# Drawing Thiessen polygons(rainfall)

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Hello there!

I have a project due Tuesday and I have to calculate the precipitation using thiessens polygons.I have georeferenced the map , drawn the basin, connected the 6 points together and formed 4 triangles.

Now im having troubles drawing "Perpendicular Bisectors of the Sides of a Triangle".In few occassions the circle is too big and lines do not meet in the center of the circle.I have tried (Osnap On midpoint)+perpendicular but the lines do not meet at all. If somebody can help me that would be great.

Sorry for the terms that im using but im trying to explain it as good as possible.

Thank you.

mepika.dwg

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Line Pick point inside triangle PERP pick triangle side, move line to mid of triangle side,

repeat for a second side.

where the lines cross is centre of circle.

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According to the laws of plane geometry, if the triangle intersects the circle at all three apexes, then a line drawn from the center of the circle to any of the three sides, at the perpendicular point on the side will bisect the triangle side. Simply, any line drawn from one point on a circle to another point on the same circle will have a midpoint that will be perpendicular to the center of the circle.

Start by turning on the center, midpoint, and perp snaps, then insure that the triangle sides actually meet at all three apexes. Then continue by drawing the circle using the 3P (three point) option, using the apex points of the triangle. Then draw three lines from the circle to the perp snap of each triangle side.

You can draw two of the lines from the midpoint of the triangle sides directly to the center of the circle, assuming everything is accurately placed. The third side needs a temporary line to provide an accurate midpoint since the third side (SE) is made up of two lines.

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If you have polar tracking on, then just pick the midpoint of a side of the triangle, and directly draw the perpendicular bisector. Then the centre of the circle will be at the intersection of these lines

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Thanks a lot for the replies. I tried drawing circles using 3 points and then drawing perpendicular lines at the midpoints of each triangle.

The red circles are perfect but the blue circles are pretty big and their center is not inside the triangle.

mepika2.dwg

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It would help AutoCAD if you were to use layers. If you draw the triangles on their own layer and then freeze all other layers, you would find no difficulty in drawing the geometry.

I think that some of your midpoint snaps might have been confused because there were more than one line where the pick was.

There is no compulsion for the centre of the circle to be inside the triangle.

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It would help AutoCAD if you were to use layers. If you draw the triangles on their own layer and then freeze all other layers, you would find no difficulty in drawing the geometry.

I think that some of your midpoint snaps might have been confused because there were more than one line where the pick was.

There is no compulsion for the centre of the circle to be inside the triangle.

Sorry but I don't know how to use layers.

I moved other lines out of the way and then I picked the midpoint and then I put everything back together.

Is there any way to convert an autocad file into .TIF becouse I think I might be able to do it in QGIS.

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Use layers and assign each a different color. It is counterproductive to draw everything on one layer (layer "0") then override the default color (white).

Create your layers in the Layer Properties Manager. Layers can be turned on/off or frozen/thawed. They can even be locked/unlocked as well as isolated/unisolated.

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There is no compulsion for the centre of the circle to be inside the triangle.

But if the center of the circle is not inside the triangle there is no way I can draw Thiessen Polygons.

I used layers for every single drawing that I made.

For the triangles in the bottom I used region command then massprop and found the centroid but the results midpoint and the perpendicular line are not in the same place.

Points-White

Triangles-Blue

Correct perpendicular bisectors- green

Correct circles-mangenta

------------------Here is the part that im having difficulties---------------------------------

Centroid of the triangle using region and massprop-red

Perpendicular-yellow

Midpoints-red

Incorrect circles(IMO)-orange

mepika3.dwg

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Your trouble lies with the positioning of the base points. To draw triangles with a circle centre within the triangle means that no angle in any triangle can be greater than 90 degrees.

You need more points!

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Thanks a lot for all the replies.

I managed to solve the problem.

I created a dfx file only with the coordinates needed and I imported it to QGIS.

Added it as a vector layer-geometric tools-Voronoi polygons.

Saved it as an image.

Imported it back to Autocad.

Created a point in the intersection of the polygons and moved it using it as a base point.

Thanks a lot for your help.

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If i had known you were going to use another software.... >>> shrug

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If i had known you were going to use another software.... >>> shrug

Well Im sorry but it was almost impossible for me.But lets look at the bright side, at least I learned how to work with layers

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