Undulation of the geoid
Encyclopedia
Undulation of the geoid is the mathematical process of determining the height in meters above the geoid
(relative to the mean sea level) from the height provided by the GPS system which uses the (WGS84) ellipsoid as reference. In maps and common use the height over the mean sea level is used to indicate the height of elevations while the ellipsoidal height results from the restrictions that apply for the GPS system.
The process of the undulation it is not standardised, as different countries use different mean sea levels as reference but mostly refers to the EGM96
geoid. Calculating the undulation factor is mathematically challenging. This is why many handheld GPS receivers have built in undulation lookup table
s to determine the height above sea level.
The deviation
between the ellipsodidal height 
and the normal height 
and can be calculated by
Geoid
The geoid is that equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest , and extended through the continents . According to C.F...
(relative to the mean sea level) from the height provided by the GPS system which uses the (WGS84) ellipsoid as reference. In maps and common use the height over the mean sea level is used to indicate the height of elevations while the ellipsoidal height results from the restrictions that apply for the GPS system.
The process of the undulation it is not standardised, as different countries use different mean sea levels as reference but mostly refers to the EGM96
EGM96
EGM96 is a geopotential model of the Earth consisting of spherical harmonic coefficients complete to degree and order 360...
geoid. Calculating the undulation factor is mathematically challenging. This is why many handheld GPS receivers have built in undulation lookup table
Lookup table
In computer science, a lookup table is a data structure, usually an array or associative array, often used to replace a runtime computation with a simpler array indexing operation. The savings in terms of processing time can be significant, since retrieving a value from memory is often faster than...
s to determine the height above sea level.
The deviation
between the ellipsodidal height and the normal height and can be calculated by