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I got contour lines for western Romania in vertical system Black Sea 1975 which is based on elevation data with 100 m resolution. Now I would like to compare that to the EU DEM with higher resolution. How can I make those two comparable? Is there a certain Z value that I can use to lower or elevate the EU DEM?

I want to compare two DEMs with two different geodetic systems or "reference sea levels". And those two reference sea levels are "Black Sea 1975" and "GRS 80 EVRS2000 with geoid EGG08". And my question is how I can make those DEMs comparable since they - as I assume - have a different 0. The 0 of "Black Sea 1975" is somewhere in the Black sea and I don't know where the 0 of GRS80 is.

Here is a famous case about using different reference sea levels. I hope that makes my question very clear now..

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  • Ok, maybe my question was not clear enough. It is not about comparing two different DTM or elevation lines with each other. My question is about the different vertical projections. They certainly use a different reference for their Z. Like black sea 1975 uses some reference in the black sea but I doubt that it is comparable to the EGG08 of the EU DEM.
    – Sepp Haunerdinger
    Nov 5, 2018 at 15:52
  • What's confusing is your use of the word "resolution", which means how detailed the data are. For example, if you had one data set of 10-ft contours and another of 100-ft contours, the 10-ft contour set has higher resolution than the 100-ft contour set. Did you mean that the black sea data set is 100 m higher in elevation than the other set? In that case you would want to subtract 100 m from the first set. It's not really clear what your question is.
    – csk
    Nov 5, 2018 at 16:00
  • Thanks for your answer. Yes, I used the word resolution for describing my overall task. So lets just assume I want to compare two DEMs with two different geodetic systems or "reference sea levels" as you could say a bit old fashioned. And those two reference sea levels are "Black Sea 1975" and "GRS 80 EVRS2000 with geoid EGG08". And my question is how I can make those DEMs comparable since they - as I assume - have a different 0. The 0 of "Black Sea 1975" is somewhere in the Black sea and I dont know where the 0 of GRS80 is.
    – Sepp Haunerdinger
    Nov 7, 2018 at 13:40
  • swissinfo.ch/ger/meereshoehe-ist-nicht-gleich-meereshoehe/… here is a famous case about using different reference sea levels. I hope that makes my question very clear now..
    – Sepp Haunerdinger
    Nov 7, 2018 at 13:42

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