I was watching a tv show about Yellowstone. Yellowstone was surveyed in the 1920’s it was surveyed again very recently. They compared the figures for feet above sea level from both surveys. The 1920’s survey was accurate within milimeters. How did they do it without todays technology, and they were 1000 miles away from the sea?
They went to the nearest town’s corporate limits and noted the elevation printed on it, then guesstimated from there. 
It’s called differential leveling. It consists of two people, one with the level rod and the other at the instrument leapfroging each other. It is highly accurate, even in the '20’s because they actually make a loop. Levels are shot from begining to end and return and the results balanced.
I am kinda suprised that the level of Yellowstone is so close because of all the geothermal activity it has gone through in the last 80 years or so, you’d think that would raise/lower the level a bit.
From Cecil’s What is sea level?:
The barometers they use must be really precise to get within millimeters.
some parts of Yellowstone had raised or lowered due to the geologicla activity in the area. they had some reference points that were very stable evidently some distance from the active areas. the measurements there were still accurate. i should have known that they used a barometer, thanks for the answer. i dont understand how they “corrected” the barometer, barometric pressure changes from hour to hour.
Quote from Cecil
[q]Because the ocean surface serves as a floor for the atmosphere and because atmospheric pressure decreases predictably with height, you can use a barometer to establish the elevation of any inland landmark, once you’ve corrected for local climatic variations. (Elevation can also be established by ground and satellite-based surveying.[/q]
Quote from Protesilaus
[q]The barometers they use must be really precise to get within millimeters.[/q]
That is why the USGS United States Geologic Surveys used and to some degree, still uses differential leveling to establish Bench Marks around the country. You’ll often see them along highways and waterways, concrete markers with a brass cap imbedded with the elevation as measured to the top of the brass cap.
I’ve got a couple altimeters/barometers but they are literally accurate to only around 20 ft. or so, and that is after setting it at the nearest benchmark and then only until the weather changes.
The standard today is GPS surveying using satelites but even they are not used when strict accuracy is needed. Scientists still prefer to use established and checked bench marks for accurate information.