How Does GPS Account for Asymmetric Geometry on Earth's Surface?

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  • Thread starter Thread starter Loren Booda
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Discussion Overview

The discussion revolves around how the Global Positioning System (GPS) accounts for the irregular geometry of the Earth's surface in relation to its center of mass. Participants explore the implications of GPS measurements, including latitude, longitude, and altitude, and how these relate to various models of the Earth's shape.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant suggests that GPS does not account for the irregular surface of the Earth, providing only latitude, longitude, and altitude based on a smooth model of mean sea level (MSL).
  • Another participant explains that GPS calculates height relative to the Earth's barycenter and converts this to a height based on the selected spheroid model (WGS84), which can lead to discrepancies when comparing to actual surface elevations.
  • A further point is made regarding the need for conversions to mean sea level based on specific coordinate systems, highlighting that certain areas, like the coast of Britain, may be several meters below sea level according to the WGS84 model.
  • One participant mentions the use of Differential GPS (DGPS) as a method to achieve better accuracy in positioning.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of GPS in accounting for the Earth's irregular surface. There is no consensus on whether GPS adequately addresses these irregularities, and multiple perspectives on the topic remain present.

Contextual Notes

Participants note the dependence on specific spheroid models and coordinate systems, as well as the potential for unresolved discrepancies in altitude measurements relative to actual surface features.

Loren Booda
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How does the Global Positioning System compensate for mapping the irregular surface of the Earth relative to its center of mass?
 
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If I understand what you are asking, it doesn't - it gives a lat, long, and altitude above msl (so the model of Earth it has is smooth and based on msl). If the surface of the Earth happens to be at that altitude, great. If not...hope you have wings...
 
Inherently it calculates a height above the Earth barycentre, essentially a distance below it's orbit.
It then converts that to a height based on whichever spheroid model you selected (for GPS=WGS84), then if you have selected a particular map grid (eg OSGB) it converts the height into the mean sea level for the default for that coordinate system.
This is all necessary to deal with bumps - the coast of the Britain is several metres below sea level according to WGS84.
 
In order to get better results you can use Differential GPS (DGPS).
 

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