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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.

Why do these calculations give me different results?

enter image description here

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I am the developer of FlowMapper plugin.

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.

Check that you point to correct coord. system (cartesian or geographic).

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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?

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.

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