[R-sig-Geo] Calculate the internal angles of polygons

[Michael Sumner]

There's code in trip for the angles, but arranging the underlying coords
and expressing those in the right form will be the crux. st_coordinates
gives what you need, or pure data frame from spbabel::sptable is what I'd
start with.

I'll take a look when I can.

Cheers, Mike


On Sat., 1 Jun. 2019, 15:41 Juan Pablo Carranza, <carranzajuanp using gmail.com>
wrote:

> Dear members,
> I'm having trouble in calculating the internal angles of a set of polygons.
> I have a sf "MULTIPOLYGON" object of 5000 land parcels that has been tagged
> according to their shape. The objective is to train an algorithm to predict
> a large dataset (over 2 million polygons) according to these
> classification. But that is a problem that comes forward.
> First I need to calculate the internal angles of each one of the polygons
> in the sample, in order to eliminate unnecesary nodes (yes, I tryed
> st_simplify and also ms_simplify, but I need to control the process by the
> angle values).
> Here is as far as I get, a few lines of code (with an example polygon). The
> next steps will be dealing with a way to generate a dataframe in wich each
> row is a line of each polygon (decomposing the geometry but preserving the
> polygon entityid in order to go back... easy to do with spdplyr?), and find
> a way to calculate the angle between adjscent lines. Need a hand, please!
>
> x =
>
> c(4293215,4293205,4293205,4293194,4293174,4293176,4293190,4293201,4293204,4293218,4293215)
> y =
>
> c(6463731,6463733,6463734,6463736,6463740,6463750,6463747,6463744,6463744,6463741,6463731)
> cords = cbind(x, y)
> library(sp)
> p = Polygon(cords)
> ps = Polygons(list(p),1)
> poly = SpatialPolygons(list(ps))
> plot(poly)
> library(sf)
> poly = st_as_sf(poly)
> lines = st_cast(poly$geometry, "LINESTRING")
> plot(lines)
> # not run: this turns the object too simple, I need to work on very complex
> shapes.
> # lines = st_simplify(lines, preserveTopology = FALSE, dTolerance = 1)
>
> Really sorry for the lousy english... Best regards.
>
> *MSc. Juan Pablo Carranza*
> Economist
>
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>
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