Unlike projections or customized SRS that can be implemented in iTwin Capture Modeler 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 iTwin Capture Modeler
Steps to add a geoid model
1) Close all iTwin Capture Modeler sessions
2) Go to the "gdal" folder of the iTwin Capture Modeler installation. This folder can be found in the following location:
- C:\Program Files\Bentley\iTwin Capture Modeler\data\gdal , if you are using the iTwin Capture Modeler Desktop.
- C:\Program Files\Bentley\iTwin Capture Modeler Center\data\gdal , if you are using the iTwin Capture Modeler 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: EPSG Geodetic Parameter Dataset
8) In the search box enter the country code for vertical datum you wish to search. In the case NZVD for "New Zealand vertical dataum"
- The resulting search will find the nearest match to your request. In this case we select the most recent vertical option.
- The next page will display your vertical CRS in the top section
- Expand the Datum link to display the code for the vertical datum
9)
- The resulting vertical datum code must be implemented in column C (DATUM_CODE). In this example "1169"
- The resulting name must be implemented in column D (DATUM_NAME). In this example "New Zealand vertical datum 2016"
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 iTwin Capture Modeler 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.