Unlike projections or customized SRS that can be implemented in ContextCapture using a PRJ or WKT file, the vertical datum can only be implemented using a transformation grid in GTX format. Here is a short procedure that describes step by step how to add a vertical datum grid (geoid model) into ContextCapture
1) Close all ContextCapture sessions
2) Go to the “gdal” folder of the ContextCapture installation. This folder can be found in the following location:
- C:\Program Files\Bentley\ContextCapture\data\gdal , if you are using the ContextCapture Desktop.
- C:\Program Files\Bentley\ContextCapture Center\data\gdal , if you are using the ContextCapture Center.
3)- Copy and Paste the GTX file of the geoid model to this folder
Note: the official national geodetic databases rarely provide the conversion grid in GTX format. This is usually in a common raster format such as GeoTIFF. The GeoTIFF can easily be converted in GIS software such as QGIS or any other software able to handle GTX export.
4) Still in the gdal folder, open the file named: vertcs.override.csv
5) Select Save as and save to a location outside of the Program Files folder (this will allow you to edit the file) and open it
6) Go to the first blank line (line 33 in the example below) and enter the description of the new vertical system preceded by a #
7) Access the EPSG registry using your web browser using the following url: http://www.epsg-registry.org/
8) Create a new query with the name of the height system and the ’type’ set to ‘vertical CRS’. Below is the case of the New Zealand Vertical Datum
9) Make a new query with the SRS name but with the ‘type’ set to ‘Vertical datum’
10) Columns E to I are filled with the same numbers as the other datum
11) Column J (PARM_1_1) needs to be filled with the GTX file name that has been pasted into the gdal folder.
After running steps 6. to 11. of the procedure, the vertcs.override.csv file should be similar to the image below image:
12) Go back to the gdal folder and change the name of the existing vertcs.override.csv to vertcs.override_backup.csv. Copy the one which has just been edited to this location.
The new vertical SRS is now ready to use in ContextCapture
13) Open your ContextCapture project, and go to the Control Point editing window of the relevant block. While selecting your spatial reference system in the database, select ‘Override vertical coordinate system’, the recently added vertical coordinate system should now be available.