[R-sig-Geo] calculating shortest distance from a set of SpatialPoints to a polygon

[Roger Bivand]

On Wed, 13 Jun 2012, Malcolm Fairbrother wrote:

> Dear list,
>
> I have a large set of locations scattered over a land area covering 
> about 200,000 km². That area borders the sea, and I'd like to find the 
> shortest distance from each point location to the sea/coast. Point 
> locations are in lat/long.
>
> I've turned the locations into a SpatialPoints object, with a 
> proj4string. And I have a SpatialPolygons object consisting of a single 
> polygon, defining the sea areas (same proj4string as for the 
> SpatialPoints object).
>
> My problem is I'm not clear on the next step. I tried using "gDistance" 
> from the "rgeos" package:
>
> dists <- gDistance(locations, sea)
>
> but got two warnings:
>
> Warning messages:
> 1: In RGEOSDistanceFunc(spgeom1, spgeom2, byid, "rgeos_distance") :
>  Spatial object 1 is not projected; GEOS expects planar coordinates
> 2: In RGEOSDistanceFunc(spgeom1, spgeom2, byid, "rgeos_distance") :
>  Spatial object 2 is not projected; GEOS expects planar coordinates
>

Wait a moment! The warnings are there for a purpose (and would also affect 
the use of functions in spatstat, which also assumes planar coordinates). 
Never delete important data (in this case the metadata of your points, 
saying which spatial reference system they possess).

If your data are in geographical coordinates, you need to state that 
specifically, and look for suitable functions. One such is perhaps 
dist2Line() in geosphere.

The alternative (and I advise doing both) is to project your points and 
coastine, and use gDistance(locations, sea, byid=TRUE), which will return 
distances in the metric of the projection (probably metres).

> So should I set the proj4string to NA? And/or is gDistance even the 
> right tool for this job? Maybe it's designed for instances where 
> distances are small, not in the hundreds of kilometres?

Never set proj4string to NA if it is known (NA means not available, it 
doesn't mean ignore). The function works on planar geometries, and 
projection distortions for well-chosed projections at that scale are not 
large. Usually, wrongly defined datum metadata are more of a problem.

Roger

>
> Any advice on how to do this appropriately would be much appreciated.
>
> - Malcolm
>
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-- 
Roger Bivand
Department of Economics, NHH Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no