recpgr_c |

Table of contents## Procedurerecpgr_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_Inputbody 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_Outputlon 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. ## Exceptions1) 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. ## FilesThis 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. ## ParticularsGiven 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 ## ExamplesThe 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; ## 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