topic Re: Judging Polygons regarding Compactness Ratio in Geoprocessing Questions

Judging Polygons regarding Compactness Ratio

MatthiasWoerner — Fri, 22 Feb 2013 19:34:37 GMT

Dear All,

while analysing polygons respective their fitness of size and form to build power plants there I am facing the following problem:

I computed Area and Perimeter of the polygons and based on these parameters the Compactness Ratio with the following formula:
[Perimeter] / (2 * 3.14159 * Sqr ([Area] / 3.14159))

As I did in other analyses before I wanted to filter the polygons with the lowest compactness (with the highest values of the Compactness Ratio). But due to the fragmented data structure of another layer the polygons are strewn with "holes" (see the attached screenshot, too). This results in rather high values of the Compactness Ratio and makes it really difficult to judge about the fitness of the polygons.

In fact I have to figure out those polygons that allow to build plants in the size of 1500m * 1300m. Does anyone have an idea of how to do that without measuring every single polygon?

Thanks a lot in advance,
Matthias

Re: Judging Polygons regarding Compactness Ratio

DanLee — Fri, 22 Feb 2013 21:45:31 GMT

Have a look of my answer to Lisa in this thread:
http://forums.arcgis.com/threads/77624-Minimum-length-amp-width-of-irregular-polygons?p=273635#post273635

She wants to fit a square, instead of rectangle. You can use 1/2 of the diagonal length in the process to find candidates.

Re: Judging Polygons regarding Compactness Ratio

MatthiasWoerner — Sat, 23 Feb 2013 19:10:32 GMT

Dear Dan,

thanks a lot for this hint. The idea is great:

1) Negative Buffering with the half of the diagonal of my rectangle.
2) Spatial Join with input polygons and output polygons.

But: It didn't work with the value of half of the diagonal of my rectangle although there obviously would be any more space inside of the input polygons - at least in some of them.

By trying with increasing negative buffering distances I found out that it worked until -370 m (I would need -992 m). I got a message that some of the features have been skipped "because a negative or very small distance resulted in no geometry". That wouldn't be a problem as long as I can push it to the limit. But still there are considerable output polygons in 20 of the input polygons but I can't buffer with bigger distance values.

Why? Is there any reason or idea about how to outsmart ArcMap?

Thanks,
Matthias

Re: Judging Polygons regarding Compactness Ratio

DanLee — Sun, 24 Feb 2013 14:56:11 GMT

When a polygon has holes, the negative buffer will be generated between the outer ring and borders of the holes. If none of these spaces is wide enough for the buffer, the polygon will be skipped. If you wish the holes to be "ignored", you can use the Eliminate Polygon Part tool to "fill" some or all of them before running Buffer.
http://resources.arcgis.com/en/help/main/10.1/index.html#//00170000005q000000

Is that what you wish to do?

Re: Judging Polygons regarding Compactness Ratio

MatthiasWoerner — Mon, 25 Feb 2013 17:07:56 GMT

Dear Dan,

that is not the point. Some of the polygons in fact have not enough space due to the holes inside. They are skipped and that is OK. But some obviously would have more space (see screenshot). And I wonder why it doesn't work with more than just -370 m buffering distance.

Regards,
Matthias

Re: Judging Polygons regarding Compactness Ratio

DanLee — Mon, 25 Feb 2013 20:27:59 GMT

Are the red buffers created by -370m? What happens when you use -500m? Is your data in meters?

Can you share your data?

Re: Judging Polygons regarding Compactness Ratio

MatthiasWoerner — Tue, 26 Feb 2013 04:59:57 GMT

Yes, the red buffers are created with -370 m. When using -375 m or more I get the mesage that it doesn't work. The data is in meters. Please find the two shapefiles attached.

Thanks,
Matthias

Re: Judging Polygons regarding Compactness Ratio

DanLee — Tue, 26 Feb 2013 20:43:02 GMT

Your data is not in meters, but in decimal degrees. Try to project your data and then try Buffer again.

I tried Africa_Equidistant_Conic projection; I can buffer with -700m and get 4 polygons in output.
All polygons are skipped using -900m; not enough room.

Re: Judging Polygons regarding Compactness Ratio

MatthiasWoerner — Wed, 27 Feb 2013 19:50:50 GMT

Dear Dan,

In fact, you are right. Stupid mistake of me. Now I projected the data and then it worked.

Thank you anyway!
Matthias

Re: Judging Polygons regarding Compactness Ratio

DanLee — Thu, 28 Feb 2013 04:11:07 GMT

No problem, Matthias. I am glad it worked out for you.
Have fun geo-processing!