What CRS is Select by location performed on in QGIS? I didn't find it in the documentation.

Is it performed on:
a) the project CRS
b) CRS of the select features from layer
c) CRS of the By comparing to the features from layer?

Do I have to make sure a), b), c) are all the same to do this operation?

  • 1
    See this here: gis.stackexchange.com/a/392388/88814
    – Babel
    Oct 25, 2021 at 8:20
  • It seems that layer are transformed to the project CRS for display. But this doesn't explain which CRS select by location is perform on? Or does it mean that both b) and c) are projected to the project CRS and select by location is done on that?
    – Sara
    Oct 25, 2021 at 8:47

1 Answer 1


No: project and layer CRS can be different to use Select by location. Even the two input layers can have different CRS. For high accuracy and precision, using the same CRS can still be of some advantage.

See the following screenshot:

  • Project CRS: EPSG:2056
  • Point Layer: EPSG:4326
  • Polygon layer: EPSG:3857

The selection points are within polygon (pseudocode) selects the correct points:

enter image description here

  • 3
    But which CRS does the operation use? Does this mean that If b) and c) are of different CRS, they will be projected to a)? And why would the analysis be more accurate if b) and c) have the same CRS?
    – Sara
    Oct 25, 2021 at 8:37
  • I guess that it is performed in project CRS, but I'm not sure about that. Is this relevant for your task?
    – Babel
    Oct 25, 2021 at 8:56
  • for my project i want to make sure that i am performing the analysis on the right CRS. For example I want to get the distance data of points in the middle states of USA, and some points in Alaska. My project CRS is NAD83 because it is suitable for both areas. But to get distance in meters, I would be using two separate Albers CRS ... I realize this wouldn't be the original selection problem. but I am trying to understand QGIS's logic.
    – Sara
    Oct 26, 2021 at 7:19

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