[R-sig-Geo] matching points to a raster

[Kamran Safi]

Hi,

I am not sure this is trivial. I think the simplest solution would be to 
find the closest grid cell centre (maybe using spDistN1 or simple vector 
length) and get the closest grid cell with environmental data and assign 
it to your animals' position. You can further complicate things by not 
taking simply the next closest point, but calculate the weighted mean 
(using the inverse of distance of the cell centres to the animals' 
location as weighing variable) or maybe most elegantly use kriging (or 
linear models), and extrapolate the value for the location. This is a 
classical geographers' problem of interpolation and not simply done by 
overlay(), which probably mimics the first solution of getting the next 
best grid cell (I am also not entirely sure whether overlay works with 
spatialPoints and spatialPixels).
Weighted mean maybe a straightforward solution.

Hope this helps,

Kami

Struve, Juliane wrote:
> Dear list,
>
> I am trying to incoporate spatial information (for example distance) into a movement model by reading an ascii -b converted raster and matching the raster with points of a movement path (for example fishmove$coord[fishmove$day==1]. Below is a section of my code to explain the problem. This code does not work because the coordinates of the movement path are not the same as those in the raster. Rounding the coordinates does not seem to solve the problem. Maybe the closest point would have to be found, but i am not sure how to approach this.  Is there another perhaps easier way ?
>
> Dist_DF=data.frame(readAsciiGrid("distance.txt"))
> Dist_shoreDF$coord = paste(round(Dist_DF$s1),round(Dist_DF$s2),sep=",")
> names(Dist_DF)[1]="Dist"
> Dist_DF$Dist[Dist_shoreDF$coord==fishmove$coord[fishmove$day==1]]
>
> I would be grateful for any help.
>
> Many thanks,
>
> Juliane
>
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