Summary

###########################################################################

	File:     geodat.icn

	Subject:  Procedures for geodetic datum conversion

	Authors:  William S. Evans and Gregg M. Townsend

	Date:     July 31, 2000

###########################################################################

   This file is in the public domain.

###########################################################################

	These procedures provide "projections" that convert among geodetic
	datums, which relate locations on the earth's surface to longitude
	and latitude coordinates.  As measurement techniques improve,
	newer datums typically give slightly different values from older
	ones.  The values returned here are used with the project()
	procedure of cartog.icn.

	geodat(s1, s2) defines a geodetic datum conversion.
	molodensky() performs an algorithmic datum conversion.
	nadcon(s1, s2) uses data files for more precise conversion.

	ellipsoid(s) return the parameters of the named ellipsoid.

###########################################################################

	geodat(f, t) returns a projection from longitude and latitude
	in datum f to longitude and latitude in datum t.
	f and t are strings.  If f and t equal "NAD83", "NAD27",
	"HARN", or "HPGN", geodat returns a nadcon projection.
	Failing that, geodat returns a molodensky projection.

	The input to the projection is a list of signed numeric values,
	angles measured in degrees, with each pair representing one
	location; longitude precedes latitude.  The output is a list
	with the same form and length as the input list.

###########################################################################

	nadcon(f, t) returns a projection from longitude and latitude
	in datum f to longitude and latitude in datum t.  The strings
	f and t must each be one of "NAD83", "NAD27", "HARN", or "HPGN".
	The projection uses our implementation of the National Oceanic
	and Atmospheric Administration's (NOAA's) North American Datum
	Conversion Utility (NADCON); for more information, see
	    http://www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.html

	nadcon() requires data grid (.loa and .laa) files, which must be
	found in the current directory or along the space-separated path
	given by the environment variable DPATH.  These files can be
	downloaded from:
	    http://www.cs.arizona.edu/icon/ftp/data/nadcon/
	    ftp://ftp.cs.arizona.edu/icon/data/nadcon/

	The projection's input and output are lists of signed numbers.
	Output is properly rounded and so may not agree exactly with
	the equivalent NOAA programs.

###########################################################################

	molodensky(dx, dy, dz, ain, fin, aout, fout) returns a projection
	from input longitude and latitude to output longitude and latitude.
	The projection uses the standard Molodensky transformation.
	The input datum is specified by an ellipsoid with parameters
	ain, the equatorial radius in metres, and fin, the flattening;
	and by three shift values dx, dy, and dz.  The output datum is
	specified by an ellipsoid with parameters aout and fout.

	If dz is null, then dx and dy are interpreted as the names of
	an input and output datum.  The names are the ID codes
	specified in NIMA TR 8350.2.

	The projection's input and output are lists of signed numbers.

###########################################################################

	ellipsoid(s) return a list [a, 1/f] containing the defining
	parameters of the standard ellipsoid model named s;  a is the
	equatorial radius and 1/f is the flattening factor.  Names are
	listed in the code; the default is "WGS84".

###########################################################################

	Ellipsoid and datum parameters are from:
		
		Department of Defense World Geodetic System 1984
		National Imagery and Mapping Agency
		Technical Report TR8350.2
		Third Edition, Amendment 1 (3 January 2000)
		ftp://ftp.nima.mil/pub/gg/tr8350.2/

###########################################################################

   Links: cartog, io

###########################################################################

This file is part of the (main) package.

ellipsoid(name)

: return [a, 1/f] for named ellipsoid

gdt_datum_add(key, fields)

gdt_datum_init()

gdt_datum_params(codename)

gdt_identity(p, L)

gdt_molo_inv(p)

gdt_molo_proj(p, L)

gdt_nadcon_bck(p, L)

gdt_nadcon_bckPoint(p, a, xpt, ypt)

gdt_nadcon_box(f, g, xcol, yrow)

gdt_nadcon_datum(x)

gdt_nadcon_findGrid(grids, xpt, ypt)

gdt_nadcon_fwd(p, L)

gdt_nadcon_fwdPoint(p, a, xpt, ypt)

gdt_nadcon_initGrids(names)

gdt_nadcon_inv(p)

geodat(f, t)

: define geodetic conversion

molodensky(dx, dy, dz, ain, fin, aout, fout)

: define geodetic conversion

nadcon(f, t)

: define NAD data conversion

gdt_datum_rec(region, name, area, eps, dx, dy, dz, ex, ey, ez)

gdt_molo(proj, inv, dx, dy, dz, ain, fin, aout, fout)

################ Standard Molodensky Datum Transformation ##################
 See NIMA TR 8350.2

 ************************ WARNING ******************************************
 NIMA TR 8350.2 contains Molodensky parameters to convert
 from an arbitrary datum to WGS84.  To convert from datum A to datum B,
 I call molodensky(Ax-Bx,Ay-By,Az-Bz,Aa,Af,Ba,Bf) where Ax,Ay,Az are the
 shift to convert A to WGS84; Bx,By,Bz are the shift to convert B to WGS84;
 Aa,Af,Ba,Bf are the ellipsoid parameters.
 ************************ WARNING ******************************************

 TODO:
	1) Add special conversion for North and South pole
	2) Add Multiple Regression Equations
	3) Add special WGS72 to WGS84 conversion

gdt_nadcon(proj, inv, grids)

 nadcon conversion record

gdt_nadcon_grid(name, offset, termLen, nc, nr, nz, xmin, xmax, dx, ymin, ymax, dy, angle)

 information about a .loa and .laa file