[R-sig-Geo] Finding distances between points and contour lines

[Barry Rowlingson]

Couple of clarifications:

 * I think you sent an HTML email which stripped out your example. Its
visible here:

http://r-sig-geo.2731867.n2.nabble.com/Finding-distances-between-points-and-contour-lines-tp7586733.html

but you shouldn't send HTML emails to the mailing list.

 * Do you want the minimal distance between the *vertices* of the
lines or the minimal distance between the lines themselves?

 For the former its just a question of extracting the vertex
coordinates from the spatial data structure and doing a distance
calculation between all pairs from each item. If you've transformed
your data to a regular cartesian coordinate system then Pythagoras
will help you.

 For the latter just turn your track into a SpatialLines object and
use gDistance from package rgeos:

trackSL = SpatialLines(LinesList=list(Lines(Line(coords=track.pts),1)))
gDistance(trackSL,contourSL)
[1] 0.02220655

this will return zero if the track defined by the points crosses the
contour, whereas the vertex distance method will return zero only if a
vertex is exactly on the contour.

# fiddle the track to cross the contour:

 track.pts[1,1]=.1

# rebuild the lines:

 trackSL = SpatialLines(LinesList=list(Lines(Line(coords=track.pts),1)))

# plot and check:

 lines(trackSL)

# distance should be zero:

 gDistance(trackSL,contourSL)
 # returns [1] 0



On Thu, Jul 10, 2014 at 10:23 PM, rhodes.uor <r.rhodes at pgr.reading.ac.uk> wrote:
> Hi all,
>
> I have two sets of data; a dataframe of lon/lat pairs which correspond to
> storm track locations, and a gridded rainfall dataset. I've taken contours
> of the rainfall dataset at a single level and saved them as a
> SpatialLinesDataFrame. I'm attempting to determine the distance between each
> of the storm track coordinates and each of the contours. Below is an example
> of what I have so far, but I can't figure out the next step:
>
>
>
> For each entry in track.pts I want to return a distance to the nearest point
> of each entry in contourSL (or in contours, for that matter). Can anyone
> recommend a way to do this?
> Thanks very much!
>
>
>
> --
> View this message in context: http://r-sig-geo.2731867.n2.nabble.com/Finding-distances-between-points-and-contour-lines-tp7586733.html
> Sent from the R-sig-geo mailing list archive at Nabble.com.
>
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