[R-sig-Geo] How to find ocean depth at specific locations?

[Paul Hiemstra]

Hi Dale,

If you have all your data in Spatial objects (sp-package) and they are 
both in the same projection you can use the overlay function. Read 
?overlay for more info. Small example:

# Script to extract the distance to the river meuse (holland) from a 
grid (meuse.grid) for the points in meuse.
library(sp)
data(meuse)
coordinates(meuse) =~x+y
data(meuse.grid)
gridded(meuse.grid) = ~x+y
spplot(meuse.grid, "dist", sp.layout = list("sp.points", meuse))
# Get indices of meuse.grid that contain the values of the grid at the
overlay(meuse.grid, meuse) 
# Get the values from the grid and attach to meuse
meuse$new_dist = meuse.grid$dist[overlay(meuse.grid, meuse)]

hope this helps!

cheers,
Paul

Dale Steele schreef:
> I would like to use ocean depth as a covariate in a spatial model
> where I have a measured catch at a specific location, but depth at
> that site was not reported in the original dataset.  The dataset
> 'scallop' in package SemiPar is a representative example:
>
>   
>> scallop[20:25,]
>>     
>    latitude longitude tot.catch
> 20 39.46667 -72.66667       722
> 21 39.43333 -72.73333        41
> 22 39.40000 -72.80000        39
> 23 39.31667 -72.73333         0
> 24 39.33333 -72.85000        94
> 25 39.26667 -72.81667         7
>
>
> I've been able to download a very large grid of depths in this area
> (33514 points) from: <http://topex.ucsd.edu/cgi-bin/get_data.cgi>.
> Given this data, I'm stuck at how to estimate the depth at each
> location in the scallops dataset.
>
> depths <- read.table("depths.txt", header=F,col.names=c("x","y","z"))
> depths$x <- depths$x - 360
>    ## subset the data keeping only negative depths
> depths <- subset(depths, depths$z < 0)
>    ## Convert depths to a positive number
> depths$z <- (-1)*depths$z
> length(depths$z)
>
> Thanks.  --Dale
>
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>   


-- 
Drs. Paul Hiemstra
Department of Physical Geography
Faculty of Geosciences
University of Utrecht
Heidelberglaan 2
P.O. Box 80.115
3508 TC Utrecht
Phone:     +31302535773
Fax:    +31302531145
http://intamap.geo.uu.nl/~paul