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I have some survey grade GPS points whose elevation is approx 55 feet lower than my contours layer elevations. I am using 1 ft contours and comparing point elevations with contours in the same location. Any idea why there's such a discrepancy?

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    Mean Sea Level, GPS, and the Geoid are measured 'slightly' differently across the world esri.com/news/arcuser/0703/geoid1of3.html – Mapperz Sep 22 '15 at 21:24
  • Contours are only a representation of the terrain, they are not physical features that can be measured so are inherently not accurate, that said 55 feet is quite a lot - assuming your contours came from accurate data (like LiDAR) and not SRTM; 55 feet sounds about right for Geoid height vs. MSL, I'd say you have two different vertical datums. Check the GPS height datum on the device if it can be changed or from the manufacturer if it cannot and refer to the metadata for your contours or the elevation data it was derived from to see what vertical datums were referenced. – Michael Stimson Sep 22 '15 at 21:34
  • That's probably it. I'm going to check both vertical datums. Thanks – Knteza Sep 22 '15 at 21:40

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