Using the EGM2008 geoid model

EGM2008 is short for Earth Gravitational Model 2008. It is the successor of EGM96 and EGM84 and it is supplied by the U.S. National Geospatial-Intelligence Agency (NGA) EGM Development Team. The EGM2008 model has a cell size of 1.0 x 1.0 minutes, resulting in a grid of 10801 rows x 21600 columns containing 4 byte IEEE float values defining the difference between the WGS84 ellipsoid height and Mean Sea Level (MSL).

The EGM2008 geoid model (image courtesy of NGA).

The EGM2008 model can be used to calculate the height offset between the WGS84 ellipsoidal height and the Mean Sea Level (MSL) on any place on the globe.

To calculate the height offset, the four points nearest to the current position are retrieved from the geoid file. With these four values, a height offset is calculated using bi-cubic interpolation. The Mean Sea Level (MSL) is calculated by subtracting the calculated offset from the WGS84 ellipsoidal height.

The relation between ellipsoidal height and EGM2008 (MSL).

Because the data file containing all grid values is very large (as it contains about 233 million data points), it would cause Hydromagic to consume 1 Gigabyte of additional memory. To solve this issue, Hydromagic is shipped with a utility called 'EGM2008Util' which allows you to extract a specified region from the EGM2008 data file, and save it as a Hydromagic geoid file.

Downloading the EGM2008 data file

Before using the utility, you need to download the large EGM2008 data file from the NGA website. You need the following file:

Und_min1x1_egm2008_isw=82_WGS84_TideFree_SE.gz

This file name might change sometimes, but it is important that you use the 1x1 minute, small endian, WGS84 variant. At this moment, the file can be downloaded using this link to the NGA website. After downloading, unzip the gzipped file, and copy the extracted file to a folder accessible to the EGM2008 Utility. Alternatively, you can find this file in the Hydromagic download archive.

Using the EGM2008 utility

The EGM2008 utility can be started from the Windows Start menu. It can also be found in the 'Program' folder in the installation directory. After starting the utility, the following screen should appear:

The EGM2008 geoid conversion utility is shipped with Eye4Software Hydromagic.

Input

Use the 'Input' section to specify the input data file downloaded from the NGA website. The utility will check the file and return an error when the incorrect file is used. Note that the utility only works with the 1x1 minute data file.

Output

Specify the file name of the Hydromagic geoid file here. The file should be placed in the '/ProgramData/Hydromagic/Geoids/' folder; otherwise, you won't be able to select it in Hydromagic. By default, this folder will be displayed in the "Save As..." dialog. The file written will have the same format as geoid files downloaded directly from the Eye4Software website. Remember the file name used for this part of the EGM2008 grid; you need it when importing it into Hydromagic.

Selection

In the 'Selection' section, you can select the area you want to export. Note that the maximum area size is 2x2 degrees (equal to 14,400 data points). To specify either a coordinate on the southern or western hemisphere, use the '-' (minus) sign. The software will warn you in case the selected area is too big.

Generating the geoid file

When ready, click 'Start' to generate the geoid file. When this process has been completed, you can close the utility using the "Close" button. The last step is adding the geoid model in Hydromagic. To do so, refer to the Manage Geoids" chapter in the manual.