# [OSRS-PROJ] problem with azimuth and distance

Pedro Morón Macías pmoron at sainsel.es
Tue Jun 6 03:04:05 EDT 2000

```OK, now it's clear to me.
Let's go farther:
Is there any way to follow a non great circle route using the proj.4
library? For example: I want to determine a point in the 80 degrees
North parallel 100 miles away from one given in the 80 degrees parallel
too.

"Gerald I. Evenden" wrote:
>
> Answer: very simple, you have a great circle route and except for
> N-S lines and E-W lines on the equator, none of these will follow
> a parallel or meridian.  Think of the inverse problem.  Take two
> points on a parallel and not on the equator.  The line will bow away
> from the parallel, reaching a maximum N-S deviation from the parallel
> midway between the points.  The forward and back azimuth will
> be less than 90 degrees (northern hemisphere) and opposite in sign.
> As the two points become more widely separated---still on the
> same parallel, the |azimuth| decreases and eventuall will become
> zero when the two points are separated by 180 degrees in longitude.
>
> Its why you fly over Greenland getting from LA to Europe.
>
> You are dealing with a geodesic line or great circle route not a
> rhumb line or loxodrome represented by straight lines on a Mercator
> projection.
>
> __________________________________________________
> Jerry Evenden and the Low Riders, Katie and  Daisy May
> gevenden at capecod.net  http://www.capecod.net/~gevenden
> ----- Original Message -----
> From: Pedro Morón Macías <pmoron at sainsel.es>
> To: <osrs-proj at remotesensing.org>
> Sent: Monday, June 05, 2000 12:01 PM
> Subject: [OSRS-PROJ] problem with azimuth and distance
>
> > Here's a little problem I have with the PROJ4 library, and since I've
> > found no answer in the documentation nor in several sources, I'm trying
> > this list.
> >
> > I have to determine latitude and longitude of a point given and initial
> > point latitude and longitude, and an azimuth and a distance from this
> > point.
> > I'm using the "geod" program to test the library and these are the
> > results:
> >
> > csp1.pmm:~/csp98/swb/proj.4/src > geod +units=kmi +ellps=WGS84 <<EOF
> > > 80dN 10dE 90d 100
> > > EOF
> > 79d51'53.25"N   19d27'54.76"E   -80d40'47.564"
> >
> > In the example I want to determine the point at 90degrees azimuth and
> > 100 nautic miles away from 80degrees North, 10degrees East.
> >
> > The given azimuth equals to 90 degrees, then, why the resulting latitude
> > doesn't equal to 80 degrees North, like the starting point does?
> >
> > Why the back azimuth doesn't equal to -90 degrees?
> >
> >
> > I'm sure this problem is not a bug of the PROJ4 library, and I want to
> > know where am I misusing the library...
> >
> >
> > Thanks a lot.
>
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---------------------------------------------------------------
Pedro Moron Macias                         Ingeniero Industrial
SAINSEL Sistemas Navales, S.A.          Manuel Velasco Pando, 7
41007 SEVILLA - SPAIN                  mailto:pmoron at sainsel.es
Tel.:+34-954936401, Fax :+34-954936433, Home Tel.:+34-639349940
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