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I have a number of vector shapefiles that I need to calculate the geodesic/ellipsoidal area for. These shapefiles are accessed and updated regularly, with the area recalculated each time they are updated. When I recalculate the areas, the planimetric area is always the same, but the ellipsoidal area often changes. The changes aren't a large amount - usually 0.00001 or 0.00002 ha (1 or 2 square metres) over hundreds of hectares. Generally the projection that I would be using is GDA94 - MGA55, with the ellipsoid being GRS 1980.

My question is, why would the ellipsoidal area be changing between each successive round of calculations? Is there a setting other than the CRS and the ellipsoid that could be affecting the area calculation function?

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    Do you have a cut down sample data set that you can include? Also, what is being updated (when you say "recalculated each time they are updated")? And where are you looking at the area? Is it in Identify Results, or somewhere else?
    – Tom Brennan
    Commented Mar 1 at 2:59
  • Are you checking the planimetric areas at double-precision values? That is, how many decimal places are being shown?
    – mkennedy
    Commented Mar 3 at 18:50

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