Concave Hull

Question: I have a point dataset with about 17,000 points. I’m looking to create a footprint of the outer most points. I see one can create a convex hull, but my points form an irregular shape and the convex hull doesn’t follow the path of the outer most points. Is there a way to do this?

I’ve attached a jpg that shows the result of a convex hull.

Thanks for your help.

Answer: I presume you are using the Minimum Bounding Geometry tool in the Features toolset in the Data Management toolbox in ArcToolbox. If you read the online help for this tool, you will see that the hull (polygon defining the extents of the input features) will always be convex, as shown in your example. You do not have the option to create, what you call a concave hull.

Instead try using the Feature Outline Mask tool in the Masking toolset of the Cartography toolbox in ArcToolbox. With this tool, along with BOX and CONVEX_HULL, you can also specify:

EXACT_SIMPLIFIED —A generalized polygon representing the exact shape of the symbolized feature. Polygons created with this method will have a significantly smaller number of vertices compared to polygons created with the EXACT method.

EXACT —A polygon representing the exact shape of the symbolized feature.

This would not work in all cases, but given the nature of your data, I think it will work for you.  This is why — given the symbology you are using, your data are clustered so close together that the mapped features coalesce into a contiguous area.  It is important to understand that the Feature Outline Masks tools will create a mask polygon around the SYMBOLIZED features, so if you used a smaller point symbol or a larger reference scale (so that you zoomed in on the area) then you would start to see pockets within the polygon.

Once you have used the Feature Outline Masks tool, use the Dissolve tool to create one or a few polygons that represent the extent of the distribution.

Note that this solution, unlike the Minimum Bounding Geometry solution, provides a polygon that extends OUTSIDE the actual extent of your point features and instead represents the extent of the SYMBOLS used to map your point features.

Below is an example using census data for the places in Michigan.

25M Points

The point distribution mapped with a 4 pt symbol at a map scale of 1:25,000,000 (this looks more like the coalescing points in your attachment.)

25M Masks

The output from the Feature Outline Masks tool for the mapped features above.

25M Dissolve

The results after using Dissolve.

5M Points

The point distribution mapped with a 4 pt symbol at a map scale of 1:5,000,000 (notice all the pockets within the mapped distribution.)

25M Masks

The output from the Feature Outline Masks tool for the mapped features above.

5M Dissolve

The results after using Dissolve.

This is the solution that was offered in the comments below by Kyaw Naing Win. This inventive solution uses TINs.

Concave Hull with TINs

 

Attachment: Convex Hull
Formerly a Mapping Center Ask a Cartographer  Q & A.
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