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 difference in elevation in one source than in another.

In the case of two topographical maps published by USGS, one in the 1900s 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 each 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 mean 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?

  • What do the maps list for the vertical datum? They could have switched between NGVD29 and NAVD88. – mkennedy Oct 3 '14 at 17:36
  • You can also see if there are NGS survey mark datasheets for the points. The data sheets often include Superseded coordinates so you can track changes over time. – mkennedy Oct 3 '14 at 17:58
  • Heights are also measured with different metrics: orthometric height, geoidal height, ellipsoidal height, etc. – Wes Oct 3 '14 at 18:07
  • @Wes - On a topographic map it would only be orthometric height (ie, height above sea level). The other two are important in geodesy, though. – Martin F Oct 5 '14 at 22:21

The most likely answer has to do with different vertical datums used, as mkennedy suggests. You should probably trust the most recent published data regarding absolute elevations. When comparing elevations, at least use values from the same era.

On topographic maps, elevation is the height above "mean sea level". In geodesy that is called orthometric height and it is related to the height above the ellipsoid and the geoid-ellipsoid separation. The important thing to note, however, is that the knowledge of where "mean sea level" is exactly -- especially far inland, like Arizona -- depends on the latest geodetic measurements and theories. Every few years, geodesists consolidate their newer measurements and theories into an agreed-upon ellipsoid and datum, the latter describing the relationship between the physical earth to the ellipsoid. (Note that often, a named Datum implies both an ellipsoid and a datum.)

(See New coordinate system? for some more explanation and reference to many diagrams.)

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