## Correcting the ICESat/GLAS Geoid Height (i_gdHt)

ICESat/GLAS elevations above the ellipsoid (i_elev) are referenced to the TOPEX/Poseidon ellipsoid. Differences between this and the WGS-84 ellipsoid are described in Frequently Asked Questions (#9). The following information applies only to the ICESat/GLAS geoid height data.

Release-12 and -14 ICESat/GLAS geoid height data (i_gdHt) were referenced to the WGS-84 ellipsoid in a tide-free system. In future releases of ICESat/GLAS data, geoid height will be referenced to the TOPEX/Poseidon ellipsoid in a mean-tide system. The following IDL program can be used to correct geoid height values from Release-12 and -14 data so that the resulting values are referenced to the TOPEX/Poseidon ellipsoid in a mean-tide system.

correct_geoid_delta_h.pro - a function that returns a double array of correction values in centimeters to add to an array of uncorrected geoid height values in centimeters. Given an array of latitudes (phi) in degrees, the function performs the following calculation:

`delta_h = q0 + q1 * sin(phi)^2 centimeters`

where:

```q0 = 83.37 q1 = -38.48```

The header in "correct_geoid_delta_h.pro" explains the derivation of q0 and q1.

As an example of how to use correct_geoid_delta_h.pro, suppose you have a long-integer i_gdHt consisting of a Release-14 GLA12 geoid height value in centimeters and a corresponding long integer i_lat consisting of a latitude value in microdegrees. If, for example, you want to create a double value (gdHt_corrected) consisting of a corrected geoid height in centimeters, then you would have the following line of IDL code:

`gdHt_corrected = i_gdHt + correct_geoid_delta_h(i_lat / 1d6)`

Page last updated: 06/12/08