Re: [R] Converting coordinates to actual distances
Paul Brewin wrote:
Hello,
I've been searching for a method of converting Lat/Lon decimal
coordinates into actual distances between points, and taking into
account the curvature of the earth. Is there such a package in R? I've
looked at the GeoR package, but this does not seem to contain what I am
looking for. Ideally the output would be a triangular matrix of
distances.
Hi Paul,
Below is an implementation of the Haversine formula that I once had to
use for calculating distances between telephone exchanges. It's in C,
but could be translated to R fairly easily. I've included a degrees to
radians conversion as well.
Jim
#define APOSTROPHE 39
#define EARTH_RADIUS 6367
typedef struct {
char *basename;
char *number[2];
double lat[2];
double lon[2];
char delimiter[4];
char *data02;
char *data03;
char *data07;
char *data08;
FILE *input;
FILE *output;
} PSTS_INFO;
/* DegreesToRadians
Converts a string of the form nnndnn'nn[NSEW] to a value in radians.
East (E) and South (S) are arbitrarily negative. Note that this routine
will generate garbage if not passed a string of the correct form,
although it is unlikely to do too much damage.
Return valuevalue of the string in radians
-10 input not correct format
*/
double DegreesToRadians(char *dms) {
int index = 0;
double degrees;
double minutes = 0;
double seconds = 0;
int mult;
degrees = atof(dms);
while(isdigit(dms[index])) index++;
if(dms[index++] == 'd') {
minutes = atof(dms[index]);
minutes /= 60;
while(isdigit(dms[index])) index++;
if(dms[index++] == APOSTROPHE) {
seconds = atof(dms[index]);
seconds /= 360;
while(isdigit(dms[index]) dms[index]) index++;
if(dms[index++] == QUOTES) {
mult = (dms[index] == 'S' || dms[index] == 'E') ? -1 : 1;
degrees += minutes + seconds;
return(mult * degrees * M_PI/180);
}
}
}
return(-10);
}
/* GreatCircleDistance
(Formula and recommendations for calculation taken from:
http://www.census.gov/cgi-bin/geo/gisfaq?Q5.1
by Bob Chamberlain - [EMAIL PROTECTED])
Calculates 'great circle' distances using the Haversine formula,
based upon a spherical earth of EARTH_RADIUS radius.
This version extracts the two latitude/longitude pairs (as radians)
from the PSTS_INFO structure.
*/
double GreatCircleDist(PSTS_INFO *psts_info) {
double a,d,dlon,dlat;
dlon = psts_info-lon[1] - psts_info-lon[0];
dlat = psts_info-lat[1] - psts_info-lat[0];
a = pow(sin(dlat / 2),2) + cos(psts_info-lat[0]) *
cos(psts_info-lat[1]) * pow(sin(dlon / 2),2);
/* The penultimate result may be calculated either way - the first
is bulletproof, the second is a bit faster */
d = 2 * atan2(sqrt(a),sqrt(1 - a));
/* d = 2 * asin(sqrt(a));*/
return(d * EARTH_RADIUS);
}
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[R] Converting coordinates to actual distances
Hello, I've been searching for a method of converting Lat/Lon decimal coordinates into actual distances between points, and taking into account the curvature of the earth. Is there such a package in R? I've looked at the GeoR package, but this does not seem to contain what I am looking for. Ideally the output would be a triangular matrix of distances. Thanks in advance, Paul Brewin Paul E Brewin (PhD) Center for Research in Biological Systems University of California San Diego 9500 Gilman Drive MC 0505 La Jolla CA, 92093-0505 USA Ph: 858-822-0871 Fax: 858-822-3631 __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] Converting coordinates to actual distances
Paul Brewin wrote: Hello, I've been searching for a method of converting Lat/Lon decimal coordinates into actual distances between points, and taking into account the curvature of the earth. Is there such a package in R? I've looked at the GeoR package, but this does not seem to contain what I am looking for. Ideally the output would be a triangular matrix of distances. Thanks in advance, Paul Brewin Paul E Brewin (PhD) Center for Research in Biological Systems University of California San Diego 9500 Gilman Drive MC 0505 La Jolla CA, 92093-0505 USA Ph: 858-822-0871 Fax: 858-822-3631 __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html See http://www.meridianworlddata.com/Distance-Calculation.asp. The calculations are trivial. -- Bob Wheeler --- http://www.bobwheeler.com/ ECHIP, Inc. --- Randomness comes in bunches. __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] Converting coordinates to actual distances
See http://finzi.psych.upenn.edu/R/Rhelp02a/archive/29117.html On 9/14/05, Paul Brewin [EMAIL PROTECTED] wrote: Hello, I've been searching for a method of converting Lat/Lon decimal coordinates into actual distances between points, and taking into account the curvature of the earth. Is there such a package in R? I've looked at the GeoR package, but this does not seem to contain what I am looking for. Ideally the output would be a triangular matrix of distances. Thanks in advance, Paul Brewin Paul E Brewin (PhD) Center for Research in Biological Systems University of California San Diego 9500 Gilman Drive MC 0505 La Jolla CA, 92093-0505 USA Ph: 858-822-0871 Fax: 858-822-3631 __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] Converting coordinates to actual distances
On Wed, 14 Sep 2005, Paul Brewin wrote: Hello, I've been searching for a method of converting Lat/Lon decimal coordinates into actual distances between points, and taking into account the curvature of the earth. Is there such a package in R? I've looked at the GeoR package, but this does not seem to contain what I am looking for. Ideally the output would be a triangular matrix of distances. Using C code in the sp package (which will be exposed at the R level shortly) and sp 0.8 and maptools 0.5: library(maptools) Loading required package: foreign Loading required package: sp xx - readShapePoly(system.file(shapes/sids.shp, package=maptools)[1]) ll - getSpPPolygonsLabptSlots(xx) # ll is a matrix of long-lat centroids of North Carolina county polygons str(ll) num [1:100, 1:2] -81.5 -81.1 -79.3 -79.8 -78.7 ... plot(ll) x - as.double(ll[,1]) y - as.double(ll[,2]) n - as.integer(length(x)) dists - vector(mode=double, length=n) lonlat - as.integer(1) res - matrix(as.double(NA), 100, 100) for (i in 1:100) res[i,] - .C(sp_dists, x, y, x[i], y[i], n, dists, + lonlat)[[6]] gives a full matrix measured in kilometers for the WGS-84 ellipsoid. Accessing the C function like this puts the responsibility for checking the argument modes on the user. If this seems scary, rdist.earth() in the fields package has an R version of this. But maybe you need the actual functions to use great circle distance instead of Euclidean, rather than just to generate a distance matrix? Hope this helps, Roger Bivand Thanks in advance, Paul Brewin Paul E Brewin (PhD) Center for Research in Biological Systems University of California San Diego 9500 Gilman Drive MC 0505 La Jolla CA, 92093-0505 USA Ph: 858-822-0871 Fax: 858-822-3631 __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html -- Roger Bivand Economic Geography Section, Department of Economics, Norwegian School of Economics and Business Administration, Helleveien 30, N-5045 Bergen, Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43 e-mail: [EMAIL PROTECTED] __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
