DART Shape (Digital Terrain) Model Bundle

Data Abstract
=============
We provide digital terrain model models and ancillary derived data products of Didymos 
and Dimorphos. These data were produced with imaging collected during the DART impact
with Dimorphos. This collection consists of digital terrain models, surface information 
(elevation relative to a geiod, slope, surface geopotential, surface gravity) and tilt 
maps. These products are accompanied with a definition of these asteroid's coordinate 
system, volume, and dart impact location.

Data Set Overview
=================
The shape of Didymos and Dimorphos is obtained using stereophotoclinometry (SPC; Gaskell 
et al. 2008) SPC combines stereo techniques with photoclinometry to derive the tilt of 
an asteroid’s  surface. This shape modeling technique estimates surface tilts at each 
pixel of a given image by initially using stereo parallax, and a photometric function, 
to constrain a relationship between a surface’s tilt and observed albedo. With this 
relationship in hand, the tilts of a piece of asteroid surface imagined at multiple 
emission and incidence angles can be obtained via least-squares that best duplicates 
the input images. Once the surface tilts are obtained, the geometric height across 
each map can be determined by integrating over the tilts in a logical manner. These
individual terrain maps (called “maplets”) of the surface can then be joined together 
to produce global and local digital terrain models. SPC processing includes the use of 
asteroid limb and terminator data to initially constrain the shape of an asteroid. 

From this model, and estimates of the mass of Didymos and Dimorphos, surface information 
critical to interpret the surface geology and tilt characteristics can be obtained 
(see Barnouin, O.S. et al. 2020). We provide both global and local products. The local 
product is centered on the DART impact site. The coordinate and other attributes for 
both Didydmos and Dimorphos are captured in the "DART Coordinate System for Didymos and 
Dimorphos" document. Users are encouraged to read the DART Shape Model SIS 
for additional information.

The data products are named according to the following convention: 
Target + “_” + Coverage Type + “_” + Ground Sample Distance + “mm”
+ “_” + SDP Area + “_” + Description + “_” +  “Center Location” +
“_” + “v” + Version + “.” + PDS Type

The Coverage Type will be:
	g - for global
	l - for local (regional). Used for both global tileset and site specific products.
	
The Ground Sample Distance (resolution) is five digits expressed in millimeters plus the appended units indicator of mm. 

The SDP Area is one of:
	SPC - product generated using stereophotoclinometry 
	
The Description legend is as follows:
	obj - Global or local shape model.
	dtm - FITS file that contains a complete set of DTM planes in simple cylindrical 
	      projection.
	For the ancillary FITS files, the description refers to the values that are stored 
	      in the binary FITS table. A value is provided for each plate in the 
	      corresponding OBJ or DTM file.Table below provides the description of 
              the variables:
              
Data Type Identifiers:
Parameter		TTYPE DATA Column	Des. string
Relative Albedo		ALBEDO	 		alb	
Normal vector X		NORMAL VECTOR_X		nvf	
Normal vector Y		NORMAL VECTOR_Y		nvf 	
Normal vector Z		NORMAL VECTOR_Z		nvf	
Gravity vector X		GRAVITY VECTOR_X		grv	
Gravity vector Y		GRAVITY VECTOR_Y		grv	
Gravity vector Z		GRAVITY VECTOR_Z		grv	
Gravitational magnitude	GRAVITATIONAL_MAGNITUDE	grm	
Gravitational potential	GRAVITATIONAL_POTENTIAL	pot	
Elevation		ELEVATION		elv	
Slope			SLOPE			slp
Facet tilt		FACET TILT		fti
Facet tilt direction	FACET TILT DIRECTION	fdi
Mean tilt		MEAN TILT		mti
Mean tilt direction	MEAN TILT DIRECTION	mdi
Tilt variation		TILT VARIATION		tiv
Tilt direction variation	TILT DIRECTION VARIATION	div
Relative tilt		RELATIVE TILT		rti
Relative tilt direction	RELATIVE TILT DIRECTION	rdi
Max relative height	MAX RELATIVE HEIGHT	mht	max height/depth within an ellipse

The Target description provides the name of the asteroid for which the product is provided:
	Dimorphos
	Didymos
	
An example of the file naming convention is:
dimorphos_l_00320mm_spc_dtm_1000n21000_v251.fits

where 1000N represents latitude of 10.00N, 210.00 represents 210.00 East longitude 
and 00320mm is for 320-mm gsd. This product was generated using stereophotoclinometry 
for target asteroid Dimorphos. The DTM means it is a plane-relative DTM. ‘v251’ is 
the product version number without decimal points.

References
==========
Gaskell et al. 2008, Characterizing and navigating small bodies with imaging data. 
Meteoritics and Planetary Science 43, 1049–1061. https://doi.org/10.1111/j.1945-5100.2008.tb00692.x
Barnouin, O.S. et al. 2020 Digital terrain mapping by the OSIRIS-REx mission. Planetary and 
Space Science 180, 104764. https://doi.org/10.1016/j.pss.2019.104764
Barnouin, O.S. et al. 2021, DART Coordinate System for Didymos and Dimorphos, PDS document XXYY
Barnouin, O.S. et al. 2021, DART Shape Model Collection SIS, PDS document XXYY