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I need to get an area of a polygon. The polygon when in GDA 2020 (7844) and GDA 2020 / MGA Zone 50 (7850) lead to significantly (20 hectares) different areas. The method for getting the area I am using is going to field calculator, creating a new field and using $area/10000 (to get Ha). I have set the project CRS to both 7844 and 7850 and the results don't change. Which area is right? Is my process for getting the area right?

Using QGIS 3.16.6-Hannover.

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    Is the difference high ? A small dif could be just be attributed to the accuracy of the crs
    – Kalak
    Commented Apr 19, 2023 at 9:32
  • @LouisCottereau, The diff is 200ha to 215ha, so yes a big difference Commented Apr 20, 2023 at 0:03
  • I have a vague memory of this problem happening to me previously, but I no longer get it, which leads me to believe it might be a 3.16 issue. Are you able to install a newer version of QGIS? im running 3.22 and dont get that issue when using MGA50
    – nr_aus
    Commented Apr 20, 2023 at 1:45
  • I have updated to 3.28 and the results are the same. Based on my methods of calculating areas using the two different CRS's, is one method 'incorrect'? I am assuming both answers cannot be right? Commented Apr 21, 2023 at 1:35

2 Answers 2

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GDA 2020 (7844) is Australia-wide and its coordinates are in degrees. GDA 2020 / MGA Zone 50 (7850) is limited in extent to between 114°E and 120°E and has its units in meters. If your area is wholly within zone 50 then that should provide your best area calculation.

The following from Geoscience Australia give some useful information: https://www.ga.gov.au/scientific-topics/positioning-navigation/geodesy/datums-projections/grid2020

Specifically it says: "The UTM system (Table 1) uses the TM projection and attempts to overcome this limitation by dividing the Earth into 60 zones, each with a width of six degrees of longitude. A central meridian is placed in the middle of each longitudinal zone. As a result, within a zone nothing is more than three degrees from the central meridian and therefore locations, shapes and sizes and directions between all features are very accurate."

By contrast, I think 7844 is a conic projection which therefore does not maintain areas over large distances.

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  • The polygon is wholly in MGA Zone 50. When I change the shape file containing the polygon between the 2 CRS's, the area calculations give an area difference of about 20 hectares (ha). Would the project CRS change the area results? Commented Apr 20, 2023 at 0:17
  • @Nick I'm fighting out of my pay grade on projections but have modified my answer. Commented Apr 20, 2023 at 0:56
  • Based on the above I would assume using 7850 as the CRS for the polygon (when area calculations are important) is the correct method. It worries me though that using 7844 results in such a large difference. Changing the project CRS does not seem to alter the area results, just the CRS of the shape file containing the polygon. Commented Apr 21, 2023 at 1:39
  • I would assume so too. That is what my answer says. Commented Apr 21, 2023 at 9:30
  • @Nick if your question is answered it is courtesy to acknowledge that. If you have further questions then feel free to ask them. Commented Apr 24, 2023 at 10:10
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What settings you have in Project-Properties-General-Measurements and in Settings-Options-CRS Handling-Planimetric measurements?

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The default is to use geodetic measurements with the selected ellipsoid and therefore changing project CRS does not affect the measurements. If you rather want planimetric measurements by using the project CRS, check the Planimetric measurements box.

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  • I have added my settings into my post. When I change the CRS of the shape file containing the polygon and redo the area calculations, the area changes by about 20 hectares. Commented Apr 20, 2023 at 0:13
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    @Nick Yes, because MGA uses transverse Mercator which is a conformal (maintains local angles/shapes) not areas. Try using 9473 (GDA2020 / Australia Albers) which uses an equal-area projection.
    – mkennedy
    Commented May 1, 2023 at 0:27
  • @mkennedy Thankyou Commented May 24, 2023 at 8:45

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