I have two similar rasters of the Mount Vesuvius: one spans 20 kilometers, and the other one 50 kilometers. Both rasters are in WGS84 geographic coordinate system, and they overlap between each other:
When I use QGIS Raster calculator to calculate the difference between them, a new raster file is created which is totally black - so it shows that there is absolutely no difference in pixel values on the part where they overlap:
However, when I project both rasters to Azimuthal equidistant projection, and again check for the difference with Raster calculator, the difference between these two reprojected rasters exists! Here is how it looks like:
Why is this so?
The difference does not exist between the two rasters when they are in WGS84 geographic coordinate system, but once they are projected to Azimuthal equidistant projection, a significant difference appears?
I used the
cubic resampling method, but I got the similar upper difference with the
Just for the sake of checking, I tried using Transverse Mercator projection (instead of Azimuthal equidistant), and again a difference exists between initial two projected rasters. Why is that so?
Additional information: To project the initial WGS84 raster files, I used the following syntax:
gdalwarp -s_srs EPSG:4326 -t_srs "+proj=aeqd +lat_ts=40.81266 +lon_0=14.414252" -r cubic -of GTiff "C:/vesuvius_radius_20KM.tif" "C:/vesuvius_radius_20KM_cubic_aeqd.tif"
To project them to Transverse Mercator:
gdalwarp -s_srs EPSG:4326 -t_srs EPSG:32633 -r cubic -of GTiff "C:/vesuvius_radius_50KM.tif" "C:/vesuvius_radius_50KM_cubic_tm.tif"
Here are the initial raster files (in WGS84):
Here are their Azimuthal equidistant projected rasters:
And the difference between two Azimuthal equidistant projected rasters: