Calculating area of polygon in WGS84 Shapefile using QGIS

Question

I am using QGIS 2.12.3 Lyon and I want to calculate the area of my polygon in m2. For that I used the tool provided by QGIS, but the result is not as it should be. I got from QGIS 50 ha, while I got from the TopMaps Viewer 20 ha – I know the result from TopMaps Viewer is right, because I have another area with almost the same size, where I already know the area size.

Then I tried to calculate the area by the field calculator and used the formula $area as explained in other questions in the GIS StackExchange (How to calculate polygon areas and perimeters using QGIS? or How to calculate polygon areas in QGIS?).
When I try this, I can’t click “ok” and I don’t know why.

Probably I should change the KBS of my shapefile, but I really have a big confusion with that. I am using Google maps as background (EPSG: 3857), my shapefile which I created uses WGS 84, EPSG 4326 and the shapefiles I downloaded from public places uses EPSG 31467 Gauss-Kruger zone 3. Is that a problem? I tried to get them on the same KBS, but then they are at the wrong place. Can that be the problem why the calculation of my area size is wrong?

bugmenot123 · Accepted Answer

To change the CRS of a layer you must not edit it in its properties but "save as..." and choose the CRS in that dialog. You will get a new file.

In QGIS if you calculate areas it always uses the units of the layer on which you operate. So in your case it tried to calculate some angular area garbage. ;) Save your file as GK3 as well and then use that, you will get square meters.

Amare · Answer

just write the name of your field like "Area_sqm" on the space for "output field name". You will see the "OK" button go active. and make sure that your data is in projected coordinate system so that you will be able to get your area in sqm. I hope this works for you.

Answered by Amare on February 13, 2021

Calculating area of polygon in WGS84 Shapefile using QGIS

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