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INTRO

A bit of tool introduction, but the problem is not related to this directly.

I have an app in CesiumJS.

I am using Cesium World Terrain, and get altitude using Sample Terrain Most Detailed

This would allow me to As is typical of the Cartographic type, the supplied height is a height above the reference ellipsoid (such as Ellipsoid.WGS84) So getting an height in WGS84.

PROBLEM

I have my WGS84, but I am using a server that require an altitude in MSL, not WGS84. Which require me to convert my WGS84 altitude to MSL.

Over the internet I found that a way would be

MSL = GPS - GEOID

I then took some measure using those sites :

And found the following difference (in meter)

GPS   |  CESIUM       | GEOID      |  DIFF (GPS / CESIUM)

669   | 705           | 49.421     | +36
52    | 88            | 37.162     | +36
51    | 36            | -15.992    | -15
0     | -86           | -98        | +86 

From what I understood, GPS and WSG84 should be the same, so my GPS and my Cesium altitude should be the same...

But it turn out that, if I use the previous formula I do find the correct value for GPS, but not with my Cesium WSG84 value.

QUESTION

When referring to altitude, are WGS84 and GPS measure identical, or is GPS just based on WGS84 but require additional conversion to retrieve a WSG84 altitude ?

  • Interesting question - if you can edit your questions down to one (and I'd suggest rephrasing it a bit) this is really worthwhile. – Simbamangu Nov 18 '19 at 14:47
  • Also have a look at other discussions on ellipsoid and GPS error. – Simbamangu Nov 18 '19 at 14:53

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