UNAVCO is a nonprofit universitygoverned 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 longterm 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.
The GPS locations of the following UNAVCO reference stations can be viewed and analyzed in my App Display Geo Data or downloaded in CSV format 20211129 UNAVCO Reference Stations.zip .
Station  X  Y  Z  Lat/Long  H  ΔH 

AMDE  2280008.627  5539233.901  2194098.501  20.24161900 67.62740400  3715.800  0.000 
BDJC  2315256.507  5569563.551  2078385.509  19.13178800 67.42744100  3808.800  +93.000 
BMWS  2234836.279  5523178.606  2278862.673  21.05930135 67.97033375  3727.091  +11.291 
BLOV  2225593.314  5577206.54  2153730.805  19.85361900 68.24544900  3735.600  +19.800 
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 algebra 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  R_{Ell}  R_{Ell}+H  Drop GPS  DropΔH  Drop Pred  ΔDrop  ΔDrop% 

BDJC  9.72°  124705.71  124707.72  6344128.34  6347844.14  1132.02  1225.02  1224.94  0.08  0.007% 
BMWS  201.48°  97382.06  97383.01  6348083.11  6351798.91  735.19  746.48  746.50  0.02  0.003% 
BLOV  304.49°  77669.01  77669.49  6369188.35  6372904.15  453.50  473.30  473.29  0.01  0.002% 
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.
As we can see, there is a measured Drop DropΔH between the stations. The Bolivia Salt Flat curves exactly as predicted by the official Globe Model WGS84.
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 algebra to calculate the drop between the stations. Lets label the AMDE reference station vector
(1) 
