Adding A Geoid Model From An External Source

Can I download geoid models from external sources and use them in Geographic Calculator? How do I add them to the Datasource so I can use them for vertical transformation?

Yes, it is possible for you to download geoid models and use them in Geographic Calculator. The ability to add custom transformations is a powerful feature in the software; you are not limited to the existing EPSG transformations. If your work requires you to use a particular geoid model, you can add it to your datasource. All you need is the geoid model file in a supported format and the appropriate usage information to ensure correct usage of the model. Geographic Calculator supports a number of different geoid file types, including .GTX and and .GGF.

If necessary, other grid formats may be opened in Global Mapper and exported as a .bgd (Blue Marble Gridded Data) file for use in vertical transformation in Geographic Calculator, as outlined here.

Bringing the Geoid Model into Geographic Calculator

Download your geoid model in a supported file format. For this example, we downloaded the Trimble geoid format (.GGF) for the Grand Cayman geoid model from this website.

Unzip the download if needed to find the geoid model file inside (e.g. GCGM0811.GGF), and place this file within your vertical shift files directory on your machine (C:\ProgramData\Blue Marble Geographics\Geographic Calculator x64\DataSource\shiftfiles\vertical).

Adding objects to the datasource

Next, you’ll typically need to add new objects for the vertical datum (e.g. GCVD54 in our example) and corresponding vertical coordinate system, if they aren’t already included in the GC datasource. Then you’ll be able to add the new transformation(s) to use the new geoid files with the appropriate coordinate system / datum.

1. Under Datsource Menu > Datum Definitions, select the Vertical folder (on the left) and use the + button (at the bottom) to add a new object. Basically, you are just adding an object with the correct name for the new datum here. At the bottom of the screen, click Save to save your addition to the Datasource before closing the window.

2. Under Datasource Menu > Coordinate System Definitions, select the Vertical folder (on the left) and use the + button (at the bottom) to add a new object. In the Details tab, the Point Style allows you to specify “height” and the desired unit of measurement. The Datum will be the corresponding object you created in step 1. Optionally, you can add the appropriate Usage area. Again, Save the Datasource.

3. Under Datasource Menu > Coordinate Transformation Definitions, select the Vertical folder (on the left​​​​​​​) and use the + button (at the bottom) to add a new object.

• In the details tab, it is important to specify the source (e.g. ellipsoidal height) and target (your new vertical coordinate system).
• Choose your vertical transform method (in this case, Trimble Geoid Grid).
• Specify the “vertical offset file” that you placed in your shift files (e.g. GCGM0811.GGF).
• You have the option to select an appropriate interpolation type.
• Specify the path (in this case, “vertical” for the vertical folder).

Click OK and Save the Datasource.

Using Your Geoid Transformation

Your custom objects added to the Datasource will appear in blue text. When using your custom added transformation, be mindful of the horizontal datum or reference frame in use, and any other transformation if applicable to ensure you are using your geoid model appropriately. Refer to your particular geoid documentation.

Test your new transformation on an appropriate point with the Interactive Conversion job. Let us know if you have any further questions.