here to Report a Problem on this Page Report a Problem Please complete all required fields Your Name Invalid Input Email Address Invalid Input What Problem are you reporting Broken Image Broken Link Spelling Other Issue Invalid Input Additional Comments Invalid Input Please enter these

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  • Manual of Geographic Information Systems Forward
    new scientific and technological developments GIS is about using the geographic approach the science and technology of capturing storing managing analyzing modeling integrating and then applying particularly to decision making geographic or geospatial information of many kinds including as primary data sources remotely sensed data and imagery The challenging times we live in have played an important role in fostering the growth of GIS We need to better understand and better manage our planet Growing human population unsustainable use of natural resources loss of biodiversity human hunger and poverty climate change rapid urbanization wars terrorism energy use natural disasters food production human health and many other issues are problems we must address GIS plays a role in dealing with each of these In dealing with these problems large resources have been devoted to creating new science and technology including both the science and technology of remote sensing and of geospatial information The development of these new capabilities has also been driven by rapid advances in technology including more powerful computer processors on ever smaller microchips high capacity storage devices high speed wide band communications improved display devices faster graphics processors new visualization techniques and the remarkable evolution and rapid growth of the Internet including wireless network access These new capabilities have strongly influenced photogrammetry and remote sensing Remotely sensed data are increasingly available and affordable they come from a widening array of sources and sensing devices Satellites can now collect staggering amounts of data and transmit high definition television autonomous and unmanned vehicles provide platforms for remote sensing on land sea and in the air and at ever higher spatial resolution We are on the threshold of creating an instrumented universe in which pervasive remote sensing will be complemented by vast numbers of networked sensors and measuring instruments located throughout the human and natural environments These developments provide many challenges Terabyte petabyte and even larger sized image databases need to be managed and used effectively and securely National and global spatial data infrastructures need to be created Users want access to authoritative data in near real time in 3D and in high definition They need searchable metadata and efficient geospatial browsers to locate these data These and other challenges provide opportunities for GIS science and technology to complement photogrammetry and remote sensing The rapid growth of GIS applications of GIS use and of the GIS industry all show that GIS is up to the task Professionals from many disciplines including photogrammetry and remote sensing have critical roles to play in these developments They must provide technical mastery developing improved technology and methods devising QA QC procedures providing authoritative content setting standards assuring interoperability increasing efficiency and managing costs They are needed in policy making management and to advise decision makers They must also do the fundamental research and educate and train the next generation of professionals Also in recent years because of the capabilities and ease of use of free intuitive viewers on Internet websites like Google Earth and Microsoft

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