# [Proj] Program Geod

Gerald I. Evenden geraldi.evenden at gmail.com
Tue Mar 17 15:18:29 EST 2009

```I noticed a factor with Karney's geodesic program Geod where the azimuth of
the geodesic at point 2, usually referred to as the "back azimuth", pointed
away from the first point.  That is, it is off by Pi.

To demonstrate what I believe to be the normal way that the back azimuth is
presented I have attached example runs from NGS and my routine geodesic.
I appologize for claiming to be a "source".
-------------------------------------------------------------------------------------------------------
Inverse from
http://www.ngs.noaa.gov/cgi-bin/Inv_Fwd/inverse2.prl
(NGS' Vincente procedure)

Ellipsoid : GRS80 / WGS84  (NAD83)
Equatorial axis,    a   =    6378137.0000
Polar axis,         b   =    6356752.3141
Inverse flattening, 1/f =  298.25722210088

First  Station : p1
----------------
LAT =  10  0  0.00000 North
LON =   5  0  0.00000 East

Second Station : p2
----------------
LAT =  60  0  0.00000 North
LON = 140  0  0.00000 East

Forward azimuth        FAZ =  21  9 40.4155 From North
Back azimuth           BAZ = 314 49 18.3799 From North
Ellipsoidal distance     S =  11287082.3529 m
----------------------------------------------------------------------------------------------------
From geodesic on charon:

\$ geodesic -- "earth ellps=wgs84" "display prec=4"
Geodesic Computer v. 2.0
geod: p1 5 10
geod: p2 140 60
Point 1
Lon:    5d00'00.000000"E
Lat:   10d00'00.000000"N
Hgt:   0.000000
Point 2
Lon:  140d00'00.000000"E
Lat:   60d00'00.000000"N
Hgt:   0.000000
Azimuth p1->p2: 21d9'40.4154961"
Distance: 11287082.3529
Azimuth p2->p1: 314d49'18.3799053"
------------------------------------------------------------------------------------------------
From Karney's program Geod

\$ ./Geod -i -d -p 4
10 5 60 140
021d09'40.41550" 134d49'18.37991" 11287082.3529

^forward azi          ^"back" azi - pi        ^dist

---------------------------------
C makes prettier output  ;-)
--
The whole religious complexion of the modern world is due
to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939) British psychologist
```