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FAQ and Tips

Chapter 9

 

 

 

GridDLL

Users can implement their own grid conversion inside a Waypoint Dynamic Link Library (DLL). Within the Developer's Kit, there is a sample DLL that can be used as a template.

9.11.1What features are available with map projections?

Waypoint’s software supports two manners of map projections:

1.Direct selection (for example, UTM, TM, Gauss Kruger, State Plane, Lambert, and so on).

2.Pre-defined and user-defined grid definitions using the Grid Manager.

For coordinate output, both methods can be used in the Export Wizard by using the Selectable Grid-East and

Selectable Grid-North variables. See Section 2.7.4, on Page 111 for more information. For Write Coordinates, only direct selection is possible.

For coordinate input that is, base stations in GrafNav and control points in GrafNet, only grid definitions can be used. Another advantage with using grid definitions is that users can manage multiple user definitions more easily that is, for each country or region.

9.12 Geoid FAQ and Tips

The geoid is the level of the sea surface (equipotential surface under land masses). For instance, a spirit level, or surveyor’s level, would follow the geoid. Because of this, vertical datums are based upon the geoid. Maps are also based on the geoid to prevent any contour lines from forming on level surfaces such as lakes.

GPS can only make measurements on the ellipsoid, and, therefore, elevations must be corrected to reference the geoid. The height above the geoid is known as an orthometric, or mean-sea-level (MSL), height. The difference between the geoid and the ellipsoid is called the geoid height or undulation value, and it changes across the earth by 100 meters. Luckily, many government agencies put out geoid undulation grid files that can be used to convert ellipsoidal to orthometric heights.

The relationship between the orthometric height, H, the ellipsoidal height, h, and the geoid height, U, is given by:

h = H + U

9.12.1What are the available geoid - related features?

Waypoint’s software supports geoids in three different formats including the U.S. National Geodetic Survey’s GEO format, the Geodetic Survey of Canada’s BIN/SLV format, and Waypoint’s own WPG format.

When correcting ellipsoidal heights to produce orthometric heights, it is very important that the geoid and processing datums match. For example, if EGM-96 is used, then the base station coordinates should be in WGS84, and this datum should also be used for processing. Be careful to use the same geoid model as used on the control sheets. This lessens the likelihood of differential errors developing.

In some cases, you may wish to process in a datum different than the one the geoid uses. For this case, accurate results can still be obtained by using the geoid model in a relative fashion. The slope should be roughly the same between ellipsoids. Between NAD27 and NAD83, there is up to 200-metre difference in geographic coordinates. In areas with very high relief, this may result in a few centimeters of orthometric height error. The same can be said for other datums with large shifts (for example, TOKYO and ED50).

In GrafNav, conversion to ellipsoidal height can be performed when entering the master coordinates. When exporting with Export Wizard, the geoid undulation will be subtracted.

For GrafNet, the process is similar. This is because you can convert orthometric heights to ellipsoidal as you enter the control/check point coordinates. Be sure to set the ellipsoidal/orthometric height flag correctly.

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Chapter 9

FAQ and Tips

 

 

Elevations are always stored as ellipsoidal inside GrafNav and GrafNet. Conversion to orthometric will only take place at time of coordinate export. Many of the export profiles use ellipsoidal height, so care should be taken when exporting elevation values.

9.12.2How can I create a WPG file?

Waypoint’s software includes command-line utilities to help create WPG files. These programs can only be used with ASCII files which have been properly formatted. Binary utilities exist but are only compatible with the source files of specific geoids, all of which are now obsolete (i.e. GSD95, Geoid96, AUSGeoid93).

The utilities provided with the software are read_ascii_geoid.exe and makegeoid.exe. The latter program is needed to produce the final WPG file. In addition, the program will also produce a bitmap file (BMP), giving a visual representation of the geoid. The input ASCII file must be in the following space-delimited format:

Latitude Longitude Undulation

60.000000000000 -188.00000000000 -3.4057000000

60.000000000000 -187.98333333320 -3.4400000000

60.000000000000 -187.96666666640 -3.4729000000

60.000000000000 -187.94999999960 -3.5042000000

The Latitude and Longitude values must be in decimal degrees and should be specified to at least 12 decimal places. Furthermore, Southern latitudes and Western longitudes should be designated as such via the use of a negative sign. The Undulations values must be in meters.

If the source file is not provided in the aforementioned format, the read_ascii_geoid.exe utility can be used to reorganize it. This utility will read in an ASCII file containing undulation values, assuming they have been sorted in a specific way. The header of this file must provide the following space-delimited fields:

S_border W_border Lat Long nrows ncols

49.0000000 202.0000000 1.666666667e-02 1.666666667e-02 721 1921

S_border and W_border represent the southern and western boundaries, respectively, of the region covered by the geoid. These values must be in decimal degrees, and negative signs should be used to indicate a Southern latitude or a Western longitude. The Lat and Long fields represent the grid spacing, in decimal degrees, of the source file. Finally, the nrows and ncols values represent the number of rows and columns covered by the source grid.

The remainder of the file must provide space-delimited undulations. The first undulation value should be at the southwest corner of the grid. The subsequent space-delimited undulations should remain along the same latitude but moving eastward by Long. Once the eastern boundary is reached, the latitude should increment by Lat and the undulations should begin from the western boundary again and move eastwards. It follows, then, that the last undulation value in the file should be that of the north-east corner of the grid.

The output file created from the read_ascii_geoid.exe utility can then be run as input for the makegeoid.exe utility.

makegeoid g2006.txt –o Geoid06 –d NAD83 –g “Geoid06” –r “Alaska”

In addition to creating a WPG file, this utility also produces a BMP image of the geoid undulation.

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