10

I am about to become desperate... I like to create buffer around several points - if possible - with a declaration of the size in km of these buffers. Just to show you what I am working with:

enter image description here

My problem seems to be the projection of my data. Its in the WGS84 projection and my background map is in the RGF93 / Lambert-93 projection (I dont know why that works together). With the WGS84 projection the distances will be shown in degrees, so i tried to change the projection of the background map, the projection of the data or both together but I dont find any other projection where they fit together.

Do you have any suggestions how to create these buffers with a clearly shown distance?

Improve this question

2 Answers 2

Reset to default
7

FYI, the buffer tool always use the input layer's Coordinate Reference System (CRS) units. In your case, to buffer your points, it will always use WGS84 in degrees (Since WGS84 is a geographic coordinate system, and not a projected one).

Notice, that changing the CRS by using "set layer CRS" does not change their actual values, It only says to QGIS "from now on, read this values as if they are on this different CRS". Wish would tell for instance that "from now on read this degrees as if they are meter", and that won't work well! :-P

Therefore, it's very important that all your layers are set with their correct CRS. If on, QGIS "On the fly transformation" will read layers from different CRSs and display them in the Project chosen CRS. (thats why your layers work together)

The solution is very simple tho, all you need is to reproject you point data to a suitable projected CRS. For what I can see, the RGF93 / Lambert-93 is suitable for your working area.

  • Right-click "points" layer, and choose "Save as...";
  • Choose "Selected CRS" instead of "Layer CRS";
  • Browse for the desired CRS. (typing 'RGF93' on the filter, it will help you to find and select the "RGF93 / Lambert-93" (EPSG:2154) CRS);
  • Choose your output shapefile location, select the "add saved file to map" an click Ok.

The resulting layer can now be used as input to buffer tool, and You can now use meters in the buffers distance.

Hope this helps!

Improve this answer
2
  • Hello Alexandre, thank you for your fast answer. Unfortunately it does not work. Maybe because I created my layers out of a self made (written in WKT) textfile? The failure message is: (OGR-Fehler: Failed to open shapefile .shp\.shp. It may be corrupt or read-only file accessed in update mode.) /edit yeah I guess the problem in this case is, that i did not use any shapefiles to create the layers but textfiles...
    – Rouven Maue
    Jul 8, 2013 at 10:00
  • 2
    SHAME ON ME, I did not renamed my file... works perfect, thank you so much!
    – Rouven Maue
    Jul 8, 2013 at 10:05
6

Background map and your data fit together because you have "on-the-fly"-reprojection enabled.

This makes sense if you want to see WGS-degree-data on projected map backgrounds. But If you want to create buffers, both layers must have the same CRS. For buffers in metres, you have to change the degree data to a projected CRS (like RGF93 Lanmbert).

So you have to save your vetor data in RGF93 Lambert with rightclick -> Save As... and select the Lambert projection with EPSG:2154, a different file name, and add the data to your canvas. Then you can do the buffering with that layer.

Improve this answer
0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.