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I am trying to create buffers around polygon centroids, and perform zonal statistics with an underlying raster layer, and the points are just not overlapping with the raster layer. I am using QGIS. I wanted to ask here what might be the cause of this, and whether they might be a correct or "incorrect" way to change the CRS of a layer, since I used the Set Layer CRS function.

I have these layers, showing a polygons layer overlapped onto a clipped raster layer.

enter image description here

This looks good to me, though the problem is when I want to make my buffers. I find the centroids of these polygons, and try to draw buffers via the Buffer tool around each of them, only to find that the Distance parameter is set in degrees, when I want meters. From my understanding, to be able to switch the unit of measurement here from degrees to meters, I would need to "project" the polygons layer. Though I am confused since it appears that my layers already are projected.

For reference, the CRS of my raster layer is:

EPSG:32618 - WGS 84 / UTM zone 18N - Projected

and the CRS of my polygons layers is:

Unknown CRS: GEOGCRS["WGS84(DD)",DATUM["WGS84",ELLIPSOID["WGS84",6378137,298.257223563,LENGTHUNIT["metre",1,ID["EPSG",9001]]]],PRIMEM["Greenwich",0,ANGLEUNIT["degree",0.0174532925199433]],CS[ellipsoidal,2],AXIS["geodetic longitude",east,ORDER[1],ANGLEUNIT["degree",0.0174532925199433]],AXIS["geodetic latitude",north,ORDER[2],ANGLEUNIT["degree",0.0174532925199433]]] - Projected

I tried changing my polygons layer to the same CRS as my raster layer, but then the polygons layer disappeared from being directly overlapped with the raster layer. That is what I don't understand. My raster layer and my polygons layer are now within the same CRS, yet do not overlap on my map. Degrees was changed to meters as the "Unit" for my polygons layer, so at least that is correct, but I cannot figure out why my polygons and raster layer do not overlap.

Where might I be going wrong here?

Could this just be an issue with my understanding of the difference between geographical and projected coordinate reference systems that I might be missing here?

TomazicM
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LostinSpatialAnalysis
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    Set Layer CRS only changes the CRS definition, it does not actually change the coordinates from one CRS to another. Set the CRS back to its original value then use the Reproject Layer tool. – user2856 Aug 11 '21 at 21:58
  • I just tried the Reproject Layer tool, and took my polygons layer and set the Target CRS as EPSG:4326 - WGS 84, and then when I looked at Properties > Information for the newly reprojected layer, I saw that the CRS was now: EPSG:4326 - WGS 84 - Geographic and Units were still Degrees. I am not sure what I did wrong there. – LostinSpatialAnalysis Aug 11 '21 at 23:49
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  • Remove your polygon layer from the project. 2. Reload it from the original data source. 3. Right click your polygon layer in the table of contents and use export-> save features as and select epsg: 32618 (a projected utm crs) to match your raster layer. You could also use Reproject layer as an alternative to step 3.
    • – Ben W Aug 12 '21 at 00:51
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    To understand the basic concepts and the difference between "changing" CRS and reprojection, see: https://gis.stackexchange.com/a/383437/88814 – Babel Aug 12 '21 at 09:57