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Detailed Magnetic DeclinationSubmitted by dylan on Fri, 2008-03-14 20:45.
Setting the magnetic declination on a compass is a critical step in accurate measurement of bearings that can be translated to map or survey. As the location of the magnetic north pole is constantly moving, the magnetic declination reported on older topographic maps or compass housings are probably out of date. Current declination values for a single point, or for large regions can be found on the NOAA sites: These resources are helpful, but a current, detailed (state-level) map of magnetic declination would be interesting. The grid of coordinates and declinations, along with a PDF map are attached at the bottom of this page. # LL location g.region n=44:00:00N s=32:00:00N w=125:00:00W e=113:00:00W res=0.5 # read in as raster: r.in.xyz in=temp/mag_dec/ca_1deg.xyz out=declin type=FCELL x=2, y=1, z=3 # read in the data as a vector: v.in.ascii in=ca_1deg.xyz out=dec_points x=2 y=1 columns="x double, y double, declin double" # note that the cells are not centered on the points d.rast declin d.vect dec_points icon=basic/point d.grid size=2 fontsize=12 # save a GMT-style grid for plotting # note syntax, and preservation of NULL cells # set the region to something just a little smaller than CA g.region n=42 s=32.5 w=-124.5 e=-115 res=0.5 r.out.bin in=declin out=- null=-9999 | xyz2grd -Gdeclin.grd -R-124.5/-115/32.5/42 -I0.5/0.5 -ZTLf -F -N-9999
#setup the contouring annotated_contour_interval=0.25 #deg #setup the contour line colors contour_color1="1/1/1" # simple map of the unique locations # define some global settings: gmtset ANNOT_FONT_PRIMARY Times-Roman \ HEADER_FONT_SIZE 16 \ ANNOT_FONT_SIZE_PRIMARY 12 \ LABEL_FONT_SIZE 14 \ BASEMAP_TYPE plain \ PLOT_DEGREE_FORMAT DF \ PAPER_MEDIA letter+ # # sample map centered on the western USA pscoast -JB-119/38/36/40/6.5i \ -R-125/-113/32.5/42.5 -B4/2 \ -Gwhite -W0.5p \ -U"Magnetic Declination (E) as of March 2008 (http://www.ngdc.noaa.gov/seg/geomag/jsp/struts/calcGridIGRF). D.E. Beaudette." \ -A250 -Dh -Na -Xc -Yc -P -K > map.eps # plot counties psxy counties.xy -M -J -R -W1/200/200/200 -P -O -V -K >> map.eps # countour the declination grid grdcontour declin.grd -J -R -GN+1 -A${annotated_contour_interval}+k$contour_color1+s9 -Wa2/$contour_color1 -O -V >> map.eps # crop map to a tight bounding-box, and convert to PDF ps2raster map.eps -A -Tf
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