## NAME

m.ll2gc - Converts geographic coordinates to geocentric coordinates.
(GRASS Data Import/Processing Program)

## SYNOPSIS

m.ll2gc lat=dd.mm.ss{n|s} lon=dd.mm.ss{e|w} [h=height] s=spheroid

## DESCRIPTION

m.ll2gc returns geocentric coordinates for geographic coordinates (latitude and longitude). Geographic coordinates are in degrees, minutes, and seconds and must include designation of north or south {n|s} and east or west {e|w}. The spheroid on which they are based must also be entered. The height (in meters) above the spheroid is optional.

The list of spheroids available is somewhat dynamic. It may not contain exactly the ones listed below. To determine the current list of possible spheroids, type in:

  m.ll2gc lat=0n lon=0w s=help

A list of available spheroids will be printed on the screen. If the spheroid desired is not on the list, the values for the semi-major axis and the eccentricity squared for the spheroid may be entered in place of a spheroid name in the following format:

  s=a=semi-major_axis,e=eccentricity_squared

### SOME POSSIBLE SPHEROIDS

(The on-line listing includes only the spheroid names)

    Spheroid     |  Commonly used for:|  Semi-major axis|  Eccentricity sqrd
    _____________|____________________|_________________|___________________
    australian   |  Australia         |  6378160.0      |  0.0066945419
    bessel       |  Japan             |  6377739.155    |  0.0066743722
    clark66      |  N. America        |  6378206.4      |  0.006768658
    clark80      |  France, Africa    |  6378249.145    |  0.0068035113
    everest      |  India, Burma      |  6377276.345    |  0.0066378466
    international|  Europe            |  6378388.0      |  0.00672267
    wgs72        |  worldwide coverage|  6378135.0      |  0.006694317778

## EXAMPLE

  m.ll2gc lat=0n lon=90w s=wgs72
Results: 4
  x=0.0
  y=6378135.0
  z=0.0

The distances designated by "x," "y" and "z" are in meters from the center of the earth. Because the sample point is on the equator and is at 90 degrees west, two of the three parameters have a value of zero (i.e., the point is at the origin of those two dimensions).

## NOTES

For a brief discussion of spheroids see m.ll2u.

Geographic coordinates (latitude/longitude) describe the location of a point on the earth relative to the equator in the north/south directions and relative to Greenwich in the east/west directions. The same point will have different coordinates depending on the spheroid used as the approximation of the shape of the earth. The geocentric coordinates given here describe the point on the basis of the same spheroid, but they use the center of the spheroid as their origin. This can be thought of as being the center of the earth, which is why they are called "geocentric." Describing the location of a point in a spheroid requires three points: x, y, and z; these are measured out from the center along three different axes. The distances given are in meters.

This is an experimental program. It is part of initial efforts to incorporate geographic coordinates into GRASS.

## SEE ALSO

m.datum.shift
m.gc2ll
m.ll2u
m.u2ll

## AUTHOR

Michael Shapiro, U.S.Army Construction Engineering Research Laboratory

Last changed: $Date: 2002/01/25 05:45:33 $