# qgis delaunay circumcircles

Is it possible to show the delaunay circumcircles in QGIS? I know how to make the Voronoi polygons and Delaunay triangles.

I found a web page that shows the circumcircles at:

http://www.scribd.com/doc/147332295/A-Gentle-Introduction-to-Gis

It appears that the author did this with QGIS, but he didn't explain how.

Image from Wikipedia:

• I think that the image has been borrowed from wikipedia to illustrate the method i.e. it was not created with QGIS. Commented Nov 1, 2013 at 6:28
• Then is there another program that shows the circumcircles?
– Jim
Commented Nov 1, 2013 at 11:54
• So you just want a way to show the circumcircles of a delaunay triangulation, it doesn't necessarily have to be within QGIS? Is this just for demonstration/teaching purposes?
– Jake
Commented Nov 1, 2013 at 14:52
• Yes, it is for a paper that I'm writing. It doesn't have to be in QGIS.
– Jim
Commented Nov 2, 2013 at 17:39

You can calculate the Delaunay triangulation and the circumcircles using Matlab (or its open-source counterpart Octave):

``````% Generate some random points
x = rand(5,1); y = rand(5,1);

% Calculate the Delaunay triangulation
tri = delaunay(x, y);

% Plot the triangulation
hold off
triplot(tri, x, y, 'k')
hold all

% Calculate the circumcircles for all triangles
for row=tri'
points = [x(row) y(row)];

% Uses the calcCircle function from http://www.mathworks.de/matlabcentral/fileexchange/19083-calccircle
[pos, r] = calcCircle(points(1,:), points(2,:), points(3,:));

% Draw the circle
rect = rectangle('Position',[pos(1)-r, pos(2)-r,2*r, 2*r], 'Curvature', 1);

% Change the color of the circle
set(rect, 'EdgeColor', [0.5 0.5 0.5])
end

% Make sure circles appear as circles
axis equal
axis off

% Plot the points on top
scatter(x,y, '.r')
``````
• Woo hoo! That's what I'm looking for. Thanks so much for your answer.
– Jim
Commented Nov 5, 2013 at 3:28

It is possible to delineate the circumcircles of a TIN in QGIS using the Minimum Enclosing Circles tool, entering the TIN as the input vector layer. The Minimum Bounding Geometry tool can also be used in the same way.