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nearpt_c
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Procedure
Abstract
Required_Reading
Keywords
Brief_I/O
Detailed_Input
Detailed_Output
Parameters
Exceptions
Files
Particulars
Examples
Restrictions
Literature_References
Author_and_Institution
Version
Index_Entries

Procedure

   void nearpt_c ( ConstSpiceDouble    positn[3],
                   SpiceDouble         a,
                   SpiceDouble         b,
                   SpiceDouble         c,
                   SpiceDouble         npoint[3],
                   SpiceDouble       * alt        )

Abstract

   This routine locates the point on the surface of an ellipsoid
   that is nearest to a specified position. It also returns the
   altitude of the position above the ellipsoid.

Required_Reading

   None.

Keywords

   ELLIPSOID,  GEOMETRY


Brief_I/O

   VARIABLE  I/O  DESCRIPTION
   --------  ---  --------------------------------------------------
   positn     I   Position of a point in bodyfixed frame.
   a          I   Length of semi-axis parallel to x-axis.
   b          I   Length of semi-axis parallel to y-axis.
   c          I   Length on semi-axis parallel to z-axis.
   npoint     O   Point on the ellipsoid closest to positn.
   alt        O   Altitude of positn above the ellipsoid.

Detailed_Input

   positn     3-vector giving the position of a point with respect to
              the center of an ellipsoid. The vector is expressed in a
              body-fixed reference frame. The semi-axes of the
              ellipsoid are aligned with the x, y, and z-axes of the
              body-fixed frame.
 
   a          is the length of the semi-axis of the ellipsoid that is
              parallel to the x-axis of the bodyfixed coordinate
              system.
 
   b          is the length of the semi-axis of the ellipsoid that is
              parallel to the y-axis of the bodyfixed coordinate
              system.
 
   c          is the length of the semi-axis of the ellipsoid that is
              parallel to the z-axis of the bodyfixed coordinate
              system.

Detailed_Output

   npoint     is the nearest point on the ellipsoid to `positn'.
              `npoint' is a 3-vector expressed in the body-fixed
              reference frame.
 
   alt        is the altitude of `positn' above the ellipsoid. If
              `positn' is inside the ellipsoid, `alt' will be negative
              and have magnitude equal to the distance between `npoint'
              and `positn'.

Parameters

   None.

Exceptions

   1) If any of the inputs a, b or c are non-positive the error
      "SPICE(BADAXISLENGTH)" will be signaled.

   2) If the ratio of the longest to the shortest ellipsoid axis
      is large enough so that arithmetic expressions involving its
      squared value may overflow, the error SPICE(BADAXISLENGTH)
      will be signaled.

   3) If any of the expressions

         a * abs( positn[0] ) / (m*m)
         b * abs( positn[1] ) / (m*m)
         c * abs( positn[1] ) / (m*m)

      where m is the minimum of { a, b, c }, is large enough so
      that arithmetic expressions involving these sub-expressions
      may overflow, the error SPICE(INPUTSTOOLARGE) is signaled.

   4) If the axes of the ellipsoid have radically different
      magnitudes, for example if the ratios of the axis lengths vary
      by 10 orders of magnitude, the results may have poor
      precision. No error checks are done to identify this problem.

   5) If the axes of the ellipsoid and the input point `positn' have
      radically different magnitudes, for example if the ratio of
      the magnitude of `positn' to the length of the shortest axis is
      1.e25, the results may have poor precision. No error checks
      are done to identify this problem.
  

Files

   None.

Particulars

   Many applications of this routine are more easily performed
   using the higher-level CSPICE routine subpt_c. 

Examples

   Example 1.

   The code fragment below illustrates how you can use CSPICE to
   compute the sub-earth point on the moon.

      /.
      Load the ephemeris, leapseconds and physical constants files
      first.  We assume the names of these files are stored in the
      character variables SPK, LSK and PCK.
      ./
      furnsh_c ( SPK );
      furnsh_c ( LSK );
      furnsh_c ( PCK );

      /.
      Get the apparent position of the Moon as seen from Earth.
      Look up this position vector in the moon body-fixed frame 
      IAU_MOON.  The orientation of the IAU_MOON frame will be 
      computed at epoch et-lt.
      ./
      spkpos_c ( "moon", et, "IAU_MOON", "lt+s", "earth, trgpos, &lt );

      /.
      Negate the moon's apparent position to obtain the 
      position of the earth in the moon's body-fixed frame.
      ./
      vminus_c ( trgpos, evec );

      /.
      Get the lengths of the principal axes of the moon. Transfer the
      elements of the array radii to the variables a, b, c to enhance
      readability.
      ./
      bodvcd_c (  399,    "RADII",  3,   &dim,  radii );
      vupack_c (  radii,  &a,       &b,  &c    );

      /.
      Finally get the point `subpnt' on the surface of the
      moon closest to the earth --- the sub-earth point.
      ./
      nearpt_c ( evec, a, b, c, subpnt, &alt );


   Example 2.

      One can use this routine to define a generalization of GEODETIC
      coordinates called GAUSSIAN coordinates of a triaxial body.  (The
      name is derived from the famous Gauss-map of classical
      differential geometry).  The coordinates are longitude, latitude,
      and altitude.
 
      We let the x-axis of the body fixed coordinate system point along
      the longest axis of the triaxial body.  The y-axis points along
      the middle axis and the z-axis points along the shortest axis.
 
      Given a point P, there is a point on the ellipsoid that is
      closest to P, call it Q.  The latitude and longitude of P is
      determined by constructing the outward pointing unit normal to
      the ellipsoid at Q.  The latitude of P is the latitude that the
      normal points towards in the bodyfixed frame. The longitude of P
      is the longitude the normal points to in the bodyfixed frame. The
      altitude is the signed distance from P to Q, positive if P is
      outside the ellipsoid, negative if P is inside. (the mapping of
      the point Q to the unit normal at Q is the Gauss-map of Q).
 
      To obtain the Gaussian coordinates of a point whose position in
      bodyfixed rectangular coordinates is given by a vector P, the
      code fragment below will suffice.
 
         nearpt_c ( p,    a,  b,     c,   q,  &alt  );
         surfnm_c (       a,  b,     c    q,  nrml  );
         reclat_c ( nrml, &r, &long, &lat           );

      The Gaussian coordinates are long, lat, alt.

Restrictions

   See the Exceptions header section above.

Literature_References

   None.

Author_and_Institution

   C.H. Acton      (JPL)
   W.L. Taber      (JPL)
   E.D. Wright     (JPL)

Version

   -CSPICE Version 1.3.2, 17-NOV-2005 (NJB) (EDW)

      The Exceptions and Restrictions header sections were updated.
      A reference to bodvar_c in the header was changed to a 
      reference to bodvcd_c.

   -CSPICE Version 1.3.1, 28-JUL-2003 (NJB) (CHA)

      Various header corrections were made.

   -CSPICE Version 1.3.0, 21-OCT-1998 (NJB)

      Made input vector const.

   -CSPICE Version 1.2.0, 15-FEB-1998 (EDW)

      Minor corrections to header.

   -CSPICE Version 1.2.0, 08-FEB-1998 (NJB)

      Removed local variables used for temporary capture of outputs.

   -CSPICE Version 1.0.0, 25-OCT-1997 (NJB)

      Based on  SPICELIB Version 1.1.0, 27-NOV-1990 (WLT)

Index_Entries

   distance from point to ellipsoid
   nearest point on an ellipsoid
Wed Apr  5 17:54:39 2017