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I have raster data with height values (z). When I try to reproject/warp the raster to Gauss Kruger 3, the z-values are changing (several meters). (The data is already GK3, but I have to reproject them, because the clipper function does not work otherwise.)

Why is that?

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    How could they not change? Consider what happens during the reprojection: because the raster is truly being warped, each cell in the new version has to reflect some combination of values from the original raster. Except for special cases (where the warping is so small that there is a one-to-one match between input and output cells and you specifically request that no interpolation be done), there cannot possibly be a perfect match of values between the rasters. Perhaps what you should ask instead is "what can I do to control how the z-values are interpolated during the reprojection?" – whuber Jun 21 '13 at 17:14
  • so its normal that the highest/lowest value changes about 10 meters? thats odd :/ – user18949 Jun 21 '13 at 17:39
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    That's not in the least unusual: the default interpolation by most systems is a form of average. Averaging, by its very nature, decreases large values and increases small ones. I have seen z-values for mountaintops decrease by over 100 meters upon reprojection. Understanding this phenomenon will help guide you towards effective strategies for maintaining raster data; in particular, it should now be obvious that you want to minimize the number of reprojection procedures when going from raw data to the final data that are mapped or analyzed. – whuber Jun 21 '13 at 18:17
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Because the comments seem to have constituted a satisfactory answer, I am copying them here for the record (and adding a few edits to improve them).


How could the values not change? Consider what happens during the reprojection. Because the raster is truly being warped, each cell in the new version has to reflect some combination of values from the original raster. Except for some special cases there cannot possibly be a perfect match of values between the rasters.

(The "special cases" are where the amount of relative warping is so small that there is a one-to-one match between input and output cells and you specifically request that no interpolation be done--that is, you use "nearest-neighbor" interpolation.)

Perhaps what you should ask instead is "what can I do to control how the z-values are interpolated during the reprojection?"

So it's normal that the highest/lowest value changes about 10 meters? That's odd :/

That's not in the least unusual:. The default interpolation by most systems uses some kind of (weighted) average, such as "bilinear interpololation." Averaging, by its very nature, decreases large values and increases small ones. I have seen z-values for mountaintops decrease by over 100 meters upon reprojection. (This happened in a 1Km resolution DEM of Haiti, for instance.)

Understanding this phenomenon will help guide you towards effective strategies for maintaining raster data; in particular, it should now be obvious that you want to minimize the number of reprojection procedures when going from raw data to the final data that are mapped or analyzed.

  • Great answer, nn/bilinear/cubic interpolation is a natural phenomenon in raster warping. As a further fact, I would add, that some of the GIS softwares (like ArcGIS) can take the vertical coordinate system in consideration, too (as far as I know). – Gabor Farkas Mar 17 '15 at 21:59
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    @Gabor I doubt the vertical datum is considered during the reprojection process. Not only would it considerably slow things down, it is generally unnecessary because the local change in the datum is so small. Moreover, exactly how the vertical datum would be used depends on how the z-values are related to elevations, which is not information that is available to ArcGIS (nor any other GIS that I know of). – whuber Mar 17 '15 at 22:02
  • Well, then vertical coord. system only affects vector reprojections (if they're defined) as they're stored internally in ArcGIS, and a one-to-one transformation can be applied on. Thank you for your answer! – Gabor Farkas Mar 17 '15 at 22:10

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