| recpgr_c |
|
Table of contents
Procedure
recpgr_c ( Rectangular to planetographic )
void recpgr_c ( ConstSpiceChar * body,
SpiceDouble rectan[3],
SpiceDouble re,
SpiceDouble f,
SpiceDouble * lon,
SpiceDouble * lat,
SpiceDouble * alt )
AbstractConvert rectangular coordinates to planetographic coordinates. Required_ReadingKERNEL NAIF_IDS PCK KeywordsCONVERSION COORDINATES GEOMETRY MATH Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- body I Body with which coordinate system is associated. rectan I Rectangular coordinates of a point. re I Equatorial radius of the reference spheroid. f I Flattening coefficient. lon O Planetographic longitude of the point (radians). lat O Planetographic latitude of the point (radians). alt O Altitude of the point above reference spheroid. Detailed_Input
body is the name of the body with which the planetographic
coordinate system is associated.
`body' is used by this routine to look up from the
kernel pool the prime meridian rate coefficient giving
the body's spin sense. See the -Files and -Particulars
header sections below for details.
rectan are the rectangular coordinates of a point. Units
are arbitrary, except that the input `re' must be
expressed in the same units.
re is the equatorial radius of a reference spheroid.
This spheroid is a volume of revolution: its
horizontal cross sections are circular. The shape of
the spheroid is defined by an equatorial radius `re' and
a polar radius `rp'. Units of `re' must match those of
`rectan'.
f is the flattening coefficient =
(re-rp) / re
where `rp' is the polar radius of the spheroid, and the
units of `rp' match those of `re'.
Detailed_Output
lon is the planetographic longitude of the input point.
This is the angle between the prime meridian and the
meridian containing `rectan'. For bodies having
prograde (aka direct) rotation, the direction of
increasing longitude is positive west: from the +X
axis of the rectangular coordinate system toward the
-Y axis. For bodies having retrograde rotation, the
direction of increasing longitude is positive east:
from the +X axis toward the +Y axis.
The earth, moon, and sun are exceptions:
planetographic longitude is measured positive east for
these bodies.
The default interpretation of longitude by this
and the other planetographic coordinate conversion
routines can be overridden; see the discussion in
-Particulars below for details.
`lon' is output in radians. The nominal range of `lon' is
given by:
0 < lon < 2*pi
-
However, round-off error could cause `lon' to equal 2*pi.
lat is the planetographic latitude of the input point. For
a point P on the reference spheroid, this is the angle
between the XY plane and the outward normal vector at
P. For a point P not on the reference spheroid, the
planetographic latitude is that of the closest point
to P on the spheroid.
`lat' is output in radians. The range of `lat' is given
by:
-pi/2 < lat < pi/2
- -
alt is the altitude of point above the reference spheroid.
The units associated with `alt' are those associated
with the input `rectan' and `re'.
ParametersNone. Exceptions
1) If the body name `body' cannot be mapped to a NAIF ID code, and
if `body' is not a string representation of an integer, the
error SPICE(IDCODENOTFOUND) is signaled by a routine in the
call tree of this routine.
2) If the kernel variable
BODY<ID code>_PGR_POSITIVE_LON
is present in the kernel pool but has a value other than one
of
'EAST'
'WEST'
the error SPICE(INVALIDOPTION) is signaled by a routine in the
call tree of this routine. Case and blanks are ignored when
these values are interpreted.
3) If polynomial coefficients for the prime meridian of `body' are
not available in the kernel pool, and if the kernel variable
BODY<ID code>_PGR_POSITIVE_LON is not present in the kernel
pool, the error SPICE(MISSINGDATA) is signaled by a routine in
the call tree of this routine.
4) If the equatorial radius is non-positive, the error
SPICE(VALUEOUTOFRANGE) is signaled by a routine in the call
tree of this routine.
5) If the flattening coefficient is greater than or equal to one,
the error SPICE(VALUEOUTOFRANGE) is signaled by a routine in
the call tree of this routine.
6) For points inside the reference ellipsoid, the nearest point
on the ellipsoid to `rectan' may not be unique, so latitude may
not be well-defined.
7) If the `body' input string pointer is null, the error
SPICE(NULLPOINTER) is signaled.
8) If the `body' input string has zero length, the error
SPICE(EMPTYSTRING) is signaled.
Files
This routine expects a kernel variable giving body's prime
meridian angle as a function of time to be available in the
kernel pool. Normally this item is provided by loading a PCK
file. The required kernel variable is named
BODY<body ID>_PM
where <body ID> represents a string containing the NAIF integer
ID code for `body'. For example, if `body' is "JUPITER", then
the name of the kernel variable containing the prime meridian
angle coefficients is
BODY599_PM
The optional kernel variable
BODY<body ID>_PGR_POSITIVE_LON
also is normally defined via loading a text kernel. When this
variable is present in the kernel pool, the prime meridian
coefficients for `body' are not required by this routine. See the
-Particulars section below for details.
Particulars
Given the body-fixed rectangular coordinates of a point, this
routine returns the planetographic coordinates of the point. The
body-fixed rectangular frame is that having the X-axis pass
through the 0 degree latitude 0 degree longitude direction, the
Z-axis pass through the 90 degree latitude direction, and the
Y-axis equal to the cross product of the unit Z-axis and X-axis
vectors.
The planetographic definition of latitude is identical to the
planetodetic (also called "geodetic" in SPICE documentation)
definition. In the planetographic coordinate system, latitude is
defined using a reference spheroid. The spheroid is
characterized by an equatorial radius and a polar radius. For a
point P on the spheroid, latitude is defined as the angle between
the X-Y plane and the outward surface normal at P. For a point P
off the spheroid, latitude is defined as the latitude of the
nearest point to P on the spheroid. Note if P is an interior
point, for example, if P is at the center of the spheroid, there
may not be a unique nearest point to P.
In the planetographic coordinate system, longitude is defined
using the spin sense of the body. Longitude is positive to the
west if the spin is prograde and positive to the east if the spin
is retrograde. The spin sense is given by the sign of the first
degree term of the time-dependent polynomial for the body's prime
meridian Euler angle "W": the spin is retrograde if this term is
negative and prograde otherwise. For the sun, planets, most
natural satellites, and selected asteroids, the polynomial
expression for W may be found in a SPICE PCK kernel.
The earth, moon, and sun are exceptions: planetographic longitude
is measured positive east for these bodies.
If you wish to override the default sense of positive longitude
for a particular body, you can do so by defining the kernel
variable
BODY<body ID>_PGR_POSITIVE_LON
where <body ID> represents the NAIF ID code of the body. This
variable may be assigned either of the values
'WEST'
'EAST'
For example, you can have this routine treat the longitude
of the earth as increasing to the west using the kernel
variable assignment
BODY399_PGR_POSITIVE_LON = 'WEST'
Normally such assignments are made by placing them in a text
kernel and loading that kernel via furnsh_c.
The definition of this kernel variable controls the behavior of
the CSPICE planetographic routines
pgrrec_c
recpgr_c
dpgrdr_c
drdpgr_c
It does not affect the other CSPICE coordinate conversion
routines.
Examples
The numerical results shown for these examples may differ across
platforms. The results depend on the SPICE kernels used as
input, the compiler and supporting libraries, and the machine
specific arithmetic implementation.
1) Find the planetographic coordinates of the point having Mars
rectangular coordinates:
X (km) = 0.0
Y (km) = -2620.678914818178
Z (km) = 2592.408908856967
(These input values have been chosen to create "simple" output
values.)
Use the PCK kernel below to load the required triaxial
ellipsoidal shape model and orientation data for Mars.
pck00008.tpc
Example code begins here.
/.
Program recpgr_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
/.
Local variables
./
SpiceDouble alt;
SpiceDouble f;
SpiceDouble lat;
SpiceDouble lon;
SpiceDouble radii [3];
SpiceDouble re;
SpiceDouble rectan [3];
SpiceDouble rp;
SpiceInt n;
/.
Load a PCK file containing a triaxial
ellipsoidal shape model and orientation
data for Mars.
./
furnsh_c ( "pck00008.tpc" );
/.
Look up the radii for Mars. Although we
omit it here, we could first call badkpv_c
to make sure the variable BODY499_RADII
has three elements and numeric data type.
If the variable is not present in the kernel
pool, bodvrd_c will signal an error.
./
bodvrd_c ( "MARS", "RADII", 3, &n, radii );
/.
Compute flattening coefficient.
./
re = radii[0];
rp = radii[2];
f = ( re - rp ) / re;
/.
Do the conversion.
./
rectan[0] = 0.0;
rectan[1] = -2620.678914818178;
rectan[2] = 2592.408908856967;
recpgr_c ( "mars", rectan, re, f, &lon, &lat, &alt );
printf ( "\n"
"Rectangular coordinates:\n"
"\n"
" X (km) = %18.9e\n"
" Y (km) = %18.9e\n"
" Z (km) = %18.9e\n"
"\n"
"Ellipsoid shape parameters:\n"
"\n"
" Equatorial radius (km) = %18.9e\n"
" Polar radius (km) = %18.9e\n"
" Flattening coefficient = %18.9e\n"
"\n"
"Planetographic coordinates:\n"
"\n"
" Longitude (deg) = %18.9e\n"
" Latitude (deg) = %18.9e\n"
" Altitude (km) = %18.9e\n"
"\n",
rectan[0],
rectan[1],
rectan[2],
re,
rp,
f,
lon / rpd_c(),
lat / rpd_c(),
alt );
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Rectangular coordinates:
X (km) = 0.000000000e+00
Y (km) = -2.620678915e+03
Z (km) = 2.592408909e+03
Ellipsoid shape parameters:
Equatorial radius (km) = 3.396190000e+03
Polar radius (km) = 3.376200000e+03
Flattening coefficient = 5.886007556e-03
Planetographic coordinates:
Longitude (deg) = 9.000000000e+01
Latitude (deg) = 4.500000000e+01
Altitude (km) = 3.000000000e+02
2) Below is a table showing a variety of rectangular coordinates
and the corresponding Mars planetographic coordinates. The
values are computed using the reference spheroid having radii
Equatorial radius: 3396.190
Polar radius: 3376.200
Note: the values shown above may not be current or suitable
for your application.
Corresponding rectangular and planetographic coordinates are
listed to three decimal places.
rectan[0] rectan[1] rectan[2] lon lat alt
--------------------------------------------------------------
3396.190 0.000 0.000 0.000 0.000 0.000
-3396.190 0.000 0.000 180.000 0.000 0.000
-3406.190 0.000 0.000 180.000 0.000 10.000
-3386.190 0.000 0.000 180.000 0.000 -10.000
0.000 -3396.190 0.000 90.000 0.000 0.000
0.000 3396.190 0.000 270.000 0.000 0.000
0.000 0.000 3376.200 0.000 90.000 0.000
0.000 0.000 -3376.200 0.000 -90.000 0.000
0.000 0.000 0.000 0.000 90.000 -3376.200
3) Below we show the analogous relationships for the earth,
using the reference ellipsoid radii
Equatorial radius: 6378.140
Polar radius: 6356.750
Note the change in longitudes for points on the +/- Y axis
for the earth vs the Mars values.
rectan[0] rectan[1] rectan[2] lon lat alt
------------------------------------------------------------
6378.140 0.000 0.000 0.000 0.000 0.000
-6378.140 0.000 0.000 180.000 0.000 0.000
-6388.140 0.000 0.000 180.000 0.000 10.000
-6368.140 0.000 0.000 180.000 0.000 -10.000
0.000 -6378.140 0.000 270.000 0.000 0.000
0.000 6378.140 0.000 90.000 0.000 0.000
0.000 0.000 6356.750 0.000 90.000 0.000
0.000 0.000 -6356.750 0.000 -90.000 0.000
0.000 0.000 0.000 0.000 90.000 -6356.750
RestrictionsNone. Literature_ReferencesNone. Author_and_InstitutionC.H. Acton (JPL) N.J. Bachman (JPL) J. Diaz del Rio (ODC Space) H.A. Neilan (JPL) B.V. Semenov (JPL) W.L. Taber (JPL) E.D. Wright (JPL) Version
-CSPICE Version 1.0.2, 10-AUG-2021 (JDR)
Edited header to comply with NAIF standard.
-CSPICE Version 1.0.1, 23-JAN-2008 (EDW)
Corrected typo in LAT range description, from:
-pi/2 < LAT < pi
- -
to:
-pi/2 < LAT < pi/2
- -
-CSPICE Version 1.0.0, 26-DEC-2004 (CHA) (NJB) (HAN) (BVS) (WLT)
Index_Entriesconvert rectangular to planetographic coordinates |
Fri Dec 31 18:41:11 2021