We know the earth’s diameter at the equator is about 25 miles larger than at the poles, therefore about 12.5 miles larger in the radii. Some large part of this phenomenon is due to the spin of the earth on its axis. The Mississippi river flows from an elevation of 1476 above msl at Lake Itasca down to sea level at the Mississippi Delta. A linear equation suggests that the sea at is about 11,200 feet further from the center of the earth at New Orleans than at the source of the Mississippi.
The longitudinal difference is 17.25 deg. or about 19.2 percent of the 90 degrees between the equator and the pole, so 19.2% of r. of 12.5 miles is 2.4 miles or 12,670 feet less the 1,476 slope = roughly 11,200 feet. (is that roughly a valid assumption?)
Before the GPS I'm assuming that the benchmarks for elevations were established empirically by surveyors performing inland surveys/traverses across the Northwest Territory with cross references being made across the great lakes.
My question is how elevations are determined accurately by GPS since, for instance, the delta is further from earth’s center (closer to the GPS for purposes of distance) than the source at Lake Itasca?
Another related item, I read (HERE) is that the sea level is rising at a (3 or for times) faster rate on the east coast of North America than other places around the world. That seems irrational to me, unless that faster rate is accounted for by a movement of some large heavy mass below the earth’s surface, under or centered nearby, and geological changes of that sort are not seen in decades but in hundreds of millennia. Any ideas why there would be isolated regional and relatively rapid in changes in sea levels such as this?