A geoid is a model of the global mean sea level that is used to measure precise surface elevations.
The United States is divided into eight regions of the continental states and individual zones for Hawaii, Alaska, and Puerto Rico.
If a geoid is used on a project, it will typically be called out on the jobsite plans.
Geoid revisions are classified by year. For example, the GEOID03 is the revision for 2003.
Using the incorrect geoid revision can result in elevation errors.
Geoid grids will overlap by approximately 150 miles (240 kilometers).
Using the wrong grid can result in elevation errors.
After determining the geoid version needed, visit the NGS website. Navigate to the GEOID section of the website, and find the correct version and type of geoid you will need. Typical applications will use hybrid geoids which converted heights from NAD83 to regional vertical datums such as NAVD88.
From the grid list, pick the grid you will be working in. Geoids should always be downloaded in Ie binary or Little-endian format.
This will save a *.bin file to your PC. For example, g2003u05.bin is the 2003 geoid for Zone 5.
Topcon 3D-MC and Pocket 3D software require a *.gff file for geoids and cannot read the *.bin file from the NGS website.
*.bin files can be converted to *.gff files using MAGNET Tools.
MAGNET Tools does not need to be licensed for this conversion to work.
After opening MAGNET Tools, navigate to Process > Geoids. Creating a new project is not required.
MAGNET Tools comes with many of the geoids preloaded into the software. If the geoid you need is not listed, the Add button can be used to import the *.bin file you downloaded.
Select the correct geoid from the list, and hit the Export... button.
Select where the file will be saved, enter a File name, and make sure the Format name is set to Topcon Geoid (*.gff).
Make sure to check the Apply legacy format checkbox.
If legacy format is not used, the following may happen:
- The *.gff file will appear in the Pocket 3D geoid list, but cannot be applied.
- The *.gff file will appear in the 3D-MC geoid list, but may result in a vertical error.
For instructions on using the geoids in 3D-MC and Pocket 3D, please see the related articles below.