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I'm trying to create a buffer on a projected file and the result is a huge blob. I've read that this can be linked to the CRS used, so I made sure that I'm using a projected CRS (EPSG:32734 - WGS 84 / UTM zone 34S - Projected). The buffer specifies meters (not degrees like when the CRS is not a projection). I'm running out of ideas ...

Windows10 64bits, qgis 3.14.16-Pi

Roads.gpkg

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    I'm getting 'Access Denied' on that GPKG. Sep 30, 2020 at 5:22
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    Please define what you mean by "I made sure that I'm using a projected CRS". Is this your project CRS? Is this the layers CRS? If so, you made you it to be the layers CRS?
    – Erik
    Sep 30, 2020 at 6:13
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    *how did you make it to be... ffs, too early in the morning.
    – Erik
    Sep 30, 2020 at 6:47
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    it sounds like the projection was set to the layer instead of reprojecting the layer to the new CRS (If it is the case, your roads would be located in Null Island, in the ocean near Africa)
    – JGH
    Sep 30, 2020 at 13:26

1 Answer 1

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I fixed the issue to access the file, sorry for that.

I also sorted out the problem. It was linked to the projection of the shapefile as @JGH suggested. My mistake was that I created the shapefile with no projection (EPSG:4326 - WGS 84), then set the projection of the layer to EPSG:32734 - WGS 84 / UTM zone 34S. The buffer created then looked wrong. When I used reproject on the original unprojected layer, then it worked fine.

What still remains obscure to me is why if I set the projection and then reproject the layer, then the buffer looks wrong. I agree that the layer is not well positioned in the world, but why does it impact on the buffer?

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  • So you have a point in degrees, let's say around long -145 lat 45. 1 degree is roughly 100km, so you get a 2nd point 100km away at -146; 45. When you set the projection, you keep these number but tell QGIS it is now in meters. The two points are now just 1 meter away and all the data is located near 0;0 (up to + or - 180 meters). You then draw a buffer of 1000 meters. It is really 1 000m, but it covers all the data underneath since they are packed around 0;0. On the other hand (re)projecting the data change the coordinates number to keep the points well located (100km away)
    – JGH
    Oct 1, 2020 at 11:26
  • Thank you @JGH. I get it now. Oct 2, 2020 at 5:49

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