re: So let me get this straight....
Read the report that I linked. EVERY pixel of White Rock they looked at with the TES data contained BOTH the light colored material and the dark material that surrounds White Rock. They had to estimate what portion of a pixel was covered by the white material and what portion by the dark material. This was because of the coarse resolution of TES.
With THEMIS considering that there are 18,000 pixels covering the structure instead of just 8 you can better try to select pixels that contain only the light colored material making up White Rock.
Bob Clark