| DATA_SET_DESCRIPTION |
Data Set Overview
=================
The High Resolution Imaging Science Experiment (HiRISE) is one of
the remote sensing instruments on the Mars Reconnaissance Orbiter
(MRO) spacecraft that acquires orbital observations of the Martian
surface during a two earth-year primary mapping phase. MRO,
successfully launched in August 2005, arrived at Mars in March
2006. Following orbit insertion the spacecraft went into an
aerobraking period to achieve a 250 x 315 kilometer near-polar
orbit suitable for the Primary Science Phase (PSP) mapping that
started in November 2006. Since the start of PSP HiRISE has been
continuously operating acquiring 10-20 observations per day.
The HiRISE team is responsible for maintaining an updated dataset
of the best version of its science data until meaningful
changes in data calibration no longer occur and to release data in
an appropriate manner for public access including their final
deposition to NASA's Planetary Data System (PDS). In carrying out
these responsibilities, the HiRISE team creates two types of
standard data products: 1) Experiment Data Record (EDR) products
and 2) Reduced Data Record (RDR) Products. An additional, derived product
is produced from HiRISE stereo image pairs: 3) Digital Terrain Model (DTM)
Products. This document describes the DTM standard products.
The Digital Terrain Model (DTM) product is an image file that represent
elevations for an area imaged in stereo by HiRISE. The source images are
orthorectified using the DTM to produce orthoimages. Both the DTM and the
orthoimages are resampled to a standard map projection They are formatted
and organized according to the standards of the PDS. The DTM is stored as
a standard PDS IMAGE object with an attached label. The orthoimages are
stored in the JPEG2000 format accepted by the PDS. The JPEG2000 images are
accompanied by a PDS detached label providing supporting information about
the observation.
The HiRISE DTM products comply with the PDS standards for file formats and
labels, specifically using the PDS image object definition. The DTM image
object uses 'IMG' as its filename extension. The PDS label is embedded
within the DTM image object. The DTM orthoimage files, formatted according
to the JPEG2000 standard, use 'JP2' as their filename extension. They are
accompanied by PDS labels; files that have the same name as the image data
file but use 'LBL' for their filename extension. The label files provides
image data characterization and science metadata information about the
observation. Additionally, the ancillary data files that accompany the DTM
products and the archive volume structure are in conformance with PDS
standards.
DTM orthoimage data are stored in the JPEG2000 ISO/IEC Part 1 standard
format (http://www.jpeg.org/jpeg2000/), which was accepted by the
PDS standard in October 2005. The JPEG2000 standard offers
benefits distinctly advantageous for storage and access to very
large images. With HiRISE RDR products reaching sizes exceeding
30,000 x 70,000 pixels the use of JPEG2000 was recognized as a
suitable solution for the storage and distribution of these data
products. Advantages include excellent compression performance,
multiple resolution levels from a single image data set,
progressive decompression quality layers, lossless and lossy
compression (HiRISE RDR products use lossless compression per the
PDS Standard), pixel datum precision up to 38 bits, multiple image
components (or bands), and selective image area access. These
features are achieved by the use of a sophisticated image coding
system based on discrete wavelet transforms (DWT) combined with
other coding techniques to generate a JPEG2000 codestream that can
be rendered to image pixel rasters using
inverse transform algorithms.
Processing
==========
The processing involved in a HiRISE DTM and associated orthoimages
is partially automated, but still involves a great deal of manual
input. For this reason, it cannot be pipelined as the other
HiRISE products are. HiRISE DTMs are being produced at several
institutions including HiROC, USGS Flagstaff, NASA Ames and JPL,
using the prodedures described here.
The image data that are the starting points for DTM production are the
radiometrically and geometrically calibrated individual RED CCDs. They
have gone through the normal HiRISE image processing pipeline up to the
HiStitch pipeline, as described in RDR_DS.CAT. Generally, the color CCDs
are not used for DTM production. The RED CCDs are reprojected into ideal
camera space using the ISIS3 (http://isis.astrogeology.usgs.gov/) program
noproj. After noproj the individual CCDs are mosaicked together and then
converted to 8-bit raw format. The 8-bit raw images are then imported into
the commercial software package Socet Set (copyright) by BAE, Inc. to
create the DTM. SPICE information for the RED5 CCD is gathered to provide
pointing information for the image to be imported into Socet Set
(copyright). The stereo pair is then triangulated and controlled to MOLA
elevation values, when possible. MOLA tracks are used as the reference
points for elevation and horizontal alignment. At equatorial latitudes,
sometimes the MOLA tracks do not completely span the HiRISE image
footprint. In polar regions, although the MOLA tracks are more densely
spaced, seasonal changes in snow/frost coverage create variability on the
scale of a few meters. In both of these cases the producer uses their best
judgement to tie the HiRISE DTM to the MOLA tracks. Terrain extraction is
performed in Socet Set (copyright) using their proprietary algorithms.
After the terrain model is produced, it is examined for obvious blunders
and artifacts. These are edited to the extent required by the project, but
within reasonable time constraints.
Socet Set (copyright) is used to create the orthoimages for each image in
the stereo pair. Orthoimages are produced at two resolutions. The first is
the full resolution of the original imagery (e.g. 0.25 m for bin1 images).
The second is produced at the same GSD as the DTM. A typical HiRISE DTM
produced from two bin1 images can have a GSD (or post spacing) of 1m.
During the project pre-processing, the stretch parameters to convert from
the native 16-bit to 8-bit format are preserved. These parameters are
documented in the label as SCALE and OFFSET should the end user wish to
restore the full dynamic range of the original image to the derived
orthoimage.
The DTM and orthoimages are exported from Socet Set (copyright) to the
ISIS3 cube format in order to populate the labels with correct scaling and
mapping values for subsequent PDS format conversion and label generation.
These products are map projected using the same mapping definitions as the
standard HiRISE RDR products.
Additionally DTM and orthoimage product browse, annotated browse,
and thumbnail jpeg images are created for HiRISE and PDS Imaging
Node web-based distribution services.
Data
====
Radiometrically and geometrically corrected RED band products are
the input source to DTM processing. Two HiRISE images are used,
comprising a stereo pair. Stereo imaging is accomplished by rolling
the spacecraft off-nadir. The first stereo image acquired typically is
a small off-nadir roll. The second stereo image is usually the larger
off-nadir roll. Elevations are derived from a priori knowledge of
the observation geometry and the degree of cross-track parallax
between the two stereo images. Stereo image targeting includes
consideration of shadow length and direction, incidence and phase
angle. The ideal stereo pair for DTM creation is acquired under
similar lighting conditions and with a stereo convergence angle
of 15 to 25 degrees.
Ancillary Data
==============
Generation of the DTM products relies on the MRO-project deliveries
for the Spacecraft Ephemeris (SPK kernels) and MRO spacecraft pointing
files (CK kernels)
Coordinate System & Cartographic Standards
==========================================
The HiRISE DTM products are compatible with the cartographic standards
and mapping conventions used by the HiRISE RDR and MRO CRISM map products.
When spatially registering map products produced by the two instrument
teams only a translation and scale change are required. The
coordinate system used is planetocentric latitude and east positive
longitude direction. The planetocentric latitude is the angle from the
equator to a point on the surface of an oblate planet. The longitude
increases from west to east (left to right).
The planetary constants used in the camera model to produce the HiRISE
RDR products are obtained from the NAIF SPICE planetary constants
kernel pck0008.tpc. The Mars constants of particular importance are
the right ascension and declination of the pole, the prime meridian,
rotation rate, and radii. The constants used are:
BODY499_POLE_RA = ( 317.68143 -0.1061 0. )
BODY499_POLE_DEC = ( 52.88650 -0.0609 0. )
BODY499_PM = ( 176.630 350.89198226 0. )
BODY499_RADII = ( 3396.19 3396.19 3376.20 )
Additionally, the SPICE kernel de405.bsp was used for the ephemeris
data for Mars.
Two map projections are used in the HiRISE DTM products:
Equirectangular and Polar Stereographic.
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