m.datum.shift

NAME

m.datum.shift - Datum shift program.
(GRASS Data Import/Processing Program)

SYNOPSIS

m.datum.shift lat=dd.mm.ss{n|s} lon=dd.mm.ss{e|w} is=input_spheroid os=output_spheroid dx=xshift dy=yshift dz=zshift

DESCRIPTION

m.datum.shift returns geographic coordinates based on a different spheroid (and datum) than the one used to obtain the original coordinates.

The input and output spheroids, is and os, are the spheroids for two different datums. The input spheroid is the one on which the original coordinates are based. The output spheroid is that on which the resultant coordinates will be based. The "shifting" occurs between the two datums. The shift values, dx, dy, and dz, are constants. They indicate the mean differences between points in the second datum versus the first as measured in meters.

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 the command:

m.datum.shift lat=0n lon=0w dx=0 dy=0 dz=0 is=help os=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.datum.shift lat=0n lon=175w is=clark66 os=wgs72 dx=-22 dy=157 dz=176
Results: 4: lat=0.00.05.72999N
lon=174.59.55.004133W

NOTES

Essentially, the program follows these steps. The original point, as defined by a latitude and a longitude, is converted to geocentric coordinates. The shift values are added to the geocentric coordinates. The summed values are then converted to latitude and longitude based on the output spheroid.

For a brief discussion of spheroids and datums see m.ll2u. For a brief discussion of geocentric coordinates see m.ll2gc.

This remains under testing is still an experimental program. It is part of an initial effort to incorporate geographic coordinates into GRASS.

AUTHOR

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