So I have been investigating the area around Jerome and Clarkdale Arizona using different USGS Topographic map sources of the same scale, but different time periods, and find that in some cases, the same control point has a different in elevation in one than source than in another. In the case of two topographical maps published by USGS, one in the 1900's and the other a number of years later, every control point I have examined (though not by any means all) the difference in values for the same control point varies by the same amount between the maps at every point I examined (which I admit is by no means all). This suggests to me some type of systemic error, perhaps recalculation of sea level. Considering another pair of maps, the difference in elevation given on the two maps for the same control point varies by a different amount than the difference in elevation given on the two maps for another control point. In the latter case, the difference in the elevations ranges from 0 to a couple of meters.

Where can I find an explanation of what the differences between these maps stem from, and how do I decide which is the most accurate elevation?