Geoid96: Converting GPS Heights above Ellipsoid (HAE) to MSL Elevations

Orthometric Height = HAE (from GPS) – Geoid correction (from GEOID96; these are negative)

GEOID.EXE, from the National Geodetic Survey, converts WGS84 HAE data to NAVD88 orthometric elevations; that's what you need for converting your post-processed, carrier-phase data from the GeoExplorers.

GEOID.EXE works on ASCII files, and accepts two reasonably easy to prepare formats. Both formats are remnants of the old keypunch days when each record had 80 characters; thus spaces count in these formats and you have to get the spaces correct. Here are the two formats:

Type 1:

--------------------- 40 characters ------------------ ------------ 41 – 80 -----------------

The first 40 columns get filled with your sample number, spaces or whatever. Columns 41 to 80 contain latitude, longitude pairs in one of three acceptable formats:

Decimal degrees 45.1234567
Integer degrees, decimal minutes 45 7.40740 a space separates 45 and 7.4_
Integer degrees, integer minutes, decimal seconds 45 7 24.44412 spaces separate 45, 7 and 24.44_

Type 2:

--------- 32 characters ------- ----- 33-40 ----- ------------ 41 – 80 -----------------

Here the first 32 columns contain latitude and longitude in any of the three acceptable formats:

Decimal degrees 45.1234567
Integer degrees, decimal minutes 45 7.40740 a space separates 45 and 7.4_
Integer degrees, integer minutes, decimal seconds 45 7 24.44412 spaces separate 45, 7 and 24.44_

Columns 33 – 40 are reserved for output; columns 41-80 hold your sample number.

 

Preparing Data for GEOID96:

If you just want to get geoid corrections for existing latitudes and longitudes, then simply move things around to fit the file types above. Another approach is to export three columns (lat, long, sample number) from your spreadsheet; you can merge the geoid values back into the complete spreadsheet later. Alternatively if you just a few points to deal with, GEOID.EXE has an interactive mode where you don’t need your data in a file.

In ExCel

In a DOS window, which you’ll need for GEOID.EXE anyhow, use DOS’ EDIT File_id to make sure the file came out ok. GEOID.EXE will go ahead and crunch inappropriate numbers if that’s what you give it.

Run GEOID.EXE at a DOS command line from the directory that contains the program and your file and provide the file names when it asks for them; subtract the geoid correction from the HAE (ellipsoid elevations) you get from the GPS units. Clean up the directory and delete your files when you are done.