[Gdal-dev] geod: azimuth to degrees
nhv at cape.com
Fri Nov 3 13:36:49 EST 2006
Charlton Purvis writes
> I have a painfully simple question to ask the group that will
> undoubtedly expose my cartographic knowledge vacuum. But be
> that as it may, I'm stumped. Fire some trig my way -- I can
> take it. But going from an azimuth to degrees is beyond my
> grasp today.
> Let's take the geod man page example as our example:
> geod +ellps=clrk66 <<EOF -I +units=us-mi 42d15'N 71d07'W
> 45d31'N 123d41'W EOF
> which gives the results:
> -66d31'50.141" 75d39'13.083" 2587.504
> where the first two values are the azimuth from Boston to
> Portland, the back azimuth from Portland to Boston followed
> by the distance.
> Now say that I'd like to draw a vector on top of Boston that
> points to Portland. (For my purpose, my points are located
> much closer together, so I want to use geod to help me draw a
> vector that is scaled by distance / time
> -- I'm drawing drifter buoys and tracks).
> Don't I have enough info from geod to do that? If so, how in
> the devil can I get a single degree value to draw my little
> vector from -66d31'50.141"
Hmm looks like you reading the geod man page good :-)
"""Geod may also be used to determine intermediate points along either a
geodesic line between two points or along an arc of specified distance from
a geographic point. In both cases an initial point must be specified with
+lat_1=lat and +lon_1=lon parameters and either a terminus point +lat_2=lat
and +lon_2=lon or a distance and azimuth from the initial point with
+S=distance and +A=azimuth must be specified."""
And use the azimuth as returned from your first invocation with an
Distance arg to get the 'scaling effect' that you want
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