Mapping Matters – 2009


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August 2009 (Download a PDF 519Kb)

Question: Data re-projection is done all the time by both GIS neophytes and advanced users but a slightly wrong parameter can wreak havoc with respect to a project’s destiny if undetected. Many update projects were originally performed in NAD27 and the client now wants the data moved to a more up-to-date datum. What happens behind the scenes when data gets re-projected? Other than embarking on an expensive ground survey effort what assurances exist to give the user confidence that what has been done is correct? What special considerations should be taken into account when data is re-projected and what are the potential pitfalls? Is every dataset a candidate to be re-projected if not why not?

Complicating the re-projection piece older projects may have been done in NGVD29 and need to be moved to NAVD88. Similar to what is above what happens behind the scenes and how do we know the result is correct? What are some of the commonly performed vertical shifts done in the industry? Is there a standardized practice to perform this task? What impact if any does this vertical shift play on contours. Why do some firms/clients/consultants feel it necessary to recollect spot elevations and regenerate the contours in the new vertical datum rather than just shifting the contours generated from the older vertical datum? Under what circumstances would a vertical shift be ill-advised?

Dr. Abdullah: I personally consider this question among the most important issues I face as a mapping scientist. Despite full awareness of the importance of coordinate and datum conversions and the role they play on the accuracy of the final delivered mapping products most users and providers have a very limited understanding and knowledge of the topic. The question accurately describes the common mistakes misunderstandings concerns and anxiety that many concerned users experience when accepting or rejecting a mapping product. I will try to address all aspects of the question as much as I can for its importance. I will start by describing “what is happening behind the scenes”.

Datums and Ellipsoids: Defined by origin and orientation a datum is a reference coordinate system that is physically tied to the surface of the Earth with control stations and has an associated reference ellipsoid (an ellipse of revolution) that closely approximates the shape of the Earth’s geoid. The ellipsoid provides a reference surface for defining three dimensional geodetic or curvilinear coordinates and provides a foundation for map projection. Here in the United States the old horizontal North American Datum of 1927 (NAD27) was replaced with a more accurate datum called the North American Datum of 1983 or NAD83. NAD83 which is a geocentric system with its center positioned close to the center of the Earth utilizes the GRS80 ellipsoid that was recommended by the International Association of Geodesy (IAG). The NAD27 on the other hand is a non-geocentric datum utilizes an old reference ellipsoid or oblate spheroid (an ellipsoid of revolution obtained by rotating an ellipse about its shorter axis) called the Clark1866 spheroid.

Conversion Types: There are two types of conversions that can occur during any re-projection: datum transformation and projection system transformation. Datum transformation is needed when a point on the Earth used to reference a map’s coordinate system is redefined. As an example of datum transformation is upgrading older maps from the old American datum of NAD27 to the newer NAD83 datum. The coordinate system (not the coordinate values) such as the State Plane may be kept the same during the transformation but the reference datum is replaced. Projection system transformation is needed when a map’s projected coordinates are moved from one projection system to another such as when a map is converted from a State Plane coordinate system to Universal Transverse Mercator (UTM). Here the horizontal datum (i.e. NAD83) of the original and the transformed map may remain the same.

Datum Transformations: In the process of updating older maps produced in reference to NAD27 a datum transformation is required to move the reference point for the map from NAD27 to NAD83. Several different methods for transforming coordinate data are widely accepted in the geodetic and surveying communities. In North America the most widely used approach is an intuitive method called NADCON (an acronym standing for North American Datum conversion) to translate coordinates in NAD27 to NAD83. NADCON uses a method in which are first and second order geodetic data in National Geodetic Services of NOAA (NGS) data base is modeled using a minimum curvature algorithm to produce a grid of values. Simple interpolation techniques are then used to estimate coordinate datum shift between NAD 83 and NAD27 at non-nodal points.. Those who utilize NADCON rarely obtain bad conversion results. Most of the common blunders and mistakes made by users while using different conversion tools result from not fully understanding the basics of geodetic geometry. As such the process of conversion should be handled by individuals who have some understanding and experience in dealing with datum and coordinates conversion.

Once the Global Positioning System (GPS) came along the discrepancies inherent in the original NAD83 which was first adjusted in 1986 and referred to as NAD83/86 to differentiate it from newer adjustments of NAD83 became apparent. New adjustments of NAD83 (HARN adjustment designated NAD83 199X where 199X is the year each state was re-adjusted) resulted in more accurate horizontal datums for North America. The multi-year HARN adjustments added more confusion to the already complicated issue of the North American Datum especially when the user had to convert back–and-forth to the World Geodetic System of 1984 (WGS84)-based GPS coordinate determination. An ellipsoid similar to the GRS80 ellipsoid is used in the development of the World Geodetic System of 1984 (WGS84) coordinates system which was developed by the Department of Defense (DoD) to support global activities involving mapping charting positioning and navigation. Moreover the DoD introduced WGS84 to express satellite positions as a function of time (orbits). The WGS84 and NAD83 were intended to be the same but because of the different methods of realization the datum differed slightly (less than 1 meter). Access to NAD83 was readily available through 250,000 or more of non-GPS surveyed published stations which were physically marked with a monument. WGS84 stations on the other hand were accessible only to DoD personnel. Many military facilities have WGS84 monuments that typically were positioned by point positioning methods and processed by the U.S. military agencies using precise ephemeris.

In 1994 the DOD decided to update the realization of WGS84 to account for plate tectonics since the original realization as well as the availability of more accurate equipment and methods on the ground. In that decision the new WGS84 was made coincident with the International Terrestrial Reference Frame (ITRF) realization known as ITRF92 and was designated WGS84(G730) where 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

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 (6.5/0.30×1000000) 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)
21 um
21 um
21 um

Similar measures have been adopted for the new digital cameras as data providers and clients alike are familiar and comfortable with the values given in Table 1. Determining the vertical accuracy from digital sensors is no different from the horizontal accuracy as we adopted the same measure we used for f lm cameras to the new sensors. As it is given in Table 2 digital imagery collected at nominal GSD of 0.15 m is considered to support 0.60 m (2 foot) contours interval accuracy or an RMSE of 0.20 m according to the ASPRS map accuracy standard. This has been practiced despite the fact that the two-foot contours support was determined in the past based on the c-factor limitation of the stereo plotters used at that time. Table 2 provides the supported map products from digital sensors collected with different ground resolutions as practiced today.

Table 2.
Image GSD (m)
Ortho GSD (m)
Supported Map Scale
Supported Contour Interval (m)

Users of digital cameras are experiencing improved map quality and accuracies that exceed those given in Table 2. In other words imagery from a good digital sensor with a GSD of 0.15 m may be suitable for map scale larger than 1:1,200 and in the future we may need a new standard for the digital camera products that ref ects the improved quality and stability of these digital sensors.

March 2009 (download a PDF 680Kb)

Question: Does lidar data support the generation of accurate one-foot contours and if it does how feasible is it to generate photogrammetricquality one-foot contours?

This answer contains graphics and tables. Please see the PDF

January 2009 (download a PDF 980Kb)

Question: The use of 3D laser scanners (or ground-based lidar) has gained momentum over the last few years among the engineering and surveying communities. Could you please elaborate on the state of this technology and its benefi ts to public- and private-service agencies?

Dr. Abdullah: Ground-based 3D laser scanners which are considered by many experts to be the new generation of survey instruments have recently become very popular and are increasingly used in providing as-built and modeling data for various applications such as land surveying highway surveys bridge and retaining wall structural surveys architectural surveys plant/factory surveys mining surveys forensic surveys reverse engineering and cultural heritage and archeological studies. In contrast to traditional surveying instruments which are limited to locating one point at a time 3D laser scanners measure thousands of points per second generating very detailed “point cloud” datasets. The point clouds can be processed further to generate very accurate and detailed 3D surface models for use in many commercial CAD packages to extract and model various design parameters and to generate as-built survey reports and analysis. 3D laser scanners of interest for highways and large structural operations are based on the following two different technological principles:

• Time-of-flight (TOF) technology measures the time it takes a laser pulse to hit its target and return to the instrument. Very advanced high-speed electronic devices are used to measure the micro time difference to compute the laser’s range or distance between the instrument and the target. The range data is then combined with extremely precise angular encoder measurements to provide a 3D location of the point from which the laser pulse was reflected. TOF technology is similar to the principle utilized in a surveying “total station” instrument. The difference between the two is the superior point measurement density of the 3D laser scanner which is capable of measuring more than 50,000 distances per second as compared to the few distances per second that can be measured by a total station device. TOF scanners are commonly used in applications that require signifi cant range measurements (typically 75 to 1000 m) such as highway surveys and other typical state department of transportation (DOT) applications.
• Phase-based technology measures the phase difference between the reflected pulse and the transmitted amplitude modulated continuous wave laser beam. The distance to the target is a function of the phase difference and the wave length of the amplitude modulated signal. Phase-based measurement scanners usually achieve a much higher number of point measurements (point cloud density) than is possible with TOF scanners. However they are limited in range (typically 25 to 100 m) which makes them best suited for inside factories and enclosed facilities.

In terms of range both TOF and phase-based scanners are outperformed by total stations that typically can handle measurements a few times greater than that of laser scanners. That said 3D laser scanners can accurately position objects at a rate of 1,000 times the speed of a total station which not only reduces survey fi eld time but also results in a more detailed site survey. The enormous survey speed of 3D laser scanners also reduces field crew exposure to all sorts of environmental hazards they are typically subjected to during traditional surveying applications; it also reduces lane closures decreases the risk of causalities and increases productivity. For certain applications and projects more than one laser scanner is needed to perform the survey. In these cases the subject of co-registering data from different scanners plays a greater role in determining the fi nal data accuracy. The process of “registration” refers to combining different point cloud datasets collected using different laser scanners at different locations into a unified coordinate system. These different datasets are joined together in correct relative position and orientation in a common coordinate system. Once joined correctly georeferencing is performed to complete data processing. Georeferencing is the process of fi xing the point clouds dataset(s) to an existing control and coordinate system and datum such as the local state plane UTM and a local site-specifi ed system.

Many commercial laser scanner manufacturers including InteliSum Leica Geosystems Optech and Trimble considered the tighter elevation accuracy requirements needed for the different transportation and highway projects around the world. Such requirement calls for an accuracy of pavement elevation measurements of 8 mm (RMSE) or better from a range of 50 to 80 m. Most scanners achieved an accuracy of 6 mm or better when tested independently by users.

Readers who are interested in more details should refer to an excellent report entitled “Creating Standards and Specifi cations for the Use of Laser Scanning in Caltrans Projects” recently published by the Advanced Highway Maintenance Construction Technology (AHMCT) Research Center of the University of California Davis in cooperation with California Department of Transportation.