# Notes and Usage Page For EGM96 Geoid Calculations

### Program constraints:

-89° 44' 24" <= Latitude <= 89° 44' 24"
-180° <= Longitude <= 360°
North positive, East positive convention

The latitude constraint is due to the spline algorithm not selecting points beyond
± 89° 44" 24'

If calculations near the poles are desired, try downloading the EGM96 Geoid Calculator program for Windows 95/NT. That GUI driven program uses spherical harmonic synthesis rather than interpolation to determine the geoid.

### Usage:

If the coordinate is negative, then place a negative sign on the degree value. Placing additional negative signs on minutes and seconds is supported but not required (unless the degree portion is zero). If degrees are positive, then both the corresponding minutes and seconds should be positive.

Examples of valid input coordinates:

•   40°   30'   30"
• -40°   30'   30"
• -40° -30' -30"
•     0° -10' -20" (Sign required on min/sec for negative coordinate with zero degree)
Examples of invalid input coordinates (combined Lat/Lon cases):
•     90°     0'       0" (Latitude polar case not allowed)
•     40° -30' -30" (Positive degress, negative min/sec)
• -185° -10' -20" (Longitude boundary exceeded)

Geoid heights can be used to convert between orthometric heights (approximately mean sea level) and ellipsoid heights according to the formula:

h   =   H   +   N
Where,
h = WGS 84 Ellipsoid height
H = Orthometric height
N = EGM96 Geoid height

Note that EGM96 applies only to the WGS 84 reference ellipsoid.

### Development:

EGM96 is based on a spherical harmonic expansion of the disturbing potential to degree and order 360. This PERL CGI is using an ANSI C program that performs spline interpolations.

Back to Geoid Calculator

Point of Contact: Office of Geomatics
phone (314) 676-9127, DSN 846-9127