[R-sig-Geo] Reordering geographical coordinates clockwise to make a polygon

[Patrick Giraudoux]

Apologize to answer to myself: the way described below would work only 
if there are no concave "peninsula" towards north or south inside the 
polygon. Even thinking about an alternate solution, e.g. the Traveling 
Salesman Problem (TSP), ie. the shortest route linking all points, one 
could get wrong since points of  a narrowpeninsula could have points 
closest to the opposite border than from the next point on the border...

Suppose we are stuck, and should redraw polygons by hand


Le 05/11/2018 à 14:02, Patrick Giraudoux a écrit :
>
> Dear listers,
>
> There is an interesting post here: 
> https://stackoverflow.com/questions/6989100/sort-points-in-clockwise-order 
> dealing on the issue. However, I would like to know if a function has 
> been already developped in a R package.
>
> I am coping with a young colleague's shapefile where borders of 
> polygons have been drawn as lines quite inconsistently. To change this 
> SpatialLinesDataFrame into a SpatialPoints object is easy. The idea is 
> to  select the points  bordering each polygon (delete the others), 
> define the point set as a Polygon, then rebuild step by step a 
> SpatialPolygonsDataFrame with all its (~25) polygons. It would be much 
> quicker than to redraw one by one the 5160 data points (two times on 
> borders shared by two polygons).
>
> The problem is that the points must be reordered clockwise (the way 
> lines making the  borders is all except clockwise) before making them 
> a polygon.
>
> Any function already developped for that ?
>
> Cheers,
>
> Patrick
>
>
>


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