Dam monitoring systems use DGPS, one receiver on the wall, the other nearby.<p>Without a local reference station, atmospheric and ionospheric disturbances would totally overwhelm the effect they are trying to measure.<p>This [1] covers it quite well.<p>[1] <a href="http:&#x2F;&#x2F;pasadena.wr.usgs.gov&#x2F;office&#x2F;hudnut&#x2F;SRL&#x2F;" rel="nofollow">http:&#x2F;&#x2F;pasadena.wr.usgs.gov&#x2F;office&#x2F;hudnut&#x2F;SRL&#x2F;</a>