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The elevation of that (project) particular landform cannot be negative (I think). Does this mean the DEM data is wrong?

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  • Did you consult the metadata?
    – Bjorn
    May 25, 2017 at 17:07
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    Could this be an unfortunate imperfection like a sink pixel?
    – Louis
    May 25, 2017 at 17:33
  • As @Bjorn said, particularly the interpolation method used which should be in metadata would be the suspect.
    – fatih_dur
    May 26, 2017 at 8:54

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The datum for most elevation is set at sea level. If you have a negative number in a DEM this usually refers to below sea level. An example of this would be Florida. Let me know if this helps

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  • thanks for your response but my location is far above sea level. i download this data from usgs website. and i am confused about its reliability. For example :- the elevation of the project i found was El. 326 masl ... and at the same point in ArcGis it shows 272.
    – Rajendra Kunwar
    May 26, 2017 at 11:55
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What area does this DEM cover? Is this a combined surface model and bathymetric data set? Is this data from someplace like Death Valley California or near the Dead Sea? If so you should expect some negative values. If not you should question the integrity of those data. The values of your DEM show the Z values above or below the origin of some vertical datum. These data might be expressed in any number of tidal datums (heights above or below mean lower low water, above or below mean sea level, mean higher high water, etc.) or any number of orthometric datums (height above a geoid). The difference between the tidal datums and the orthometic datums in my part of the world is about 12 feet so even if I do not know the vertical datum of a DEM I can assume a <=12 foot error.

What is the unit of measure of these data? If the unit of measure is centimeters and you are near a shoreline then the values seem quite reasonable.

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  • thanks for your response but my location is far above sea level. i download this data from usgs website. and i am confused about its reliability. For example :- the elevation of the project i found was El. 326 masl ... and at the same point in ArcGis it shows 272.
    – Rajendra Kunwar
    May 26, 2017 at 11:56
  • @Rejendra Kunwar can you post a link to the source of the data and the coordinates of the location?
    – GBG
    May 26, 2017 at 14:26
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You could use RASTER CALCULATOR to identify cell(s) containing suspicious values. You could end up realizing that odd value(s) are not that odd after all. For instance, I happen to deal with an area in which a deep quarry was captured by the DEM, and that accounted for negative values. But, in order for you to get an exact idea of those values, you may want to pinpoint them first (using what I have suggested above). I will give you further guidelines in case you wish to proceed in that way, or you can follow the procedure (using ArcGIS) showed in this video I found on Youtube (https://www.youtube.com/watch?v=MvepuPiSoJk).

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  • good videos there. thanks for your response but my location is far above sea level. i download this data from usgs website. and i am confused about its reliability. For example :- the elevation of the project i found was El. 326 masl ... and at the same point in ArcGis it shows 272.
    – Rajendra Kunwar
    May 26, 2017 at 11:55
  • It is not clear to me what DEM you are actually using. Can you provide some more info about that? Please note that sometime DEMs come with accompayining documents about the accuracy, possible errors, and the like.
    – NewAtGis
    May 27, 2017 at 5:20

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