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Geoids

 

 

 

 

Each survey has a Coordinate Reference System (CRS) which determines the relationship between geographic coordinates (latitude, longitude and ellipse height) and rectangular coordinates (X, Y and elevation). The CRS uses an ellipsoid (mathematical representation of the earth), a horizontal datum (to reference geodetic to rectangular coordinates), a vertical datum (currently only NGVD 1929) or geoid and a map projection (to convert the earth's spherical surface to a flat surface).


Elevation vs Ellipse (GPS) Height

Elevations and ellipsoid heights are related by the following equations.

h = N + H,  H = h - N

where:

N = geoid height = height of geoid above the ellipsoid (geoid undulation)

h = height above the ellipsoid

H = height above the geoid (elevation = MSL height)

Geoids help us relate ellipsoid heights (shown in the figures as h) with elevations (shown in the figure as H) by computing a geoid undulation for any geodetic position.

By default, TPC selects a geoid model called None, and no geoid computations are performed. For small surveys, a simple translation of ellipsoid heights to elevation works great, but there are lots of reasons for needed a geoid model.

 

As soon as you select a geoid, coordinate elevations get computed automatically from geodetic ellipsoid heights and visa versa.

 

You can even change geoids and watch TPC compute slightly difference elevations and ellipsoid heights as you do.


TPC Coordinate Points

In TPC, if a survey point we not derived from geodetic lat/lon/ht, it is a coordinate point based on X,Y,Z.

If you have selected a geoid for your survey, coordinate points compute a corresponding ellipsoid height based on their elevation.

If you have not selected a geoid for your survey, TPC copies the elevation of coordinate points into the ellipse height of their geodetic position.


TPC Geodetic Points

In TPC, points that were created by importing or entering lat,lon,ht or points that are computed via a geodetic computation are geodetic points. TPC holds their geodetic positions and computers their coordinates.

If you have selected a geoid for your survey, geodetic points will compute a corresponding elevation based on the geoid and their ellipsoid height.

If the elevation of that point is protected, TPC will not overwrite that protected elevation.

If you have not selected a geoid for your survey, TPC copies the ellipse ht of geodetic points into the elevation of their coordinates.


When are Geoid Heights Computed

TPC has a basic routine that it calls as needed to transform a survey point, computing its geodetic position if it is a coordinate point or computing it's coordinates if it is a geodetic point. Geoid heights are computed in this basic routine. So anytime lat/lon is computed, ellipse height is computed. Anytime X/Y is computed, elevation is compute.


Turning off Geoid Computations

To turn off geoid computations, look for the Geoid section and click the Model drop down list and choose None. When you choose None, some of the items in the Geoid section will be disabled.


Geoid Models

Before TPC can compute geoid heights, it must have a geoid model to work with.  This is selected as part of the Coordinate Reference System for the survey.  See Geoid Models.


Related Topics

Coordinate Reference Systems

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