On Thu, 23 May 2013, Zack Holden wrote:
Karen,
you could try converting your polygons to raster using rasterize() in the
raster library, and then use the extract() function to retrieve the cell
values corresponding to your XY locations. I've used this workaround for
similar problems in the past.
No, not a good idea. The error message from over() was given for a reason,
that if the points and the polygon are not known to be in the same
projection, then the outcome may be misleading. So my guess would be that
the GPS are geographical coordinates and the coastal basins in State Plane
appropriate for the location. Look at proj4string() of each object, and if
they differ in predictable ways, use spTransform() to convert points to
the polygons' projection, or vice-versa. over() does know what it is
doing.
Hope this clarifies,
Roger
Zack
On Thu, May 23, 2013 at 9:00 AM, karengrace84 <kgfis...@alumni.unc.edu>wrote:
I am new to mapping with R and having some trouble. Here is what I am
trying
to accomplish. I am looking at fish tagging data. I have gps coordinates of
where each fish was tagged and released, and I have a map of 10 coastal
basins of the state of Louisiana. I am trying to determine which basin each
fish was tagged in, so that I can perform further analysis from there.
I've used the sp and rgdal packages to input my shp files of the coastal
basins, and I have my list of gps coordinates where the fish were tagged.
I've tried using the point.in.polygon() function and the over() function
from the sp package unsuccessfully. Can someone please tell me how to use
one of these functions correctly, or if there is a better way to reach my
goal?
For the point.in.polygon() function, I am unsure of how to get my data in
the correct format for the function. The generic form of the function is as
follows:
point.in.polygon(point.x, point.y, pol.x, pol.y, mode.checked=FALSE)
I have no problem with the point.x and point.y inputs. My problem is with
the pol.x and pol.y input. My polygon is currently in the form of a
SpatialPolygonsDataFrame. How do I get a numerical array of the x- and
y-coordinates from my polygon that will go into the point.in.polygon
function?
Or for the over() function (x %over% y), with x as my SpatialPoints (the
gps
coordinates) and y as my SpatialPolygonsDataFrame (the basin), I receive
the
following error message, Error: identicalCRS(x, y) is not TRUE.
Thanks for any suggestions.
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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.biv...@nhh.no
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