# Create diagonals in polygon with QGIS or ArcGIS [closed]

How is it possible to create the diagonals in/of polygons and to calculate the length of this diagonals afterwards.

It's for a lot of polygons so maybe a fast/automatic method would be nice.

I prefer the software QGIS or ArcGIS.

The polygons are created by following streets that cross each other. So the shape is very weird, not a typically square. Some polygons has 4 vertices, some only 3 vertices.

The green lines on the second figure shows the objective, it's to create this lines and calculate the length of this lines...

I am asking this question because of the following situation: I digitalise this polygon on raster data of year 1950. Then i search the same ways in the raster data of year 2014. I try to digitalize the same polygon as in the year 1950.

But unfortunate the area (inside the ways) aren't perfect the same. So i search something to compare the polygons of the same location of the two different years. I was thinking to compare the area or perimeter, but isn't perfect. so my next idea was to create the diagonals of the polygons and calculate the length. If the difference in the length between two diagonals (of same polygon but different years) is to large, the two polygons aren't the same and so not possible to use in the further analysis... maybe there is an easier way to compare the polygons?  ## closed as too broad by PolyGeo♦Jan 25 '17 at 10:07

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• Would you be able to include some pictures to show us what you mean by the diagonal of a few different polygons, please? – PolyGeo Mar 9 '15 at 13:23
• Are the polygons squares? – Jakob Mar 9 '15 at 13:24
• Do you mean a the diagonals of the bounding box of the polygons, or do you want the BND diagonals clipped against the figure. Please take one of the shapes above, and using Paint (or whatever) draw what you are looking to obtain. – Vince Mar 9 '15 at 15:28
• Your illustration seems to just show the vertices on half a dozen polygons so I am still none the wiser about what you mean by the diagonal of a polygon. – PolyGeo Mar 9 '15 at 23:28
• It looks like you first want to define corners, and there will be three or four for each shape? But do you see how that's tricky? Your northernmost shape, for example, has a rounded top that at some point is not going to be seen as a corner... But if the polygon edges are from road lines, are the corners always where two roads cross? That might make it easier if your data has that info. – Spacedman Mar 10 '15 at 8:18