This is the definition adopted by the PDS Small Bodies Node for the           
format that should be used for shape models of small bodies.  The model       
of the nucleus of comet Hartley 2 follows this format.                        
                                                                              
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.)             
                                                                              
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 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.