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I am rather new to ArcGIS and I have a simple question regarding a problem I cannot fathom.

I am attempting to generate a polygon 'dartboard' shapefile. Radially, there should be a circle from 0 to 20 km, an annulus from 20 km to 30 km, and an annulus from 30 km to 80 km. These three concentric shapes are divided into 8 segments to yield 24 sectors.

Within ArcGIS, within a new shapefile, I have generated three overlapping circles, with the same origin. However, I do not know how to take these multiple features and convert them into a single feature set (e.g. separate outer ring, middle ring, and inner circle).

I have attempted to use the Overlay tools in Arc Toolbox but to no effect.

Equally, I have no idea, once that is done, how to divide this shape up into 45 degree sectors.

Any advice regarding the best ways to dissect a polygon shape would be very much appreciated.

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3 Answers

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Clipping the circles can be done using the editing tools. First, select the innermost circle and click the 'clip' command under the 'Editor' menu. Then select the middle circle (which will now actually be an annulus) and click the 'clip' command again.

For the sectors, you should first make a new shapefile of 8 isosceles triangles radiating out from the center point. You can then intersect the triangles with the annuli to arrive at your final shapes.

The coordinates for the points of your first triangle (clockwise from north) will be:

x_1 = x_circle_center (x-coordinate of the center of the circles)
y_1 = y_circle_center
x_2 = (x_circle_center + cos(0 * pi / 180) * length)
y_2 = (y_circle_center + sin(0 * pi / 180) * length)
x_3 = (x_circle_center + cos(45 * pi / 180) * length)
y_3 = (y_circle_center + sin(45 * pi / 180) * length)

For the second triangle you will use 45 and 90 in place of 0 and 45, and so on.

The length that you use must go sufficiently past the outside of the outer annulus so that the shortest leg of the triangle does not cut into it. Just use something like 1.5 times the longest radius (in your example, 1.5 * 80 km = 120 km), which will be way more than enough.

All of this can be implemented in Python using arcpy geometries, but you can also do this manually using the editing tools in ArcMap. Just calculate the coordinates for your triangles and enter them into the Edit Sketch Properties window.

And be sure to use a projected coordinate system for this (not latitude and longitude). If you want to be very accurate with your angles and distances, you could reconstruct your circles and create your triangles in an azimuthal equidistant projection centered on the circle center. Depending on your needs, that might not be necessary.

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Many thanks for your suggestions and clear instructions. As I am not yet up to speed with arcpy, I have followed these through 'manually, and have been able to generate the dartboard plot in exactly the desired format. Again, many thanks for your help. –  Steve Feb 7 at 13:16
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This ArcUser article may point you in the right direction. The final output is a ring map, which seems similar to the "dartboard" you are trying to achieve. It has a download link to a Ring Map toolbox and practice datasets.

After further digging, this question references the same ArcUser article.

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Thanks for the suggestion. Ring Maps are similar to what I was wanting, but I would have needed an additional segmented bullseye. The solution beginning 'Clipping the circles ...' proved to work very much as desired. With that said, certain data that myself and colleagues will want to present can very neatly be presented in this Ring Map format and so I will certainly investigate this further. Again, thanks for the suggestion and for introducing me to Ring Maps. –  Steve Feb 7 at 13:23
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I would build the dartboard with a projected coordinate system, perhaps azimuthal equidistant with its defining parameter set to use the center point. Buffer it to form the three rings you need.

To create the 45 degree X that you need, use the Create Fishnet tool. You'll need to set the origin to the lower left of your area of interest, so something like (-100 km,-100 km) or if the coordinate system units are meters, (-100000 m, -100000 m). Use the Y Axis coordinates to set the 45 degree angle that you need like (0,0). Then you can either set the number of rows and columns to zeroes and set the upper right corner, or define the upper right corner of the grid.

Note: I have not tried this. Thinking about it further, you may need to experiment with the start point in order to have the fishnet cover the buffered rings.

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Thanks for the suggestions. I originally looked at the create fishnets option but was thrown by the examples with ArcGIS help showing square sections and not annular sections. However, I will look into this further as it does look a handy tool. –  Steve Feb 7 at 13:29
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