Table of contents
CSPICE_LATSRF maps an array of planetocentric longitude/latitude
coordinate pairs to surface points on a specified target body.
The surface of the target body may be represented by a triaxial
ellipsoid or by topographic data provided by DSK files.
Given:
method a short string providing parameters defining the computation
method to be used.
help, method
STRING = Scalar
In the syntax descriptions below, items delimited by brackets
are optional.
`method' may be assigned the following values:
'ELLIPSOID'
The surface point 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 surface point 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 the name of the target body.
help, target
STRING = Scalar
`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 the epoch for which target surface data will be selected, if the
surface is modeled using DSK data.
help, et
DOUBLE = Scalar
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 the name of a body-fixed reference frame centered on the target
body.
help, fixref
STRING = Scalar
If the target shape is given by DSK data, `fixref' may
designate 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).
When the target surface is modeled as an ellipsoid,
the reference frame designated by `fixref' (described
below) must have its coordinate axes aligned with the
respective principal axes of the reference ellipsoid.
The string `fixref' is case-insensitive, and leading
and trailing blanks in `fixref' are not significant.
The output surface points in the array `srfpts' will be
expressed relative to this reference frame.
lonlat an array of pairs of planetocentric longitudes and latitudes of
surface points.
help, lonlat
DOUBLE = Array[2,N]
Elements
lonlat[0,i]
lonlat[1,i]
are, respectively, the planetocentric longitude and
latitude of the ith surface point.
The units of longitude and latitude are radians.
the call:
cspice_latsrf, method, target, et, fixref, lonlat, srfpts
returns:
srfpts an array of target body surface points corresponding to the
pairs of coordinates in the input `lonlat' array.
help, srfpts
DOUBLE = Array[3,N]
Elements
srfpts[0,i]
srfpts[1,i]
srfpts[2,i]
are the Cartesian coordinates, expressed in the
reference frame designated by `fixref', of the surface
point corresponding to the ith pair of input
coordinates, where `i' ranges from 0 to n-1.
If there are multiple solutions for a given input
coordinate pair, this routine will return the point
at those coordinates having the greatest distance
from the origin of the coordinate system.
None.
Any numerical results shown for this example may differ between
platforms as the results depend on the SPICE kernels used as input
and the machine specific arithmetic implementation.
1) In the following example program, a DSK file containing a
type 2 segment is used to provide a plate model representation
of the surface of Phobos.
Find the surface points on a target body corresponding to a
given planetocentric longitude/latitude grid.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File: latsrf_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.
PRO latsrf_ex1
;;
;; Set target, reference frame, and epoch.
;;
target = 'phobos'
fixref = 'iau_phobos'
et = 0.D
;;
;; Use both a reference ellipsoid and DSK data
;; to represent the surface.
;;
method = [ 'ELLIPSOID', 'DSK/UNPRIORITIZED' ]
;;
;; Load the meta-kernel.
;;
cspice_furnsh, 'latsrf_ex1.tm'
;;
;; 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.
;;
lat = (dindgen(2)*90.d - 45d )*(-1.d)
lon = (dindgen(3)*60.d)
n = 0L
grid = dblarr(2, n_elements(lat) * n_elements(lon) )
for i = 0, n_elements(lat)-1 do begin
for j = 0, n_elements(lon)-1 do begin
grid[0,n] = lon[j]
grid[1,n] = lat[i]
n = n+1
end
end
grid = grid * cspice_rpd()
;;
;; Find the surface points corresponding to the grid points.
;;
;; Compute outward normal vectors at the surface points,
;; using both surface representations.
;;
for i = 0, 1 do begin
cspice_latsrf, method[i], target, et, fixref, grid, srfpts
for j = 0, n-1 do begin
;;
;; Use cspice_recrad rather than cspice_reclat to produce
;; non-negative longitudes.
;;
cspice_recrad, srfpts[*, j], xr, xlon, xlat
print, method[i]
print, 'Intercept for grid point ', j
print, ' Cartesian coordinates: ', srfpts[*,j]
print, ' Latitudinal Coordinates:'
print, ' Longitude (deg): ', xlon*cspice_dpr()
print, ' Latitude (deg): ', xlat*cspice_dpr()
print, ' Radius (km): ', xr
print
print, ' Original Grid Coordinates:'
print, ' Longitude (deg): ', grid[0,j]*cspice_dpr()
print, ' Latitude (deg): ', grid[1,j]*cspice_dpr()
print
endfor
endfor
;;
;; It's always good form to unload kernels after use,
;; particularly in IDL due to data persistence.
;;
cspice_kclear
END
When this program was executed on a Mac/Intel/IDL8.x/64-bit
platform, the output was:
ELLIPSOID
Intercept for grid point 0
Cartesian coordinates: 7.4550105 0.0000000
7.4550105
Latitudinal Coordinates:
Longitude (deg): 0.0000000
Latitude (deg): 45.000000
Radius (km): 10.542977
Original Grid Coordinates:
Longitude (deg): 0.0000000
Latitude (deg): 45.000000
ELLIPSOID
Intercept for grid point 1
Cartesian coordinates: 3.5966446 6.2295712
7.1932892
Latitudinal Coordinates:
Longitude (deg): 60.000000
Latitude (deg): 45.000000
Radius (km): 10.172847
Original Grid Coordinates:
Longitude (deg): 60.000000
Latitude (deg): 45.000000
ELLIPSOID
Intercept for grid point 2
Cartesian coordinates: -3.5966446 6.2295712
7.1932892
Latitudinal Coordinates:
Longitude (deg): 120.00000
Latitude (deg): 45.000000
Radius (km): 10.172847
Original Grid Coordinates:
Longitude (deg): 120.00000
Latitude (deg): 45.000000
ELLIPSOID
Intercept for grid point 3
Cartesian coordinates: 7.4550105 0.0000000
-7.4550105
Latitudinal Coordinates:
Longitude (deg): 0.0000000
Latitude (deg): -45.000000
Radius (km): 10.542977
Original Grid Coordinates:
Longitude (deg): 0.0000000
Latitude (deg): -45.000000
ELLIPSOID
Intercept for grid point 4
Cartesian coordinates: 3.5966446 6.2295712
-7.1932892
Latitudinal Coordinates:
Longitude (deg): 60.000000
Latitude (deg): -45.000000
Radius (km): 10.172847
Original Grid Coordinates:
Longitude (deg): 60.000000
Latitude (deg): -45.000000
ELLIPSOID
Intercept for grid point 5
Cartesian coordinates: -3.5966446 6.2295712
-7.1932892
Latitudinal Coordinates:
Longitude (deg): 120.00000
Latitude (deg): -45.000000
Radius (km): 10.172847
Original Grid Coordinates:
Longitude (deg): 120.00000
Latitude (deg): -45.000000
DSK/UNPRIORITIZED
Intercept for grid point 0
Cartesian coordinates: 7.1816610 0.0000000
7.1816610
Latitudinal Coordinates:
Longitude (deg): 0.0000000
Latitude (deg): 45.000000
Radius (km): 10.156402
Original Grid Coordinates:
Longitude (deg): 0.0000000
Latitude (deg): 45.000000
DSK/UNPRIORITIZED
Intercept for grid point 1
Cartesian coordinates: 3.5819949 6.2041972
7.1639898
Latitudinal Coordinates:
Longitude (deg): 60.000000
Latitude (deg): 45.000000
Radius (km): 10.131412
Original Grid Coordinates:
Longitude (deg): 60.000000
Latitude (deg): 45.000000
DSK/UNPRIORITIZED
Intercept for grid point 2
Cartesian coordinates: -3.6853577 6.3832268
7.3707154
Latitudinal Coordinates:
Longitude (deg): 120.00000
Latitude (deg): 45.000000
Radius (km): 10.423766
Original Grid Coordinates:
Longitude (deg): 120.00000
Latitude (deg): 45.000000
DSK/UNPRIORITIZED
Intercept for grid point 3
Cartesian coordinates: 8.0268854 0.0000000
-8.0268854
Latitudinal Coordinates:
Longitude (deg): 0.0000000
Latitude (deg): -45.000000
Radius (km): 11.351730
Original Grid Coordinates:
Longitude (deg): 0.0000000
Latitude (deg): -45.000000
DSK/UNPRIORITIZED
Intercept for grid point 4
Cartesian coordinates: 3.3335904 5.7739480
-6.6671808
Latitudinal Coordinates:
Longitude (deg): 60.000000
Latitude (deg): -45.000000
Radius (km): 9.4288175
Original Grid Coordinates:
Longitude (deg): 60.000000
Latitude (deg): -45.000000
DSK/UNPRIORITIZED
Intercept for grid point 5
Cartesian coordinates: -3.7985851 6.5793424
-7.5971702
Latitudinal Coordinates:
Longitude (deg): 120.00000
Latitude (deg): -45.000000
Radius (km): 10.744021
Original Grid Coordinates:
Longitude (deg): 120.00000
Latitude (deg): -45.000000
This routine is intended to be used for target body surfaces that
have a unique radius for each pair of planetocentric longitude
and latitude coordinates.
If the target surface is represented by topographic data, it is
possible for there to be multiple surface points at a given
planetocentric longitude and latitude. For example, this can
occur if the surface has features such as cliffs, caves, or
arches.
For more complex surfaces, the routine
DSKSXV {DSK, ray-surface intercept, vectorized}
may be more suitable. That routine works with rays having vertices
anywhere outside of the target body.
Planetocentric coordinates
==========================
Planetocentric longitude and latitude are defined as follows:
Longitude of a point P is the angle between the prime meridian
and the meridian containing P. The direction of increasing
longitude is from the +X axis towards the +Y axis.
Latitude of a point P is the angle from the XY plane of the
ray from the origin through the point.
Using DSK data
==============
DSK loading and unloading
-------------------------
DSK files providing data used by this routine are loaded by
calling cspice_furnsh and can be unloaded by calling cspice_unload or
cspice_kclear. See the documentation of cspice_furnsh 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'
1) If the target body name 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 cspice_latsrf, 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 coordinates provided in `lonlat' are not
available, an error is signaled by a routine in the call
tree of this routine.
10) If a surface point cannot be computed because the ray
corresponding to a longitude/latitude pair fails to intersect
the target surface as defined by the plate model, the error
SPICE(NOINTERCEPT) is signaled by a routine in the call tree
of this routine.
11) If the surface point corresponding to a longitude/latitude
pair in `lonlat' does not have matching longitude and latitude
(because it is on the opposite side of the origin), the error
SPICE(SHAPENOTSUPPORTED) is signaled by a routine in the call
tree of this routine.
12) If the radii are not available in the kernel pool, an error is
signaled by a routine in the call tree of this routine.
13) 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.
14) If any of the input arguments, `method', `target', `et',
`fixref' or `lonlat', is undefined, an error is signaled by
the IDL error handling system.
15) If any of the input arguments, `method', `target', `et',
`fixref' or `lonlat', is not of the expected type, or it does
not have the expected dimensions and size, an error is
signaled by the Icy interface.
16) If the output argument `srfpts' is not a named variable, an
error is signaled by the Icy interface.
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 cspice_furnsh.
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 cspice_furnsh.
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 cspice_furnsh.
- 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.
1) This routine assumes that the origin of the body-fixed
reference frame associated with the target body is located in
the interior of that body.
2) The results returned by this routine may not be meaningful
if the target surface has multiple surface points associated
with some (longitude, latitude) coordinates.
ICY.REQ
FRAMES.REQ
PCK.REQ
TIME.REQ
None.
J. Diaz del Rio (ODC Space)
M. Liukis (JPL)
E.D. Wright (JPL)
-Icy Version 1.0.1, 01-JUN-2021 (JDR)
Edited the header to comply with NAIF standard. Added example's
problem statement. Reduced the number of grid points to compute in code
example. Added -Parameters, -Exceptions, -Files, -Restrictions,
-Literature_References and -Author_and_Institution sections.
Removed reference to the routine's corresponding CSPICE header from
-Abstract section.
Added arguments' type and size information in the -I/O section.
-Icy Version 1.0.0, 15-DEC-2016 (ML) (EDW)
map latitudinal coordinates to Cartesian surface points
map latitudinal coordinates to DSK surface points
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