1. About pyegt

pyegt is an open source program developed by NCEAS to look up the geoid, tidal, or geopotential model height above the ellipsoid in order to convert model-referenced heights to ellipsoid height (i.e. compatible with Cesium) and vice-versa.

The following figure demonstrates the difference between geoid, ellipsoid, and topographic ground surface:

Diagram showing conceptual model of ellipsoid height h, geoid height H, and height of geoid relative to ellipsoid N along with topographic surface (grey).

Ellipsoidal height (h) is generally used in global projections such as Cesium due to its small digital footprint and ease of calculation relative to systems based on gravity or geopotential height. However, gravity and tides are influenced by local differences in Earth’s density and other factors. Therefore some projects prefer reference systems that use height referenced to a geoid or tidal model (H) which provides a much easier framework to understand height relative to, for example, modeled mean sea level or sea level potential. Converting from H to h requires knowing the height difference between the geoid and the ellipsoid (N). Conversion is then a simple addition of these values (H + N = h).

The lookup performed by the pyegt.height.HeightModel class will use either the NGS or VDatum API to look up model height relative to the ellipsoid (N).

Note

pyegt is a wrapper around the NGS and VDatum APIs, but not all functionality of those APIs has been included in this software. Please read the documentation and consider contributing to the codebase if you feel strongly about missing features.