# geod¶

geod (direct) and invgeod (inverse) perform geodesic (“Great Circle”) computations for determining latitude, longitude and back azimuth of a terminus point given a initial point latitude, longitude, azimuth and distance (direct) or the forward and back azimuths and distance between an initial and terminus point latitudes and longitudes (inverse). The results are accurate to round off for |f| < 1/50, where f is flattening.

invgeod may not be available on all platforms; in this case call geod with the -I option.

## Synopsis¶

geod +ellps=<ellipse> [ -afFIlptwW [ args ] ] [ +args ] file[s]

invgeod +ellps=<ellipse> [ -afFIlptwW [ args  ]  ]  [  +args  ] file[s]


## Description¶

The following command-line options can appear in any order:

-I     Specifies that the inverse geodesic computation is to be
performed. May be used with execution of geod as an
alternative to invgeod execution.

-a     Latitude and longitudes of the initial and terminal
points, forward and back azimuths and distance are  output.

-ta    A specifies a character employed as the first character
to denote a control line to be passed through without
processing.

-le    Gives a listing of all the ellipsoids that may be
selected with the +ellps= option.

-lu    Gives a listing of all the units that  may  be  selected
with the +units= option.

-[f|F] format
Format is a printf format string to control the output
form of the geographic coordinate values (f) or distance
value (F). The default mode is DMS for geographic
coordinates and "%.3f" for distance.

-[w|W]n
N is the number of significant fractional digits to
employ for seconds output (when the option is not
specified, -w3 is assumed). When -W is employed the fields
will be constant width with leading zeroes.

-p     This option causes the azimuthal values to be output as
unsigned DMS numbers between 0 and 360 degrees. Also
note -f.


The +args command-line options are associated with geodetic parameters for specifying the ellipsoidal or sphere to use. See proj documentation for full list of these parameters and controls. The options are processed in left to right order from the command line. Reentry of an option is ignored with the first occurrence assumed to be the desired value.

One or more files (processed in left to right order) specify the source of data to be transformed. A - will specify the location of processing standard input. If no files are specified, the input is assumed to be from stdin.

For direct determinations input data must be in latitude, longitude, azimuth and distance order and output will be latitude, longitude and back azimuth of the terminus point. Latitude, longitude of the initial and terminus point are input for the inverse mode and respective forward and back azimuth from the initial and terminus points are output along with the distance between the points.

Input geographic coordinates (latitude and longitude) and azimuthal data must be in decimal degrees or DMS format and input distance data must be in units consistent with the ellipsoid major axis or sphere radius units. The latitude must lie in the range [-90d,90d]. Output geographic coordinates will be in DMS (if the -f switch is not employed) to 0.001” with trailing, zero-valued minute-second fields deleted. Output distance data will be in the same units as the ellipsoid or sphere radius.

The Earth’s ellipsoidal figure may be selected in the same manner as program proj by using +ellps=, +a=, +es=, etc.

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.

If points along a geodesic are to be determined then either +n_S=integer specifying the number of intermediate points and/or +del_S=distance specifying the incremental distance between points must be specified.

To determine points along an arc equidistant from the initial point both +del_A=angle and +n_A=integer must be specified which determine the respective angular increments and number of points to be determined.

## Examples¶

The following script determines the geodesic azimuths and distance in U.S. statute miles from Boston, MA, to Portland, OR:

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.

An example of forward geodesic use is to use the Boston loca- tion and determine Portland’s location by azimuth and distance:

geod +ellps=clrk66 <<EOF +units=us-mi
42d15'N 71d07'W -66d31'50.141" 2587.504
EOF


which gives:

45d31'0.003"N 123d40'59.985"W 75d39'13.094"


Note

lack of precision in the distance value compromises the precision of the Portland location.