On Fri, 7 Dec 2012, Sarah Goslee wrote:
Hi Simon,
I've copied this back to the list, as is encouraged.
On Fri, Dec 7, 2012 at 9:41 AM, O'Hanlon, Simon J
<simon.ohan...@imperial.ac.uk> wrote:
Hi Sarah, thank you.
My data points are located in West Africa, so I think a good projection
would be Lambert Azimuthal Equal Area.
This does raise one more question for me. I also have some raster data
that I wish to use as covariates which is also in geographic
coordinates. If I try to simply reproject a raster, I think it would
screw up the regular grid. Does it make sense if I convert the lat-long
raster to a SpatialPointsDataFrame and transform this to LAEA so I
could then create a grid in the desired planar coordinates from scratch
and use over() to assign values from the reprojected spatial points
onto the raster?
A reprojected raster is still a raster, and thus a regular grid. Or am
I missing something?
Yes, a warped raster is a raster, but a reprojected raster will in general
be a set of irregular points. So resampling is invoved one way or the
other. Whether one queries the raster as-is with spatial points, and then
projects the output, or warps the raster doing spatial query with
projected points will depend a bit on the support(s) of the data sets.
Maybe see: ?projectRaster in raster for warping.
Roger
Sarah
Thanks again for your help.
Cheers,
Simon
-----Original Message-----
From: Sarah Goslee [mailto:sarah.gos...@gmail.com]
Sent: 07 December 2012 14:18
To: O'Hanlon, Simon J
Cc: r-sig-geo@r-project.org
Subject: Re: [R-sig-Geo] Distance between two points
Precisely. You should use great-circle distances with lat-lon coordinates,
rather than Euclidean distance, because the actual length varies with position
on the globe.
Converting to UTM or something similar is one solution if your points are not
too far apart.
There are many other R solutions: searching for "great circle distance" at
rseek.org will get you quite a list.
Sarah
On Fri, Dec 7, 2012 at 8:10 AM, O'Hanlon, Simon J
<simon.ohan...@imperial.ac.uk> wrote:
Dear list,
I am using the package geoRglm to do some predictive mapping. There is a
function that calculates the distance between observed data points and the
prediction locations using a .C call to a function which eventually calculates
the length of the hypotenuse between one location and the other given the
vertical and horizontal separation distance of those points.
My question is, is this method of distance-finding incompatible with long-lat
style coordinates? Should I first transform my data and prediction locations
into something where the unit of measurement is in metres rather than decimal
degrees?
Many thanks,
Simon
--
Sarah Goslee
http://www.functionaldiversity.org
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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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