This is the general definition adopted by the PDS Small Bodies Node as the
format that should be used for shape models of small bodies.
Different users of (pre-SPICE) triangular plate shape models require
different pieces of information. (e.g. some want the number of vertices
and facets denoted explicitly, some want the connectivity of the facets
defined, and some only want the coordinates of the vertices.) The SBN
defined the formats described in this file to accomodate all users, with
the information included in the label providing the pointers to the
specific pieces of information. Thus users can use the label to access the
relevant data as needed for their individual software. This data format is
also easily converted to newer, commercial formats for use in 3D printers,
etc.
The format described below is a simple set of tables.
As a convenience to users, the SBN has also been including a VRML wrapper
around these tables to allow the user to easily view the model with publicly
available software. This wrapper does not affect the data tables themselves,
which can still be read using the information in the data label.
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General description of the model
Shape models are defined using a set of points distributed across
the surface of the body. These points, or vertices, are linked
together in threes to form triangular plates or facets that
approximate the local surface topography. Each vertex is
incorporated into as many plates as necessary to define the shape of
the body (or at least the parts of it that have been modeled). The
total number of vertices and plates that are used is dependent on
the spatial resolution that is available for defining the shape of
the body.
Coordinate systems
The vertex positions are defined in a body-centered system using either
cartesian coordinates or planetocentric spherical coordinates.
Spherical coordinate systems should follow the IAU definition for
small bodies. By this definition, the positive pole lies in the
direction of the rotational angular momentum vector, and positive
latitudes increase in that direction. Longitudes should be
measured from 0 to 360 degrees in a right-hand coordinate system
from a designated prime meridian.
Whenever possible, the cartesian coordinate system should maintain a
logical relationship to the rotational reference frame (e.g., +Z
corresponds to the positive spin axis, +X corresponds to the prime
meridian). If the rotational properties are not known, then a
description of the adopted coordinate system should be included to
allow for a transformation to a different system if the rotational
properties are derived in the future.
Model format
The format for the model is defined to be an ascii file that is
divided into three different, consecutive tables.
Table 1 consists of a single line with two columns:
Column 1 -- Total number of vertices that are included for defining
the surface (N_vert)
Column 2 -- Total number of plates that are used to represent the
body's surface (N_plate)
Table 2 consists of N_vert lines, each of which defines the
position of one vertex. Each line contains three or more
columns:
Column 1-3 -- Position of a vertex in the adopted coordinate
system.
Column 4-n -- (Optional) Additional information about the vertex.
(e.g., a flag denoting a point derived from
observational data vs. an unobserved point that has
been filled in by assuming a triaxial ellipsoid.)
If these columns are included in the data, they will be
described in the data label.
Table 3 consists of N_plate lines, each of which identifies three
vertices from Table 2. The three vertices are connected to form a
triangular plate on the body's surface. Each line contains three or
more columns:
Column 1-3 -- Vertex numbers denoting three vertices that are connected to
form a plate. Each number is the offset from the first line
from Table 2 (0 is the first vertex, 1 is the second, etc.).
The vertices should be listed in an order such that when
connected in the direction defined by the right hand rule,
the resulting surface normal points to the exterior of the
body.
Column 4-n -- (Optional) Additional information about the plate.
(e.g., a flag denoting that the plate contains
vertices derived from observational data vs. those
from an assumed triaxial ellipsoid. The column may also
contain "-1" which denotes the end of a triangle in the VRML
format.)
If these columns are included in the data, they will be
described in the data label.
If the entire surface has been modeled, the connectivity should be defined
such that when the plates are assembled, the body forms a closed surface. For
example, longitude 359 deg should connect back to the previously defined
point at 0 deg, rather than to a mirrored point at 360 deg, and all of the
plates that have a corner at a pole should connect to a single vertex at
latitude (+/-)90 deg.