Distance discrepancies

[david_jarvis]

Hello All.

Can anyone help with Distances/Lengths?

I use Mapinfo and Autocad Map 3D and I am trying to find the distance to the coast (Point to Line).

Maybe I should point out I'm in Britain and use OSGB (Grid) coordinates.

I plot the distance to the coast in Mapinfo which also produces a line. I pull this line into Alteryx and find it's length, this disagrees with Mapinfo by some 31 meters over 47 kilometers.

To test I produce end points in Alteryx and plot these in Autocad, Autocad and Mapinfo agree.

Plotting these points created in Alteryx, in Alteryx I get a distance that almost agrees with the Alteryx line length (less by 0.26 meters).

The minor differences I could cope with putting these down to differing algorithms. I'd like to use Alteryx for the whole query as it's much faster and more can be automated in the workflow but the difference of 31 meters is less convincing.

Mapinfo Cartesian Length                47871.90014

Mapinfo Spherical Length                 47777.5798

Alteryx Length                                         47746.50612

Alteryx Distance between points 47746.24269

Can anyone point out what I'm doing wrong or explain the differences?

Much appreciated.

David J

[tom_montpool]

I suspect that what you're seeing is due to either or both of the following:

1) Alteryx converts spatial objects to WGS84

2) Alteryx uses a 6 decimal place precision for spatial operations

Someone at Alteryx would have to confirm to be 100% sure.

[david_jarvis]

I wondered if WGS84 had anything to do with it, that was why I created points and fed them back into Autocad. The points would be derived from Alteryx. The point coordinates appear to be correct, the lengths differ.

Alteryx outputs distance in kilometres, six decimal places is stil millimetre accuracy?

Thanks for the feedback.

[Philip]

I've noticed the same discrepancy between Alteryx and R spatial functions. I haven't checked ArcGIS, yet. I'm curious as to why Alteryx differs from other systems, too.

[RobertB · Accepted solution]

Alteryx version 10 includes a fix for mis-calculated lengths of lines with very long segments, so if you are using a pre-10 version, that may be responsible for the discrepancy between length of line and distance between endpoints.

For distance between points (or line endpoints in 10+)  Alteryx calculates distances "ellipsoidally", i.e. on the surface of the WGS84 ellipsoid.  This will produce slightly different (more accurate) results than a spherical distance calculation.

As an aside: The "6 decimal points" in reference to spatial precision refers to spatial objects Alteryx produces, which use 6 decimal places of WGS84 lat/lon to represent points (i.e. millionths of a degree).  This can introduce round-off up to a maximum of ~5 centimeters, so shouldn't be relevant here.

[Philip]

Thanks for that explanation, Robert.

[david_jarvis]

Robert,

Yes, using version 9.5.

Unfortunately a reason to instal v10....

Thanks for looking into this.