In QGIS I georeferenced a 1970 US Army contour map for part of southern Laos (20 m intervals) with the following indicated properties:

Horizontal datum Indian datum 1960
Verical datum ="approximate mean sea level"

It shows elevations about 12 m lower than my SRTM tif which I set to WGS84 EPSG:4326. The Army map fits other information I have for my project better. I guess I could just adjust the base for my project by 12 m but I hate to do this without understanding the big difference.

I'm new to all this. Any ideas on how to reconcile the two?

  • SRTM is in EGM96 vertical datum lta.cr.usgs.gov/SRTM1Arc, your map is in MSL.. thus 12m is the difference between ellipsoidal and sea level height datums. Note that the vertical accuracy of SRTM is 10m www2.jpl.nasa.gov/srtm/statistics.html which is about the geoid separation. – Michael Stimson Oct 12 '18 at 1:16
  • @MichaelStimson EGM96 is approximately MSL, not ellipsoidal heights. – mkennedy Oct 12 '18 at 17:06
  • @radouxju I downloaded the Japanese ALOS tif which some say is better than the SRTM. It agrees within a meter or two with the SRTM for my area in Southern Laos and so also indicates about 12 m higher elevations compared to the 1970 US ARMY map which appears of high quality, similar to USGS 7.5 min quads. Haven't tried ASTER yet. So my problem remains unsolved. – Dick Meehan Oct 13 '18 at 23:00

Elevation is measured using a vertical coordinate system (vertical datum), and this vertical coordinate system can differ even if your ground coordinate system (horizontal datum) is the same. There are two main types of coordinate system for the elevation, above ellipsoid or above sea-level (which can be further divided into local seal level and geoid) :

  • Height above ellipsoid (HAE) the elevation is measured from the datum of the geographic coordinate system that gives your position on the Earth surface. This is typically the case of the raw elevation coordinates that you get from a GNSS system. The differences can be very large (roughly between -100 to +80 m, note that Southern India is -100m).

  • Height above local mean sea level (MSL): the sea level is a local parameter which can differ between two neighboring countries. Rigourous measurement takes the average of the tides over nearly two decades, at one point (for India, it seems to be at Cochin, Southwest Coast of India). The coordinates are then obtained by levelling. The countries that maintain a levelling network usually provide an altimetric correction grid because the difference between MSL and HAE is not constant. However, if you don't have such a grid, using a constant correction factor will remove the bias, which is the main component of the errors, but local differences will still remain (especially on a large area). Just to give you an idea, their can still be a difference of two meters after removing the bias in Belgium (about 300 km distance). This vertical datum is often used with topographic maps.

  • Orthometric height : the elevation is measured above the geoid, an imaginary surface defined based on the gravity field. As mentioned by @Michael Stimson, SRTM uses the EGM96. This is global surface fitted on the MSL, therefore it differs from local MSL datums which are only based on one point of reference.

That being said, your mentioned difference (12m) between the "approximate MSL" and the EGM96 seems quite large to me. It is in many places around 2 m, so part of the difference could be due to the combined uncertainty of the contour map and of the SRTM. It is however possible because India is quite large, so local and global MSL could differ. If you need to know what to trust, take some GPS coordinates and compare them with elevation on your maps (using EGM96 data to convert from HAE to orthometric height).

Remark: For (very) precise measurement, one should also take into account the definition of the verticality. It can be defined as the normal to a given surface or as the direction of the gravity field.

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