8

I created a circle with a radius of 3 km, calculated the area using field calculator and got a different result from real area.

The QGIS result is 28273078.8 while manually calculated area is 28274333.9 which is also the same number I got when I calculated the area in ArcMap.

"On the fly" projection is disabled and project CRS is set to utm32

Any ideas what is causing this, how to get more accurate area calculations?

  • 3
    How did you create the circle, in particular how many vertices did you use to approximate the circle? – Jake Jan 2 '16 at 14:32
  • I tried two methods, the first was buffering a circle from a point with 99 segments to approximate and the second was creating a circle in ArcMap and using it in QGIS. Both methods produced the same issue. – A. Jousef Jan 2 '16 at 14:41
  • It depends on the radius, but I prefer to use at least 181 vertices if the area is important to me. – Vince Jan 2 '16 at 14:45
  • See my answer in gis.stackexchange.com/questions/168168/… for area accuracy by vertex density. – Vince Jan 2 '16 at 14:51
  • Vince already gave you the answer, but I'm still intrigued by the result QGIS gave you. With 99 vertices and 3 km radius, the result should be 28255356.207 m2. Could you check whether the distance from the circle center to one of the vertices is 3 km? Maybe the vertices are actually a little bit further out in order to get a better approximation. – Jake Jan 2 '16 at 15:09
15

All recent versions of ArcGIS cheat -- the underlying shape type knows when it was generated as a circle, and reports a perfect circle area, even when the vertex count would make it an octagon.

I just whipped up some test code using the ArcSDE 'C' API and got the following results (all generated as a circle from point {0,100000} in PCS_WGS_1984_UTM_32N):

ArcSDE 10.2 Dynamic ASCII Table Utility  Sat Jan 02 15:37:30 2016
------------------------------------------------------------------------
         1
              Radius: 3000.00
              nVerts: 9
               Shape: Area shape (1 part, 9 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 25455900.00000000
          pct_ACTUAL: 90.03182924
         2
              Radius: 3000.00
              nVerts: 13
               Shape: Area shape (1 part, 13 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 26999987.41125000
          pct_ACTUAL: 95.49292133
         3
              Radius: 3000.00
              nVerts: 19
               Shape: Area shape (1 part, 19 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 27703641.02895000
          pct_ACTUAL: 97.98158692
         4
              Radius: 3000.00
              nVerts: 37
               Shape: Area shape (1 part, 37 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 28131014.34700000
          pct_ACTUAL: 99.49311083
         5
              Radius: 3000.00
              nVerts: 91
               Shape: Area shape (1 part, 91 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 28251381.25285000
          pct_ACTUAL: 99.91882168
         6
              Radius: 3000.00
              nVerts: 99
               Shape: Area shape (1 part, 99 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 28254960.01385000
          pct_ACTUAL: 99.93147896
         7
              Radius: 3000.00
              nVerts: 100
               Shape: Area shape (1 part, 100 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 28255353.61820000
          pct_ACTUAL: 99.93287105
         8
              Radius: 3000.00
              nVerts: 181
               Shape: Area shape (1 part, 181 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 28268601.72120000
          pct_ACTUAL: 99.97972663
         9
              Radius: 3000.00
              nVerts: 361
               Shape: Area shape (1 part, 361 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 28272903.62000000
          pct_ACTUAL: 99.99494148
        10
              Radius: 3000.00
              nVerts: 385
               Shape: Area shape (1 part, 385 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 28273075.88680000
          pct_ACTUAL: 99.99555075
        11
              Radius: 3000.00
              nVerts: 386
               Shape: Area shape (1 part, 386 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 28273079.37910000
          pct_ACTUAL: 99.99556310
        12
              Radius: 3000.00
              nVerts: 387
               Shape: Area shape (1 part, 387 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 28273082.09195000
          pct_ACTUAL: 99.99557270
        13
              Radius: 3000.00
              nVerts: 396
               Shape: Area shape (1 part, 396 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 28273146.91000000
          pct_ACTUAL: 99.99580194
        14
              Radius: 3000.00
              nVerts: 397
               Shape: Area shape (1 part, 397 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 28273144.96360000
          pct_ACTUAL: 99.99579506
        15
              Radius: 3000.00
              nVerts: 398
               Shape: Area shape (1 part, 398 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 28273150.34970000
          pct_ACTUAL: 99.99581411
        16
              Radius: 3000.00
              nVerts: 721
               Shape: Area shape (1 part, 721 vertices)
          PiRSquared: 28274333.88230814
        Area_PERFECT: 28274333.88230814
         Area_ACTUAL: 28273981.39730000
          pct_ACTUAL: 99.99875334

16 rows read in 6.69 ms

Area_PERFECT was generated using SE_shape_get_area(), and Area_ACTUAL was generated by converting the polygon to a simple line shape and back to a polygon before calling the area function. (I had to go back to the 9.3.1sp2 API to get an ACTUAL area from the circle shape's area function.)

I'm not quite sure what QGIS is doing, since the area reported in the question (28273078.8 m2) and the one reported in comments (28273147.55 m2) don't exactly correspond to 99 and 397 vertices (or 98, 100, 384, 385, 386, 387, 396, or 398, for that matter). There might be a geodetic calculation involved (though I don't have an easy way to evaluate that without the exact coordinate data), or the value might have been calculated to a 32-bit float.

I guess the lesson here is: If you really care about geometry area, you should make sure circles contain a lot of vertices.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.