[R-sig-Geo] Polygon width

[Paulo Flores Ribeiro]

Thank you, Barry. I am using planar coordinates (meters) and Euclidean 
distances. The shape or size of the polygons is not important (for 
illustration purposes, imagine that polygons are farm boundaries and 
that each farm is a single polygon). I know how to extract the maximum 
distance in a single polygon by calculating the distance matrix and 
selecting the maximum value. My "difficulty" is exactly in the coding of 
the loop process (or in the construction of the "function" to be used in 
the apply approach), so that I can apply it "automatically" to 25,000 
polygons. I am using the rgadl package, but I can switch to sf.
Thanks for any help.
Cheers,
PauloFR

Às 12:27 de 25-04-2018, Barry Rowlingson escreveu:
> Do you want great-circle distance or is your space small enough that 
> you can use planar coordinates?
>
> Are your polygons all single rings or are there islands and/or holes? 
> Does that matter?
>
> The straightforward way would be to coerce the polygons to points, 
> compute the distance matrix, then take the maximum. Depending on if 
> you are reading your shapefile into sp or sf classes, the functions 
> would be a bit different. You should try and implement the 
> straightforward way, test it for correctness, and then worry about the 
> "best way" if the straightforward way isn't what you need. Its often 
> the case that "best" ways need fancy data structures or complex 
> algorithms with more opportunity for bugs. Start simple, work up.
>
> For example, using sf, here's the max distance between any points in 
> the first feature of `pcs`
>
> > max(st_distance(st_cast(st_geometry(pcs[1,]),"POINT")))
> 172.556 m
>
> loop from 1 to N or otherwise apply over the features, and you're done.
>
> Barry
>
>
>
> On Wed, Apr 25, 2018 at 11:26 AM, Paulo Flores Ribeiro 
> <paulo.flores.mail at gmail.com <mailto:paulo.flores.mail at gmail.com>> wrote:
>
>     Hello,
>
>     I have a shapefile with ca. 25000 polygons. Each polygon has an
>     average of 40 vertices (nodes). I would like to extract, for each
>     polygon, the distance separating the two most distant vertices
>     (aka "polygon diagonal" or "maximum polygon width"). It is not
>     important whether the polygon is convex or concave, so the lines
>     connecting the vertices can be inside or outside the polygon. The
>     desired result would be a two-column array, with a number of rows
>     equal to the number of polygons, and where the first column is the
>     id of the polygons, and the second the "maximum width" of the
>     corresponding polygon.
>
>     What would be the best way to do this, considering that the
>     calculation will probably require frequent updates (e.g. due to
>     changes in the shape of the polygons)?
>
>     Thanks in advance,
>
>     PauloFR
>
>     _______________________________________________
>     R-sig-Geo mailing list
>     R-sig-Geo at r-project.org <mailto:R-sig-Geo at r-project.org>
>     https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>     <https://stat.ethz.ch/mailman/listinfo/r-sig-geo>
>
>
>
> <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient> 
> 	Sem vírus. www.avg.com 
> <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient> 
>
>
> <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>


	[[alternative HTML version deleted]]