[R-sig-Geo] Identifying adjacent polygons within larger polygon groupings

[Edzer Pebesma]

On 01/04/2021 17:01, Sean Trende wrote:
> Hello,
> 
> I am using sf for the project below, but am open to non-sf solutions.
> 
> Given an sf data.frame of polygons sorted into n contiguous groups (e.g., a column in the df identifies each polygon as a member of group (1, . . ., n), where within each group all polygons are neighbors), is there a simple way to identify all polygons in, say, Group 5 that are neighbors to polygons in, say, Group 8 (assuming groups 5 and 8 are neighbors)?
> 
> E.g., if I have an sf data.frame of U.S. counties with a column that identifies the state in which each county is located, is there a way to identify all counties on the border between, say, Kansas and Nebraska?

Yes, look into using st_intersects (or st_touches) with x the counties 
of Kansas and y the counties of Nebraska.

What you get back is a list of length length(x), with integer vectors, 
the i-th vector containing the indexes of Nebraska counties 
touching/intersecting county i of Kansas.

> 
> Thank you so much,
> 
> Sean
> 
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-- 
Edzer Pebesma
Institute for Geoinformatics
Heisenbergstrasse 2, 48151 Muenster, Germany
Phone: +49 251 8333081