I'm not sure, I did some searching. Here is an article that contains some technical detail. https://www.xyht.com/surveying/subsidence/ https://www.xyht.com/su

I'm not sure, I did some searching. Here is an article that contains some technical detail. https://www.xyht.com/surveying/subsidence/ https://www.xyht.com/surveying/subsidence/ "As a point of clarification, the sign Dr. Poland is holding says this is the change detected in BM S661. However, according to the NGS datasheet for that bench mark (PID GU0103), it was not set until 1942. I have been told by reliable sources at the USGS that the pre-1942 subsidence, back to 1925, was a linear estimate. " This appears to be the data sheet for the bench mark used. https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=GU0103 https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=GU0103 Here is a recent analysis from 2018 regarding that area, the photo, and the data used. http://www.itrc.org/papers/pdf/subsidence.pdf http://www.itrc.org/papers/pdf/subsidence.pdf Which seems to conclude that the photo is possibly very wrong, they include a new updated photo at the bottom that only goes back as far as there is reliable data.