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I have an image where I know the distance between two points in meters (say 800).

How do I import this image in QGIS (3.8) so that I get this distance when I measure with the tool in the toolbar?

Note: The image shows just a small part of the planet, so there is no distortion to speak of. So unlike other questions, I'm not trying to map a whole planet. If this matters, the planet has the size of Earth.

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You probably want to georeference your map/image first to a fictive or already existing Coordinate Sytem in meters. You can do this with the GeoreferrencerPlugin. The link gives you a comprehensive description on how this plugin works.

You can enter fictive GCP coordinates for the locations that you know, considering your desired distances. For example, if city A and city B are 500 meters away from each other in the same latitude, you could enter a GCP in A (x=50,y=50) and one in B (x=550, y=50).

After saving your new image with the chosen coordinate system, you can just load it in QGIS as a raster layer, define the layer projection if needed, and use the measurement tool as usual.

  • The plugin is installed but there is no toolbar button or menu item ...? I'm using QGIS 3.8. There is also no button to "add points" :-( – Aaron Digulla Jul 17 at 15:40
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    Ah. It's not enough to install the plugin, you also have to enable it by selecting the checkbox next to the name... – Aaron Digulla Jul 17 at 15:44
  • I'm glad you figured it out :) I updated my answer with some more hints/details. I hope it works – Ruben FV Jul 17 at 15:51
  • QGIS always wants me to select a CRS. Which one is "safe" to use to load the image so I can select "No projection" in the project properties? – Aaron Digulla Jul 17 at 16:07
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    You can select a UTM projected system and work on that area of the real earth, f.ex. 17S Ecuador, as if your imaginary world was there. this can work well if your map distances are not that large. You should also use the same projection in the project properties. – Ruben FV Jul 17 at 16:21

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