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I have a raster layer which contains mean erosion values in tonnes/hectare/year. The layer is in EPSG:4283-GDA94 with degrees as units (the green layer below). When I save it (i.e. reproject it) to EPSG:28356 - GDA/MGA zone 56 - projected, the values in the raster change (the blue raster below). Please see the image below.

enter image description here

My question is: What calculation has been done on the values in the raster during the reprojection from degrees to metres. I need to know how the values in the reprojected raster are related to the source raster (which is in degrees) so that I can use the data in some raster calculations.

Obviously during the reprojection the cell size will change but how does QGIS re-calculate each pixel values ?

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  • Probably using "nearest neighbour" resampling (gis.stackexchange.com/q/223799/2856). Use the "Warp (reproject)" tool if you want to specfy a different resampling method.
    – user2856
    Commented Oct 9, 2019 at 4:33
  • It is not a matter of the passage from degrees to meters. Reprojection causes a calculation of the value of the destination pixels, but it is the same if they were two metric systems. The difference you are showing is in the visualization of the layer, which is configured from the Symbology tab, but that does not necessarily imply a big difference in the data. If you could attach the statistics of the layers from the Information tab instead, we could draw some conclusion.
    – Gabriel De Luca
    Commented Oct 10, 2019 at 0:43
  • In fact, you could save the style of the original layer (in a .qml file) and load it into the new layer and both would have the same display, which does not necessarily imply that the data in both datasets is the same.
    – Gabriel De Luca
    Commented Oct 10, 2019 at 1:03

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