I have some locations in EPSG:2284:


These are in US Survey Ft, however I don't understand what the elevation is relative to?

The reason this confused me if I have the following image showing these locations from the air and the EXIF data in the image reports an Absolute Altitude (which I assume is height above ellipsoid) of approximately 200m - which would make it just under these points?

  • Are you certain your image is giving you an absolute altitude, and not an height above surface? – Midavalo Mar 2 '17 at 20:32
  • It gives both, height above surface is 91m. Absolute altitude is 200m – nickponline Mar 2 '17 at 20:53

EPSG:2284 only specifies the horizontal coordinates, the altitudes depend on the system the surveyor (or whoever captured the points) used. It could be in NAVD88, Mean Sea Level, etc. You'll have to check the data's metadata or ask whoever did the data collection.

For more on vertical datums see the National Geodetic Survey's page

  • They appear to be in MSL. But that doesn't explain why they are above the image location right? – nickponline Mar 2 '17 at 20:40
  • If the ellipsoid surface is significantly below the topography, sure, the absolute altitude will be bigger than the height above surface. I found a control point near your first point (Lynchburg area?). Its NAVD88 height is 208.6 m (but it's not very accurate, ellipsoid height is around 177 m, so that doesn't match up. – mkennedy Mar 2 '17 at 23:01

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