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I`m using Qgis 2.2 and 2.4 and loaded a wms-map using the coordinate system ERTS89 UTM 32N. I set 8 points (all in on shp-fileand in UTM 32N) on various locations on the map. After that i created a buffer for each of the points with 72 and later 99 cricle-segments, because i thought this is more accurate according to a real circle. Each buffer has a radius of 50m.Then i calculated the area of this buffer-polygons using the fieldcalculator.

Now the problem.

Not only that the calculated areas differ from a mathematic calculated circle (with a radius of 50m, the area should be around 7853m² --> fieldcalc gives areas around 7830m²). The areas of each of 8 polygons differ between each other. And the more circle-segments i use, the more deviation between the results, alldough the difference between calculated area and mathematical area gets smaller. WHY does that deviation exist, alldough each polygon should have the same radius and therefore the same area.

I understand the difference between qgis-calculated area and mathematic area, but not the difference between the calculated ones. I tried to use different coordinate-systems like WGS84, Nad83 (NSRS2007), Lambert, DHDN GK4,...... All with the same problem.

Is it a qgis-discrepancy i have simply to deal with or did i something wrong?

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  • I suspect this is linked to the use of map datums and projections. gis.stackexchange.com/questions/664/… Depending on where your features are they could end up with different areas. This game highlights the difficulties in projecting a sphere onto a flat surface: flowingdata.com/2013/01/31/mercator-map-puzzle
    – MAJ742
    Commented Nov 25, 2014 at 12:49
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    Yes, you are right with that point. But this difference according to the projection-typ shouldn`t produce area-differences of 20m² in rather small scale. If you look over a large area (lets say germany) then a difference between mapped polygon-areas seem logical to me, but not in the scale of 50kmx50km. Overall, i tried verious proj.-types. What else can i do?
    – Chris
    Commented Nov 25, 2014 at 13:08
  • It might be a rounding error on the calculations. What field type are you using to do the calculations? - For example a short integer could lead to rounding errors as it only stores whole numbers whereas a Double would be better as it allows for decimal places. resources.arcgis.com/en/help/main/10.1/index.html#//…
    – MAJ742
    Commented Nov 25, 2014 at 13:11
  • I use real/double with 9 fields and 3 decimal places. But a rounding error of 20m²? Isn`t it a little much? When i use 72 circle-segments the variance is 4-5m². Using 99 (max.) the variance is 15-20m², but closer to the mathematical result.
    – Chris
    Commented Nov 25, 2014 at 13:20
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    I feel your pain. GIS can be super frustrating at times. My final suggestion is to use the Grass command of v.buffer.distance and see what happens. Also try the calculations without a basemap to doubly ensure that isn't affecting the projections. (and compare the steps to what your colleagues are doing) grass.osgeo.org/grass65/manuals/v.buffer.html
    – MAJ742
    Commented Nov 25, 2014 at 13:45

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