| srfnrm_c |
|
Table of contents
Procedure
srfnrm_c ( Map surface points to outward normal vectors )
void srfnrm_c ( ConstSpiceChar * method,
ConstSpiceChar * target,
SpiceDouble et,
ConstSpiceChar * fixref,
SpiceInt npts,
ConstSpiceDouble srfpts[][3],
SpiceDouble normls[][3] )
AbstractMap array of surface points on a specified target body to the corresponding unit length outward surface normal vectors. The surface of the target body may be represented by a triaxial ellipsoid or by topographic data provided by DSK files. Required_ReadingDSK FRAMES PCK SPK TIME KeywordsCOORDINATES DSK GEOMETRY SURFACE Brief_I/O
VARIABLE I/O DESCRIPTION
-------- --- --------------------------------------------------
method I Computation method.
target I Name of target body.
et I Epoch in TDB seconds past J2000 TDB.
fixref I Body-fixed, body-centered target body frame.
npts I Number of surface points in input array.
srfpts I Array of surface points.
normls O Array of outward, unit length normal vectors.
SPICE_DSK_PTMEMM
P Default point-surface membership margin.
Detailed_Input
method is a short string providing parameters defining
the computation method to be used. In the syntax
descriptions below, items delimited by brackets
are optional.
`method' may be assigned the following values:
"ELLIPSOID"
The normal vector computation uses a triaxial
ellipsoid to model the surface of the target
body. The ellipsoid's radii must be available
in the kernel pool.
"DSK/UNPRIORITIZED[/SURFACES = <surface list>]"
The normal vector computation uses topographic
data to model the surface of the target body.
These data must be provided by loaded DSK
files.
The surface list specification is optional. The
syntax of the list is
<surface 1> [, <surface 2>...]
If present, it indicates that data only for the
listed surfaces are to be used; however, data
need not be available for all surfaces in the
list. If absent, loaded DSK data for any surface
associated with the target body are used.
The surface list may contain surface names or
surface ID codes. Names containing blanks must
be delimited by double quotes, for example
SURFACES = \"Mars MEGDR 128 PIXEL/DEG\"
If multiple surfaces are specified, their names
or IDs must be separated by commas.
See the -Particulars section below for details
concerning use of DSK data.
Neither case nor white space are significant in
`method', except within double-quoted strings. For
example, the string " eLLipsoid " is valid.
Within double-quoted strings, blank characters are
significant, but multiple consecutive blanks are
considered equivalent to a single blank. Case is
not significant. So
"Mars MEGDR 128 PIXEL/DEG"
is equivalent to
" mars megdr 128 pixel/deg "
but not to
"MARS MEGDR128PIXEL/DEG"
target is the name of the target body. `target' is
case-insensitive, and leading and trailing blanks in
`target' are not significant. Optionally, you may
supply a string containing the integer ID code for
the object. For example both "MOON" and "301" are
legitimate strings that indicate the Moon is the
target body.
When the target body's surface is represented by a
tri-axial ellipsoid, this routine assumes that a
kernel variable representing the ellipsoid's radii is
present in the kernel pool. Normally the kernel
variable would be defined by loading a PCK file.
et is the epoch for which target surface data will be
selected, if the surface is modeled using DSK data.
In this case, only segments having time coverage that
includes the epoch `et' will be used.
`et' is ignored if the target is modeled as an
ellipsoid.
`et' is expressed as TDB seconds past J2000 TDB.
fixref is the name of a body-fixed reference frame centered
on the target body. `fixref' may be any such frame
supported by the SPICE system, including built-in
frames (documented in the Frames Required Reading)
and frames defined by a loaded frame kernel (FK). The
string `fixref' is case-insensitive, and leading and
trailing blanks in `fixref' are not significant.
The input surface points in the array `srfpts' are
expressed relative to this reference frame, as are
the normal vectors computed by this routine.
npts is the number of surface points in the array `srfpts'.
srfpts is an array of target body surface points. Elements
srfpts[0][i]
srfpts[1][i]
srfpts[2][i]
are the Cartesian coordinates, expressed in the
reference frame designated by `fixref', of the ith
surface point in the array. Each surface point
represents an offset from the center of that frame.
All surface points must actually be "on" the surface,
that is, the distance of each point from the surface
must be less than a small margin. See the -Parameters
section below for a description of this margin.
Detailed_Output
normls is an array of unit length, outward normal vectors
corresponding to the points in `srfpts'. Elements
normls[0][i]
normls[1][i]
normls[2][i]
are the Cartesian coordinates, expressed in the
reference frame designated by `fixref', of the ith
normal vector in the array.
Parameters
SPICE_DSK_PTMEMM
is the default point-surface membership margin. This
margin limits the distance an input point can be from
a surface and still be considered to lie on that
surface.
The details of the application of
SPICE_DSK_PTMEMM
are implementation-dependent. In the DSK case, roughly
speaking, a point-surface distance limit within a DSK
segment is set to
SPICE_DSK_PTMEMM * MAXR
where MAXR is the radius of an outer bounding sphere
for the segment.
For shapes modeled as ellipsoids, the expression
above is applied to the maximum radius of the
ellipsoid.
See the header file
SpiceDtl.h
for the declaration of SPICE_DSK_PTMEMM. This margin
can be overridden. See this header file
and the routine dskstl_c for details.
Exceptions
1) If the target body name specified in the input string cannot
be converted to an integer ID code, the error
SPICE(IDCODENOTFOUND) is signaled by a routine in the call
tree of this routine.
2) If the input target body-fixed frame `fixref' is not recognized,
the error SPICE(NOFRAME) is signaled by a routine in the call
tree of this routine. A frame name may fail to be recognized
because a required frame specification kernel has not been
loaded; another cause is a misspelling of the frame name.
3) If the input frame `fixref' is not centered at the target body,
the error SPICE(INVALIDFRAME) is signaled by a routine in the
call tree of this routine.
4) If data are not available to convert between the frame
`fixref' and the frame of a DSK segment of interest, an error
is signaled by a routine in the call tree of this
routine.
5) If the input argument `method' cannot be parsed, an error
is signaled by either this routine or a routine in
the call tree of this routine.
6) If the computation method specifies an ellipsoidal target
model, and if triaxial radii of the target body have not been
loaded into the kernel pool prior to calling srfnrm_c, an error
is signaled by a routine in the call tree of this routine.
7) If the computation method specifies an ellipsoidal target
model, and if any of the radii of the target body are
non-positive, an error is signaled by a routine in the call
tree of this routine. The target must be an extended body.
8) If `method' specifies that the target surface is represented by
DSK data, and no DSK files are loaded for the specified
target, an error is signaled by a routine in the call tree
of this routine.
9) If `method' specifies that the target surface is represented by
DSK data, and data representing the portion of the surface
corresponding to the surface points provided in `srfpts' are
not available, an error is signaled by a routine in the
call tree of this routine.
10) If an input surface point is not within a small tolerance of
the specified surface, the error SPICE(POINTNOTONSURFACE) is
signaled by a routine in the call tree of this routine. See
the -Parameters section for details.
11) If the radii are not available in the kernel pool, an error is
signaled by a routine in the call tree of this routine.
12) If the target shape is "ellipsoid" and not all radii of the
ellipsoid are strictly positive, the error
SPICE(BADAXISLENGTH) is signaled by a routine in the call tree
of this routine.
13) If any of the `method', `target' or `fixref' input string
pointers is null, the error SPICE(NULLPOINTER) is signaled.
14) If any of the `method', `target' or `fixref' input strings has
zero length, the error SPICE(EMPTYSTRING) is signaled.
Files
Appropriate kernels must be loaded by the calling program before
this routine is called.
The following data are required:
- Shape data for the target body:
PCK data:
If the target shape is modeled as an ellipsoid,
triaxial radii for the target body must be loaded into
the kernel pool. Typically this is done by loading a
text PCK file via furnsh_c.
DSK data:
If the target shape is modeled by DSK data, DSK files
containing topographic data for the target body must be
loaded. If a surface list is specified, data for at
least one of the listed surfaces must be loaded.
- Target body orientation data: these may be provided in a
text or binary PCK file. In some cases, target body
orientation may be provided by one more more CK files. In
either case, data are made available by loading the files
via furnsh_c.
The following data may be required:
- Frame data: if a frame definition is required to convert
between the body-fixed frame of the target and the frame of
a DSK segment providing topographic data, that definition
must be available in the kernel pool. Typically the
definition is supplied by loading a frame kernel via furnsh_c.
- Surface name-ID associations: if surface names are specified
in `method', the association of these names with their
corresponding surface ID codes must be established by
assignments of the kernel variables
NAIF_SURFACE_NAME
NAIF_SURFACE_CODE
NAIF_SURFACE_BODY
Normally these associations are made by loading a text
kernel containing the necessary assignments. An example of
such a set of assignments is
NAIF_SURFACE_NAME += 'Mars MEGDR 128 PIXEL/DEG'
NAIF_SURFACE_CODE += 1
NAIF_SURFACE_BODY += 499
- SCLK data: if the target body's orientation is provided by
CK files, an associated SCLK kernel must be loaded.
In all cases, kernel data are normally loaded once per program
run, NOT every time this routine is called.
Particulars
Using DSK data
==============
DSK loading and unloading
-------------------------
DSK files providing data used by this routine are loaded by
calling furnsh_c and can be unloaded by calling unload_c or
KCLEAR. See the documentation of furnsh_c for limits on numbers
of loaded DSK files.
For run-time efficiency, it's desirable to avoid frequent
loading and unloading of DSK files. When there is a reason to
use multiple versions of data for a given target body---for
example, if topographic data at varying resolutions are to be
used---the surface list can be used to select DSK data to be
used for a given computation. It is not necessary to unload
the data that are not to be used. This recommendation presumes
that DSKs containing different versions of surface data for a
given body have different surface ID codes.
DSK data priority
-----------------
A DSK coverage overlap occurs when two segments in loaded DSK
files cover part or all of the same domain---for example, a
given longitude-latitude rectangle---and when the time
intervals of the segments overlap as well.
When DSK data selection is prioritized, in case of a coverage
overlap, if the two competing segments are in different DSK
files, the segment in the DSK file loaded last takes
precedence. If the two segments are in the same file, the
segment located closer to the end of the file takes
precedence.
When DSK data selection is unprioritized, data from competing
segments are combined. For example, if two competing segments
both represent a surface as sets of triangular plates, the
union of those sets of plates is considered to represent the
surface.
Currently only unprioritized data selection is supported.
Because prioritized data selection may be the default behavior
in a later version of the routine, the UNPRIORITIZED keyword is
required in the `method' argument.
Syntax of the `method' input argument
-------------------------------------
The keywords and surface list in the `method' argument
are called "clauses." The clauses may appear in any
order, for example
DSK/<surface list>/UNPRIORITIZED
DSK/UNPRIORITIZED/<surface list>
UNPRIORITIZED/<surface list>/DSK
The simplest form of the `method' argument specifying use of
DSK data is one that lacks a surface list, for example:
"DSK/UNPRIORITIZED"
For applications in which all loaded DSK data for the target
body are for a single surface, and there are no competing
segments, the above string suffices. This is expected to be
the usual case.
When, for the specified target body, there are loaded DSK
files providing data for multiple surfaces for that body, the
surfaces to be used by this routine for a given call must be
specified in a surface list, unless data from all of the
surfaces are to be used together.
The surface list consists of the string
SURFACES =
followed by a comma-separated list of one or more surface
identifiers. The identifiers may be names or integer codes in
string format. For example, suppose we have the surface
names and corresponding ID codes shown below:
Surface Name ID code
------------ -------
"Mars MEGDR 128 PIXEL/DEG" 1
"Mars MEGDR 64 PIXEL/DEG" 2
"Mars_MRO_HIRISE" 3
If data for all of the above surfaces are loaded, then
data for surface 1 can be specified by either
"SURFACES = 1"
or
"SURFACES = \"Mars MEGDR 128 PIXEL/DEG\""
Double quotes are used to delimit the surface name because
it contains blank characters.
To use data for surfaces 2 and 3 together, any
of the following surface lists could be used:
"SURFACES = 2, 3"
"SURFACES = \"Mars MEGDR 64 PIXEL/DEG\", 3"
"SURFACES = 2, Mars_MRO_HIRISE"
"SURFACES = \"Mars MEGDR 64 PIXEL/DEG\", Mars_MRO_HIRISE"
An example of a `method' argument that could be constructed
using one of the surface lists above is
"DSK/UNPRIORITIZED/SURFACES = \"Mars MEGDR 64 PIXEL/DEG\", 3"
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) Compute outward normal vectors at surface points on a target
body, where the points correspond to a given planetocentric
longitude/latitude grid. Use both ellipsoid and DSK shape
models.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File: srfnrm_ex1.tm
This meta-kernel is intended to support operation of SPICE
example programs. The kernels shown here should not be
assumed to contain adequate or correct versions of data
required by SPICE-based user applications.
In order for an application to use this meta-kernel, the
kernels referenced here must be present in the user's
current working directory.
The names and contents of the kernels referenced
by this meta-kernel are as follows:
File name Contents
--------- --------
pck00010.tpc Planet orientation and
radii
phobos512.bds DSK based on
Gaskell ICQ Q=512
plate model
\begindata
KERNELS_TO_LOAD = ( 'pck00010.tpc',
'phobos512.bds' )
\begintext
End of meta-kernel
Example code begins here.
/.
Program srfnrm_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
/.
Local constants
./
#define MAXN 10000
#define META "srfnrm_ex1.tm"
/.
Local variables
./
SpiceChar * fixref;
SpiceChar * method [2];
SpiceChar * target;
SpiceDouble dlat;
SpiceDouble dlon;
SpiceDouble et;
static SpiceDouble grid [MAXN][2];
SpiceDouble lat;
SpiceDouble lat0;
SpiceDouble lon;
SpiceDouble lon0;
static SpiceDouble normls [2][MAXN][3];
SpiceDouble nrmlat;
SpiceDouble nrmlon;
SpiceDouble nrmrad;
static SpiceDouble srfpts [2][MAXN][3];
SpiceDouble xlat;
SpiceDouble xlon;
SpiceDouble xr;
SpiceInt i;
SpiceInt j;
SpiceInt n;
SpiceInt nlat;
SpiceInt nlon;
/.
Set target, reference frame, and epoch.
./
target = "phobos";
fixref = "iau_phobos";
et = 0.0;
/.
Use both a reference ellipsoid and DSK data
to represent the surface.
./
method[0] = "ELLIPSOID";
method[1] = "DSK/UNPRIORITIZED";
/.
Set the grid dimensions.
./
nlon = 3;
nlat = 2;
/.
Derive evenly spaced grid separations and starting
values in the longitude and latitude dimensions.
Units are degrees.
./
lat0 = 90.0;
lon0 = 0.0;
dlat = 180.0 / (nlat + 1);
dlon = 120.0 / (nlon + 1);
/.
Load the meta-kernel.
./
furnsh_c ( META );
/.
Now generate the grid points. We generate
points along latitude bands, working from
north to south. The latitude range is selected
to range from +45 to -45 degrees. Longitude
ranges from 0 to 120 degrees. The increment
is 90 degrees for latitude and 60 degrees for
longitude.
./
n = 0;
for ( i = 0; i < nlat; i++ )
{
lat = rpd_c() * ( lat0 - (i+1)*dlat );
for ( j = 0; j < nlon; j++ )
{
lon = rpd_c() * ( lon0 + j*dlon );
grid[n][0] = lon;
grid[n][1] = lat;
++n;
}
}
/.
Find the surface points corresponding to the grid points.
Compute outward normal vectors at the surface points,
using both surface representations.
./
for ( i = 0; i < 2; i++ )
{
latsrf_c ( method[i], target, et,
fixref, n, grid, srfpts[i] );
srfnrm_c ( method[i], target, et,
fixref, n, srfpts[i], normls[i] );
}
/.
Print out the surface points in latitudinal
coordinates and compare the derived lon/lat values
to those of the input grid.
./
printf ( "\n" );
for ( i = 0; i < n; i++ )
{
/.
Use recrad_c rather than reclat_c to produce
non-negative longitudes.
./
recrad_c ( srfpts[0][i], &xr, &xlon, &xlat );
printf ( "\n"
"Surface point for grid point %d:\n"
" Latitudinal Coordinates:\n"
" Longitude (deg): %12.6f\n"
" Latitude (deg): %12.6f\n"
" Ellipsoid Radius (km): %12.6f\n",
(int)i,
xlon*dpr_c(), xlat*dpr_c(), xr );
recrad_c ( srfpts[1][i], &xr, &xlon, &xlat );
printf ( " DSK Radius (km): %12.6f\n",
xr );
recrad_c ( normls[0][i], &nrmrad, &nrmlon, &nrmlat );
printf ( " Ellipsoid normal vector direction:\n"
" Longitude (deg): %12.6f\n"
" Latitude (deg): %12.6f\n",
nrmlon * dpr_c(),
nrmlat * dpr_c() );
recrad_c ( normls[1][i], &nrmrad, &nrmlon, &nrmlat );
printf ( " DSK normal vector direction:\n"
" Longitude (deg): %12.6f\n"
" Latitude (deg): %12.6f\n",
nrmlon * dpr_c(),
nrmlat * dpr_c() );
}
printf ( "\n" );
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Surface point for grid point 0:
Latitudinal Coordinates:
Longitude (deg): 0.000000
Latitude (deg): 30.000000
Ellipsoid Radius (km): 11.580383
DSK Radius (km): 11.051271
Ellipsoid normal vector direction:
Longitude (deg): 0.000000
Latitude (deg): 49.678570
DSK normal vector direction:
Longitude (deg): 341.192386
Latitude (deg): 47.321056
Surface point for grid point 1:
Latitudinal Coordinates:
Longitude (deg): 30.000000
Latitude (deg): 30.000000
Ellipsoid Radius (km): 11.329953
DSK Radius (km): 11.148352
Ellipsoid normal vector direction:
Longitude (deg): 36.898722
Latitude (deg): 47.413981
DSK normal vector direction:
Longitude (deg): 9.491613
Latitude (deg): 45.091562
Surface point for grid point 2:
Latitudinal Coordinates:
Longitude (deg): 60.000000
Latitude (deg): 30.000000
Ellipsoid Radius (km): 10.874268
DSK Radius (km): 11.161069
Ellipsoid normal vector direction:
Longitude (deg): 66.059787
Latitude (deg): 43.718565
DSK normal vector direction:
Longitude (deg): 34.240727
Latitude (deg): 54.938460
Surface point for grid point 3:
Latitudinal Coordinates:
Longitude (deg): 0.000000
Latitude (deg): -30.000000
Ellipsoid Radius (km): 11.580383
DSK Radius (km): 12.654808
Ellipsoid normal vector direction:
Longitude (deg): 0.000000
Latitude (deg): -49.678570
DSK normal vector direction:
Longitude (deg): 3.751991
Latitude (deg): -46.672667
Surface point for grid point 4:
Latitudinal Coordinates:
Longitude (deg): 30.000000
Latitude (deg): -30.000000
Ellipsoid Radius (km): 11.329953
DSK Radius (km): 11.428638
Ellipsoid normal vector direction:
Longitude (deg): 36.898722
Latitude (deg): -47.413981
DSK normal vector direction:
Longitude (deg): 38.665944
Latitude (deg): -50.347745
Surface point for grid point 5:
Latitudinal Coordinates:
Longitude (deg): 60.000000
Latitude (deg): -30.000000
Ellipsoid Radius (km): 10.874268
DSK Radius (km): 10.410058
Ellipsoid normal vector direction:
Longitude (deg): 66.059787
Latitude (deg): -43.718565
DSK normal vector direction:
Longitude (deg): 69.560790
Latitude (deg): -44.917308
RestrictionsNone. Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) Version
-CSPICE Version 1.0.1, 10-AUG-2021 (JDR)
Edited the header to comply with NAIF standard. Reduced
the number of grid points to compute in code example.
Corrected the parameter name.
Added entries #11 and #12 in -Exceptions section.
-CSPICE Version 1.0.0, 20-MAR-2016 (NJB)
Index_Entriesmap Cartesian surface points to normal vectors compute normal vectors on topographic surface compute normal vectors on DSK surface |
Fri Dec 31 18:41:13 2021