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I need to make sure the scale bar i am using on this map on QGIS 3.22 is accurate. None of the tutorials available online teach how to check the accuracy of a scale bar, only how to set it and customize it, and similar questions asked here are too specific and not related enough to what i need.

I only started using this software a few days ago, so general advice on the subject is welcome. The map uses the same CRS as the project (EPSG: 4326 - WGS 84) and the scale i'm currently using is 1900000.

I need to adjust the scale bar so it is accurate: How do I do it?

Image of the map in question, with the items bar visible

Babel
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Canary Sparks
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2 Answers2

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You are rigth there is a accuracy issue on your scale bar, but it is not due to the scale bar but to your map projection system. As you work with WGS 84 coordinate system x and y axis unit are in degree. As the earth is quite a sphere the distance to move from longitude -50 to -51 will not be the same if you are near the equator or near the pole. On your map you have the same distortion at a smaller scale.

To show the accuracy is from your CRS I created a gird based on WGS 84, then I have reprojected the layer and then I have shown as label the size of horizontal segment. You see the size change depend on your map position.

enter image description here

Conclusion

If you want to have accurate map to measure the same in real life and in your map you have to change your coordinate system. The CRS SIRGAS 2000 seems to be adapted. I am not GIS user from brazil so it has to be confirmed it is a good one for your project extent.

(to keep info about WGS84 coordinate you can do map in a metric CRS and then add over it a gird to show coordinate in degree)

Corentin Lemaitre
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    Does the scalebar in print composer show cartesian or ellipsoidal lengths? Based on what CRS? Project CRS? OP's screenshot shows EPSG:4326 as map CRS, but scalebar is still in km, not in degrees... In any case, worth to remember that any projection distorts lengths and accurate length measurements can at best be expected for shorter distances (in local CRS, as you mention) or in special cases, e.g. along a line (equator in Mercator projection). See here for some links https://gis.stackexchange.com/a/434196/88814 – Babel Aug 24 '22 at 06:57
  • Thank you both for the insights. With this information, i will probably ditch the scale bar entirely, as it is not required for this project. – Canary Sparks Aug 24 '22 at 17:54
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You do not have to change the properties of the map itself, just set the properties of the scale bar to point to the map.

First you need to right click the scale bar in the items list and select item properties (1), you should see that the scale bar properties are being shown where the arrow points rather than the map.

Next look at the top under Main Properties -> Map and make sure that your base map is selected from the drop down menu (2).

enter image description here

Kartograaf
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    I think the question is more abotu the accuracy of the lengths shown by the scalebar, not how to set it. So this is more a question about CRS, distortion based on projection, planar vs. ellipsoidal measurements etc. – Babel Aug 23 '22 at 21:49