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I have a problem with my map scale. It shows 0:1 instead of proper scale.

I am not capable to make any measurements, as the values go wrong. When I click on the measurement tool I get an error:

This map is defined with a geographic coordinate system (latitude/longitude) but the map extents suggest that it is actually a projected coordinate system (e.g., Mercator). If so, the results from line or area measurements will be incorrect. To fix this, explicitly set an appropriate map coordinate system using the Settings:Project Properties menu.

and after starting measurements some weird things are populated i.e from Stockholm to Kopenhagen I have 4300km.

enter image description here

I entered the Settings and project, but the only thing I can do there is changing the CRS.

What is the reason behind it?

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  • As the warning says your data's projection is wrong, you need to fix it
    – Ian Turton
    Jun 11, 2021 at 15:57
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    How did you add the base map, did you change the CRS in the base map properties ?
    – J.R
    Jun 11, 2021 at 16:04
  • yes I did. Basemap is WGS 84
    – Geographos
    Jun 12, 2021 at 9:45

1 Answer 1

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The short answer

Set the layer-CRS of your background map back to EPSG:3857 and the project-CRS to a CRS appropriate for your area of interest - I suggest EPSG:23033 = UTM zone 33N. Than use the measure tool and compare values for cartesian and ellipsoidal measurement: they should be more or less the same.

See the following link for more details about which CRS to use for distance measurements.

Explanation

There are two (three) problems involved, all connected to projection-issues. In detail:

  1. You have set the layer CRS to EPSG:4326. Never change the layer CRS (if you're not 100% sure about what you do), see: https://gis.stackexchange.com/a/383437/88814 and https://gis.stackexchange.com/a/392388/88814. You have to reproject - that's something fundamentally different. Either use right-click on layer / export, than choose the CRS you want to re-project to. Or use Menu Processing / Toolbox / Reproject layer. This, however, makes no sense with the raster-tile basemap and you probably won't be able to save it locally. But if you have your own data, this could make sense. But to just measure distances, it's enough to change the project CRS.

  2. And never use either EPSG:4326 (WGS 84) nor EPSG:3857 (WebMercator, used for most webmaps like Google Maps/Satellite, OpenStreetMap etc.) for measurements: you will get meaningless values. Choose a CRS appropriate for distance measurements in your area of interest. The local UTM zone for example is generally a good option.

  3. Choose between cartesian and ellipsoidal measurement. Cartesian measures the distance on the projected map-canvas (that's probably not what you want). Ellipsoidal measurement consideres the distance on the ellipsoid used for the CRS - that's more accurate. See the Screenshot: Google maps as basemap in layer in EPSG:3857, project CRS in EPSG:4326 and ellipsoidal distance of ca. 522 km from Stockholm to Kopenhagen. Cartesian distance in the same setting: 732km, thus you see that this CRS returns values far from accurate.

As a rule of thumb: the smaller the difference between cartesian and ellipsoidal distance measurements are, the better the CRS is appropriate for this task. Re-projecting to EPSG:23033 = UTM zone 33N, distance measurements for both should be almost the same.

screenshot: Google maps as basemap-layer in EPSG:3857, project CRS in EPSG:4326 and ellipsoidal distance of ca. 522 km from Stockholm to Kopenhagen. But better use a CRS that is appropriate for your area of interest: enter image description here

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