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I try to measure area in QGIS, but encounter some problems. The results seem to be quite inexact. When testing it with some administrative units (polygons from OpenStreetMap) and compare the results with the known real surface, results in some cases are quite close, while in other cases differ up to 20% or more from the "real" surface. So there seems to be a problem with my measurement. I saw some answers in this forum connected to projection choice. I couldn't understand which projection (the name I have to choose) is adequate for area measurements?

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Your projection should be in the same unit as your area for example projection in meters and then calculate area in square meters.

I don't know where is your area, so I can't tell you what projection you should choose. As example if polygon is located in Poland you can use projection with epsg 2180

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  • Thanks. I'm working on Eastern Ukraine, so I tried CRS EPSG:6387 UCS-2000 / Ukraine TM zone 13. But now, calculating areas on my layer of the Oblasts (first-level administrative units, with an average surface of ca. 10'000 to 30'000 square km) with field calculator and $area, in the preview I got a value of like 3.066, but when applying, all the field are just 0. I have no idea what CRS would be good to use for calculating large areas in eastern Ukraine. – Babel Dec 29 '16 at 10:29
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    Make sure that you save layer in your projection, not just display in that projection – ami Dec 29 '16 at 10:39
  • It does not help saving the layer, same result as before. To try something else, I changed to EPSG:5561, did the same as before and: the results are much more accurate. Except for e few cases, the calculated areas of oblasts are between 99.4% and 100.3% of the known area. So EPSG:5561 works quite good, 6387 does not. Any idea why? And how to interpret the small remaining difference of ca. 0.6%? Different methods of calculation? OpenStreetMap-Polygons not quite accurate? – Babel Dec 29 '16 at 12:03
  • Keep in mind that OpenStreetMap is good source in many areas, but it's not official data from government institution or something similar so it could be not 100% accurate – ami Dec 29 '16 at 12:07
  • OK, thank you! Any idea what could be the problem wit EPSG:6387? – Babel Dec 29 '16 at 12:14

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