Convert Coordinates from Grid to Latitude/Longitude
Convert geographic coordinates from Universal Transverse Mercator system to Latitude/Longitude.
grid2latlong(input)
input |
A data frame with columns named |
input |
either an |
Longitude/latitudes are not a grid-based coordinate system: latitudes are equidistant but the distance between longitudes varies.
Either a data frame with the corresponding longitude and latitude, or a SpatialPolygons object with the coordinates changed.
Rough conversion of US lat/long to km (used by GeoBUGS): (see also forum.swarthmore.edu/dr.math/problems/longandlat.html). Radius of earth: r = 3963.34 (equatorial) or 3949.99 (polar) mi = 6378.2 or 6356.7 km, which implies: km per mile = 1.609299 or 1.609295 a change of 1 degree of latitude corresponds to the same number of km, regardless of longitude. arclength=r*theta, so the multiplier for coord\$y should probably be just the radius of earth. On the other hand, a change of 1 degree in longitude corresponds to a different distance, depending on latitude. (at N pole, the change is essentially 0. at the equator, use equatorial radius. Perhaps for U.S., might use an "average" latitude, 30 deg is roughly Houston, 49deg is most of N bdry of continental 48 states. 0.5(30+49)=39.5 deg. so use r approx 6378.2*sin(51.5)
Lance A. Waller
coord <- data.frame(rbind( # Montreal, QC c(-6414.30, 5052.849), # Vancouver, BC c(-122.6042, 45.6605) )) grid2latlong(coord)
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