Procedure

   convrt_c ( Convert Units ) 

   void convrt_c ( SpiceDouble       x,
                   ConstSpiceChar  * in,
                   ConstSpiceChar  * out,
                   SpiceDouble     * y    )

Abstract

   Take a measurement X, the units associated with
   X, and units to which X should be converted; return Y ---
   the value of the measurement in the output units.

Required_Reading

   None.

Keywords

   CONVERSION
   UNITS

Brief_I/O

   VARIABLE  I/O  DESCRIPTION
   --------  ---  -------------------------------------------------
   x          I   Number representing a measurement in some units.
   in         I   The units in which x is measured.
   out        I   Desired units for the measurement.
   y          O   The measurement in the desired units.

Detailed_Input

   x           is a number representing a measurement in the units
               specified by in.

   in          represents the units associated with a measurement x.
               Acceptable units are:

               Angles:                 "RADIANS"
                                       "DEGREES"
                                       "ARCMINUTES"
                                       "ARCSECONDS"
                                       "HOURANGLE"
                                       "MINUTEANGLE"
                                       "SECONDANGLE"

               Metric Distances:       "METERS"
                                       "M"
                                       "KILOMETERS"
                                       "KM"
                                       "CENTIMETERS"
                                       "CM"
                                       "MILLIMETERS"
                                       "MM"

               English Distances:      "FEET"
                                       "INCHES"
                                       "YARDS"
                                       "STATUTE_MILES"
                                       "NAUTICAL_MILES"

               Astrometric Distances:  "AU"
                                       "PARSECS"
                                       "LIGHTSECS"
                                       "LIGHTYEARS" julian lightyears

               Time:                   "SECONDS"
                                       "MINUTES"
                                       "HOURS"
                                       "DAYS"
                                       "JULIAN_YEARS"
                                       "TROPICAL_YEARS"
                                       "YEARS" (same as julian years)


               The case of the string in is not significant.


   out         represents the units desired for the measurement x.
               See the description of in.

               The case of the string out is not significant.

Detailed_Output

   y           is the input measurement converted to the desired units.

Parameters

   None.

Exceptions

   1)  If the input units, output units, or both input and output
       units are not recognized, the error SPICE(UNITSNOTREC) is
       signaled by a routine in the call tree of this routine.

   2)  If the units being converted between are incompatible, the
       error SPICE(INCOMPATIBLEUNITS) is signaled by a routine in the
       call tree of this routine.

   3)  If any of the `in' or `out' input string pointers is null, the
       error SPICE(NULLPOINTER) is signaled.

   4)  If any of the `in' or `out' input strings has zero length, the
       error SPICE(EMPTYSTRING) is signaled.

Files

   None.

Particulars

   This routine converts a measurement x given in units specified by
   in to the equivalent value y in units specified by out.

   If a unit is not recognized, an error message is produced that
   indicates which one was not recognized.

   If input and output units are incompatible (for example angle
   and distance units) and error message will be produced stating
   the requested units and associated types.

Examples

   The numerical results shown for this example 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) Convert 300 miles (statute miles) to kilometers and determine
      the number of lightyears in a parsec.

      Example code begins here.


      /.
         Program convrt_ex1
      ./
      #include <stdio.h>
      #include "SpiceUsr.h"

      int main( )
      {

         /.
         Local variables.
         ./
         SpiceDouble               distsm;
         SpiceDouble               onpars;
         SpiceDouble               result;

         /.
         Convert 300 miles (statute miles) to kilometers.
         ./
         distsm = 300.0;
         convrt_c ( distsm, "statute_miles", "km", &result );
         printf ( "300 miles in km  : %16.6f\n",  result   );

         /.
         Determine the number of lightyears in a parsec.
         ./
         onpars = 1.0;
         convrt_c ( onpars, "PARSECS", "LIGHTYEARS", &result );
         printf ( "Lightyears/parsec: %16.6f\n",  result     );

         return ( 0 );
      }


      When this program was executed on a Mac/Intel/cc/64-bit
      platform, the output was:


      300 miles in km  :       482.803200
      Lightyears/parsec:         3.261564

Restrictions

   1)  This routine does not do any checking for overflow. The caller
       is required to make sure that the units used for the
       measurement are such that no floating point overflow will
       occur when the conversion is performed.

   2)  Some of the units are not "defined" quantities. In such a case
       a best estimate is provided as of the date of the current
       version of this routine. Those estimated quantities are:

          AU               The astronomical unit. The value was taken
                           from the JPL ephemeris DE125. This value
                           is an approximation and should not be used
                           for high-accuracy work. It agrees with the
                           value used in the JPL planetary ephemeris
                           DE430 (149597870.700 km) at the 100m
                           level.

          TROPICAL_YEARS   The tropical year is the time from equinox
                           to equinox. This varies slightly with
                           time.

          PARSECS          The parsec is the distance to an object
                           whose parallax angle is one arcsecond. Its
                           value is dependent upon the value of the
                           astronomical unit.

Literature_References

   None.

Author_and_Institution

   N.J. Bachman        (JPL)
   C.A. Curzon         (JPL)
   J. Diaz del Rio     (ODC Space)
   H.A. Neilan         (JPL)
   W.M. Owen           (JPL)
   W.L. Taber          (JPL)
   I.M. Underwood      (JPL)

Version

   -CSPICE Version 2.0.1, 13-AUG-2021 (JDR)

       Edited the header to comply with NAIF standard. Added
       complete code example.

   -CSPICE Version 2.0.0, 13-MAY-2016 (NJB)

       Updated to support expanded names of metric distance
       units, as well as to support the abbreviation "M"
       indicating meters.

   -CSPICE Version 1.0.0, 17-MAY-1999 (NJB) (CAC) (HAN) (WMO) (WLT) (IMU)

Index_Entries

   convert units

Link to routine convrt_c source file convrt_c.c