Wind barbs are all of equal length, but that length may be adjusted by the scale parameter.
Wind barbs assume velocity data is given in knots. Actually it doesn't care, but effectively maxes out at velo=150. When the style is set to wind barbs, the scale parameter becomes a magnitude multiplier instead of changing the size of the barb. In this way scale=1.9438 can be used to convert data stored in m/s to knots without altering the data.
# Spearfish dataset g.copy vect=bugsites,dummy_map v.db.addcol map=dummy_map \ columns='direction DOUBLE PRECISION, magnitude DOUBLE PRECISION' v.db.update dummy_map column=direction value='cat * 4.0' v.db.update dummy_map column=magnitude value='cat / 2.0' g.region n=4918590 s=4914780 w=591510 e=595650 d.erase d.barb input=dummy_map direction=direction magnitude=magnitude \ aspect=compass style=barb
# r.surf.volcano module from GRASS-addons g.region -d r.surf.volcano output=gauss method=gaussian sigma=1 r.colors -ne gauss color=bcyr r.slope.aspect elev=gauss slope=gauss.slope aspect=gauss.aspect d.barb dir=gauss.aspect mag=gauss.slope scale=20 color=aqua
d.graph << EOF color 230:230:210 polygon 80 5 80 35 97 35 97 5 color black polyline 80 5 80 35 97 35 97 5 80 5 EOF d.barb legend_velo=5,10,15,20,25 \ legend_at=90,30,90,25,90,20,90,15,90,10 \ color=black legend_fontsize=20 style=arrow
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