[Gdal-dev] geod: azimuth to degrees

Norman Vine 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.
> Fantastic!
> 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"
> 75d39'13.083"?

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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