Mobile Mapping Tips and Tricks Projections, Datums and Geoids, May 2017 Introduction This guide is designed to provide a basic understanding of coordinate system projections, datums and geoids. The guide will also show you how to apply these in TerraScan and GrafNav (where applicable) and how to transform from one coordinate system to another in TerraScan. Pre-Requisites Software This guide has been developed specifically for the following software (although older software should still apply): GrafNav 8.7 MMProcess 17.1.24 TerraScan 17.007 User Experience Basic GrafNav, MMProcess and TerraScan experience is required. Page 1 of 13
The Conversion Process To convert from global coordinates to local coordinates the following steps are usually applied: 1. Global datum to local datum conversion 2. Projection 3. Local grid shift (geoid correction and Easting, Northing shifts for example) Note: geoid corrections can sometimes be applied at the beginning Datums Global datums represent the whole earth, with examples including WGS84, ITRF2014 and ETRS89. Local datums represent an ellipsoid best-fit to only a portion of the earth, with examples including OSGB36, RGF93 and NAD83. The default datum used for the Global Positioning System (GPS) is known as WGS84. A more precise datum for GPS is called ETRS89. The main reason for using local datums is because of the movement of land mass, due to continental drift, subsidence and diurnal movement (caused by the moon and tides). Local datums often get upgraded after a few years, hence the use of the year in their name. Local datums are usually applied during GNSS processing (GrafNav for example). Using Datums in GrafNav When adding a base station in GrafNav it is important to select the correct datum for that reference station (Figure 1). The datum that the provided coordinates are in will be available from the base station supplier. Figure 1: Base station datum Page 2 of 13
It is equally important to select the correct processing datum in GrafNav (Figure 2). This should be the datum used by your local coordinate system. Some examples are listed in Table 1 below: Country Coordinate System Datum UK OSGB36 National Grid ETRS89 USA State Plane NAD83(2011) France CC42-50 RGF93 Australia MGA48-58 GDA94 Belgium Lambert 72/2008 ETRS89 Table 1: Common processing datums Figure 2: Processing datum During export there is also the option to output to a different coordinate system as shown in Figure 3. If the correct processing datum was used then there should be no need to convert during export. Figure 3: Output datum Page 3 of 13
Projections A map projection is a transformation of the latitudes and longitudes of locations on the surface of the Earth into 2D Cartesian coordinates on a plane. Common projection types include: Transverse Mercator Lambert conic conformal Hotine oblique Mercator The projection will be based upon a reference ellipsoid, with common ellipsoids including: GRS80 (also known as the WGS84 ellipsoid, used by GPS) Bessel 1841 Clarke 1866, 1880 WGS72 Using Projections in MMProcess Some coordinate systems are built-in to MMProcess such as UK National Grid and UTM. But additional coordinate systems can be added from the coordinate systems tab from application settings (Figure 4). Figure 4: Coordinate systems tab in application settings To add a new system right click on the list and select add coordinate system. A list of available coordinate systems will be shown (Figure 5). You can search for a system by entering a keyword in the search dialog. Click on the add button to add the system to the list. Page 4 of 13
Figure 5: adding a coordinate system Note: coordinate systems in MMProcess do not include local grid shifts, with the exception of the built-in UK National Grid (uses the ostn02 xy and geoid correction grid). Page 5 of 13
Using Projections in TerraScan Some coordinate systems are built-in to TerraScan such as UK National Grid, UTM, US State Planes and Belgium LB72 to name just a few. Projections which are not built-in can be created by defining a new system from the User Projection Systems which is found under the Coordinate Transformations folder. Click on Add, and then enter the required parameters (Figure 6). Figure 6: TerraScan user projection system The ellipsoid is defined by semi-major axis and inverse flattening rather than by name, so ensure these values are entered correctly based upon the required reference ellipsoid. Note: coordinate systems in TerraScan may not include local grid shifts, please check with your TerraSolid distributor for more information. Page 6 of 13
Using TerraScan to Transform from one Projection to Another Sometimes it will be necessary to transform from one coordinate system to another, for example UK National Grid to UTM31. Ensure the source and target coordinate systems are enabled in TerraScan settings. From the transformations settings, found in TerraScan settings, add a new transformation with type Projection Change (Figure 7). Enter a name for the projection, and select the From and To coordinate systems and specify whether to change xyz or xy only. This new transformation can then be applied either during importing of points for a new project or for transforming an existing project (or loaded points). When transforming an existing project you will need to create a macro to transform the blocks and you will also need to transform the block definitions too (from the project dialog). Figure 7: Project change Page 7 of 13
Local Grid Shifts Some local coordinate systems require an additional shift in easting and northing, and most coordinate systems will also require a shift in height to adjust to orthometric height (generally mean sea level) which requires a gravity geoid correction file. Correction files for these shifts will usually be provided by your country s mapping agency, although some software packages such as TerraScan provide some local grid correction files. Gravity is not constant over large areas and therefore orthometric heights need to be corrected for local gravity changes. A geoid file, consisting of elevation corrections over a specified area, is used to transform the ellipsoidal height to orthometric height. The file can be in various formats and various sizes, some covering the entire world (EGM2008 for example), some covering entire continents (Denker Europe for example), and others just covering a single country (RAF98, SA201 for example). Using Geoids in GrafNav Although easting and northing shifts cannot be applied in GrafNav, orthometric elevations can be output instead of ellipsoidal by applying geoid correction files. The following website has geoids that can be downloaded in GrafNav format for various countries: http://www.novatel.com/support/waypoint-support/waypoint-geoids/ When exporting the GrafNav solution make a copy of the AEROCTRL profile (Figure 8). Figure 8: Create a new copy of the AEROCTRL export profile Page 8 of 13
Modify the new profile by replacing ellipsoidal height with orthometric height using the formatting shown in Figure 9. Figure 9: Replace ellipsoidal height with orthometric height During the export wizard you will now be asked to provide a geoid file (Figure 10). This file will need to be in a GrafNav compatible format (.wpg). Figure 10: Select the geoid during the export wizard Page 9 of 13
Using Local Grid Shifts in TerraScan There are some local grid shifts available in TerraScan which will correct for both Easting and Northing and will apply a geoid correction, these are listed in Table 2. These grid shifts correction files can be found in the coordsys folder in the TerraScan installation folder. Coordinate System XY Z Belgium 72 Xygridlb72.dat Hbg03.dat UK National Grid (OSTN02) Ostn02.txt Ostn02.txt UK National Grid (OSTN15) Ostn15.txt Ostn15.txt Northern Ireland (OSGM02) - Osgm02_ni.txt Northern Ireland (OSGM15) - Osgm15_belfast.gri Republic of Ireland (OSGM02) - Osgm02_roi.txt Republic of Ireland (OSGM15) - Osgm15_malin.gri Netherlands RD/NAP2008 X2c.grd, y2c.grd Nlgeo04.grd Table 2: grid shifts available in TerraScan The user does not need to use any special tools to use these corrections, they are automatically applied when you use one of the built-in projection systems. Page 10 of 13
Using Geoids in TerraScan To use a geoid in TerraScan the geoid file format should be ASCII, space delimited, with the following three fields: Easting Northing ΔHeight Geoid corrections can be applied to loaded points using the Adjust to Geoid command from the tools menu in the main TerraScan toolbar, and for project points from the tools menu in the project dialog (Figure 11). Figure 11: Adjust to geoid on loaded points and to a project It is also important to remember to apply the geoid correction to the trajectories and to the image list if necessary (Figure 12). Note: if you have created an image list from trajectories that have already had a geoid correction applied then do not apply the geoid to the image list! Figure 12: Adjust to geoid on a project Page 11 of 13
Converting a Geoid in TerraScan TerraScan has a tool that can convert some freely available geoids to TerraScan compatible geoids. For example, the Earth Gravitation Model 2008 (EGM2008) can be downloaded from the internet and converted from global coordinates to a smaller local coordinate system (UTM31 for example). The links below can be used to download EGM2008 and EGM96 respectively. http://earth-info.nga.mil/gandg/wgs84/gravitymod/egm2008/und_min1x1_egm2008_isw=82_wgs84_tidefree.gz http://earth-info.nga.mil/gandg/wgs84/gravitymod/egm96/binary/ww15mgh.dac To convert a geoid use the convert geoid model command from the tools menu in the main TerraScan toolbar. Select the source geoid and browse to the geoid source file (Figure 13). Chose which local coordinate system to use. You can select a fence so that the new geoid only covers a specific area, this is useful when large coordinates become a problem with the limited coordinate resolution of Microstation. Enter a new filename for the local geoid that TerraScan will create. Figure 13: Converting a global geoid to a local geoid Page 12 of 13
About the Author Dr. Christopher Cox has over 11 years' experience in the LiDAR industry. He was awarded a PhD in 2003 by the University of Nottingham for his thesis entitled ``The Use of Computer Graphics for Visual Impact Assessment``. Chris joined 3D Laser Mapping in 2005 and specialises in mobile mapping and airborne LiDAR research and development, as well as software support and training. Since joining the company, Chris has tested numerous MMS components, both hardware and software, from various suppliers from all over the world. Page 13 of 13