UNAVCO is a non-profit university-governed consortium that facilitates geoscience research and education using Geodesy and supports geoscience research around the world.
The UNAVCO GAGE Facility manages a community pool of high accuracy portable GPS/GNSS receiver systems that can be used for a range of applications. These complete systems – receivers, antennas, mounts, power and optional communications – can be deployed for days in episodic campaigns or for many months long-term investigations. Systems are also available for precision mapping applications.
This GPS/GNSS reference stations provide their location to sub cm accuracy in Earth Centered Earth Fixed (ECEF) cartesian coordinates. We can use the 3D positions of this stations to measure the curvature drop, or lack of, to find out whether the Salt Flat are as flat as they look, or curve according to the Globe Model.
The reference station AMDE is located on the Isla Incahuasi in the middle of the Bolivian Salt Flat. Three more stations, BDJC, BMWS and BLOV, are located around the Salt Flat, see Image.
All values except Lat/Long are given in meters.
Lat/Long are calculated from the X,Y,Z coordinates using the WGS84 globe model. They are not used for any of the following calculations. They are only provided to find the locations on any map. You can copy the values in the Lat/Long field and paste it in Google Earth to find the corresponding location.
X,Y,Z and Lat,Long,H describe exactly the same location in 3D space in 2 different coordinate systems. If the earth is flat, than all locations lie on a flat plane in 3D space, no matter which coordinate system we use. If the earth is a globe, the locations lie all on the surface of a spheroid in any coordinate system. So we can use the 3D X,Y,Z coordinates to find out the shape the points lie on.
Using the X,Y,Z coordinates and vector geometry we can calculate the drop of the stations around the Salt Flat with respect to the station AMDE in the center of the Salt Flat. If the earth is flat then the calcultions will show no drop between the stations. If the earth is a globe, then the drop must match the prediction of the globe model. So lets see what the data leads to.
The following table shows the drop calculations from the Station AMDE in the middle of the Salt Flat to the other 3 stations:
|Station||Azim||Dist Chord||Dist Surf||REll||REll+H||Drop GPS||Drop-ΔH||Drop Pred||ΔDrop||ΔDrop%|
The values in the yellow fields are the final Drop calculations from the GPS measurements, all with respect to the reference station AMDE. The values in the green fields are the predicted values for the WGS84 globe model. If the earth were flat, all the values in the yellow fields would be 0.
To correctly predict the Drop for the globe model, we have to know the radius of curvature between the reference stations.
The WGS84 Globe Model describes the earth as an oblate spheroid, the so called Reference Ellipsoid. Contrary to a sphere, an ellipsoid has not only one radius. In fact the radius of curvature depends not only on latitude, but also on the direction it is measured. To get an accurate drop value prediction, we have to use the directional radius between 2 stations, increased by the elevation of the station AMDE. The calculation of the Azim direction and the directional radius is very complicated. I used my WGS84 Calculator for that.
The Bolivian Salt Flat show a Drop in every direction as predicted by the Globe Model. The Earth is NOT FLAT.
There is another salt lake in Utah, the Bonneville Salt Flats. I have the GPS data from a car driving along Rte 80 recording 1274 data points (thanks Jesse). They clearly show the curvature of the earth too. There are even images and a video showing this curvature:
We can use the GPS (x,y,z) coordinates of 2 reference stations and vector geometry to calculate the drop between the stations. Lets label the AMDE reference station vector