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I'm finding some trouble with the length calculation in the Flow Mapper plugin. As you can see in the picture below the length of the 2 lines

C001-C021 = 4233.7986...

C001-C009 = 6069.3053...

is wrong, as you can tell even just looking at the map. To confirm this I've output the distant matrix using the tool found in the vector menu -> analysis -> distance matrix that you can see in the excel spreadsheet in the top-right of the picture.

Do someone have a clue about why these calculation give me different results?

Thanks for your help! enter image description here

  • Did you try disabling the 'on-the-fly' CRS Transformation (Project > Project Properties > CRS) as this sometimes solves issues with calculating lengths and areas of shapefiles? – Joseph Oct 29 '14 at 14:22
  • Hello @Joseph, I've try it right now, same results, both in the length field and the distance matrix. By the way the CRS of all layers is EPSG:23032 - ED50 / UTM zone 32N. Any other idea? – Matteo Mandrile Oct 29 '14 at 14:42
  • Does that include the Project CRS? – Joseph Oct 29 '14 at 14:44
  • Yessir ;) layers and project have all the same CRS. – Matteo Mandrile Oct 29 '14 at 14:48
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I am the developer of FlowMapper plugin. Hope you find it useful. Regarding your question:

FlowMapper offers two types of flow length caculations:
(i) for cartesian coordinates (e.g. UTM x y)
(ii) for geographic coordinates (e.g. WGS84 easting northing)

User must properly choose either CARTESIAN or GEOGRAPHIC calculation depending on the type of coordinates that reside in the node coordinates file.


For the 1st option calculation is pretty straight forward. len=sqrt((x1-x2)^2+(y1-y2)^2)
For the 2nd option a spherical (greatest circle) length calculation is performed with reference to WGS84 ellipsoid.

Please check that you point to correct coord. system (cartesian or geographic) Hope it works, thanks.

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I get the solution of the problem directly from the developer of the plugin, I'll post here the answer I get:

FlowMapper offers two options for distance caculation: (i) cartesian coordinates (e.g. UTM x y) (ii) geographic coordinates (e.g. WGS84 easting northing)

As far as I can see, you are using geographic coordinates (x-y). Please check that you select CARTESIAN option on the create flow lines form? I hope it works.

Indeed, selecting the option "CARTESIAN" to generate the flow lines solved the problem, and now both distances from the Flow Mapper plugin and the ones from the distance matrix shows the same results.

Thanks you for your support!

Matteo

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