Voronoi/Buffer/Createcircle() mismatch?

[ericwd...@gmail.com]

I used the same set of points to generate Voronoi polygons (thin blue line); circles with createcircle() (thin red line); and buffer w/same radius as createcircle() (wide green line).  This is a lat/lon projection.  I think all of these are supposed to be calculated spherical projections, so why is it that the points where the circles intersect do not lie on the Voronoi boundaries (the picture shows just a few, but there are a lot that are waaay off)?  They should be equidistant from the associated points, so I'm not sure what is happening...

[Peter Horsbøll Møller]

That’s quite odd.

 

I tried to repeat this with a projected dataset (using British National Grid)

In this case the result seems to match much better. The buffer intersections lie nicely on the Voronoi line.

 

I’m not sure why it would behave differently for lat/long data.

It seems that the Voronoi line between the two pair of point isn’t perpendicular to the line that connects the two points.

 

Peter Horsbøll Møller

www.precisely.com

 

—

Peter Horsbøll Møller

Principal Sales Engineer - Distinguished Engineer

Precisely.com

 

From: mapi...@googlegroups.com <mapi...@googlegroups.com> On Behalf Of ericwd...@gmail.com
Sent: 10. marts 2021 19:39
To: MapInfo-L <mapi...@googlegroups.com>
Subject: [MI-L] Voronoi/Buffer/Createcircle() mismatch?

 

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I used the same set of points to generate Voronoi polygons (thin blue line); circles with createcircle() (thin red line); and buffer w/same radius as createcircle() (wide green line).  This is a lat/lon projection.  I think all of these are supposed to be calculated spherical projections, so why is it that the points where the circles intersect do not lie on the Voronoi boundaries (the picture shows just a few, but there are a lot that are waaay off)?  They should be equidistant from the associated points, so I'm not sure what is happening...

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[ericwd...@gmail.com]

I grabbed a subset of the points (Test) and created a Circle (using createcircle(X,Y,35)), Buffer (offset 35), and Vor table.  The points are all in the US, so I was originally using Lat/Lon NAD 83 EPSG4269, but also tried with UTM Zone 14 EPSG 26914 and had the same problem.  I've zipped up the tables, which are pretty small.

[Uffe Kousgaard]

I have reprojected your data into UTM Zone 14 EPSG 26914 and did the
voronoi calculations. Looks correct to me.

Then I added your output and it is not the same.

Red is your voronoi-table, black is mine.

I guess the voronoi calculations simply require projected input.
Just like my voronoi-library does (https://www.routeware.dk/toolbox.php)

Regards
Uffe Kousgaard

[ericwd...@gmail.com]

Thank you!  I thought that was what I did when I reset the map to EPSG26914--I didn't realize I actually needed to resave the data table as EPSG26914.  I re-ran the Voronoi with the "new" point file and it lines up nicely.  Appreciate both of your insight and assistance.