Is there truly an accurate way to measure the sea level of a location?

Why is it so difficult to accurately measure the sea level of a given location, say like Denver, Colorado? its always been called the “mile-high city”, but is this an accurate description? scientists have always had a difficult time giving an accurate measurement to a given location. it is because of the shifting tides? even sattelites seem to be of no help. so, is Denver really a “mile-high city” or could the numbers be off be a foot or two?