Reproduced from Cartography and Geographic Information Systems,
Special Issue:  IMPLEMENTING THE SPATIAL DATA TRANSFER STANDARD,
American Congress on Surveying and Mapping,
Vol. 19, No. 5, Dec. 1992.
Developing a Raster Profile for the Spatial Data Transfer Standard
         David D. Greenlee
ABSTRACT.  The Spatial Data Transfer Standard (SDTS) was designed to
transfer  both vector and raster data sets.  In the early development
of the SDTS, the designers recognized that there was a need to
transfer raster data in addition to the more challenging vector data.
As a result, the SDTS includes a "raster module" that accommodates
a variety of raster data structures and formats.  A raster profile
is being developed that will exercise a selected subset of SDTS
capabilities in order to provide a simple-to-use transfer of complete
raster data sets.
KEYWORDS: raster, spatial data, standards
           Introduction
During the decade of Spatial Data Transfer Standard (SDTS)
developments, many advancements were made in hardware and software
that are used for spatial analysis (e.g. image processing and
geographic information systems).  Some systems can now efficiently
and directly process both raster and vector data using an integrated
data model.  This has placed new requirements on the raster transfer
process.  To facilitate this process, a raster profile for the SDTS
is being developed.
It is apparent that many users are spending excessive time and effort
dealing with the problems of geographic referencing and registration.
This situation is often aggravated by poorly documented data sets,
inconsistent terminology, and incomplete transfer of ancillary
georeferencing information.  As users of geographic information and
image processing systems attempt to process spatial data in both
raster and vector form, a simple but more complete raster transfer
process is needed.
             Objective
The objective of the raster profile is to provide an SDTS-prescribed
method for encoding and transferring raster data.  The U.S.
Geological Survey (USGS) requires such a profile to distribute data
such as Digital Elevation Models, Digital Orthophotos, and remotely
sensed images.
The design of the raster profile will take advantage of the existing
Topological Vector Profile (Fegeas, 1992), and will use it as a model
for all commonly defined SDTS modules  (e.g. data quality
terminology, data set lineage, and geographic referencing).
A goal of the raster profile is to integrate raster data sources with
SDTS data that are transferred in vector form.
USGS Requirements for a Raster Profile
The development of an SDTS raster profile will involve the
participation of Federal agencies, State and local governments, the
private sector, and academic institutions that have a vested interest
in implementing the SDTS for raster data transfers.  The actual
process to accomplish this activity will be similar to that for
developing the vector profile:  design of a prototype profile, test
and demonstration of sample data sets and software, refinement of the
profile, and FIPS approval.
At this point, however, a prototype raster profile has not yet been
defined.  Therefore, this paper will focus on known requirements for
a raster profile from the perspective of the USGS.
     Digital Elevation Models
The USGS has been distributing raster elevation data as Digital
Elevation Models (DEM) for more than 15 years (USGS, 1990).  DEM's
are typically referenced to the Universal Transverse Mercator
projection, or directly to latitude and longitude, and are made up
of a regularly spaced set of elevations.  These DEM's contain a
complete set of georeferencing information in the header records.
The information in the DEM header should be preserved in the raster
profile transfer process.
Provision should also be made for inclusion of additional information
that can be provided to assist in making appropriate use of the DEM
(e.g., accuracy as determined during validation procedures or an
indication of the source of the DEM (Gestalt Photo Mapper, scanned
hypsography, or digital profiles)).  This provision will require
additional research by the data provider, in this case the USGS.
As expressed in a DEM, elevation (referred to as altitude by the
SDTS) values will typically require a 16-bit (two byte) signed
integer binary data representation.
        Digital Orthophotos
The USGS is working cooperatively with the U. S. Department of
Agriculture to implement a national interagency digital orthophoto
program.  The product of this program is a digitized photo from the
National Aerial Photography Program digitized at 25 microns.  The
digital image will then be geographically registered to a standard
map projection to create a Digital Orthophoto Quadrangle (DOQ) (USGS,
1992).  The result will require an 8-bit unsigned byte (range 0-255)
to represent each monochromatic brightness value (pixel) that
corresponds to a 1-meter square on the ground.
As with DEM's, provision should be made to include the header
information that accompanies each DOQ.  The DOQ header carries
sufficient information to satisfy SDTS data quality, lineage and
georeferencing requirements.
For color images that can be scanned with filters, multiband data
sets will be required.  The SDTS allows for virtually any
organization, including the commonly used Band Interleaved by Line,
and Band SeQuential.  One or both of these band organization methods
will be chosen for the raster profile.
                      Remotely Sensed Images
Data transfers of remotely sensed data will require the capability
to transfer unsigned (8-bit) byte data.  This capability will also
accommodate most digitally enhanced data and classification results
(classes ranging from 0-255).  The raster profile will not provide
a mechanism for transferring images of complex numbers (e.g. Fourier
transforms), real valued data (e.g. some aeromagnetic data), or some
raw remotely sensed data (e.g. raw Advanced Very High Resolution
Radiometer data that require 10 bits).
Multispectral scanner data will require multiple bands.  Often these
data sets contain 4 to 7 bands, but some data sets can contain as
many as 224 bands (e.g. Advanced Visible and Infrared Imaging
Spectrometer).  Also, remotely sensed data are often transferred as
a geometrically registered time series, and this increases the number
of coregistered bands.  The raster profile will require a decision
on the maximum number of bands allowed in a single transfer.
To facilitate the registration of raster data with other data sources
(and profiles), a geographic referencing procedure will be required
for data transferred with the raster profile.  This may require
resampling of raw sensor data to a rigorously defined map projection
and the confinement of errors to subpixel accuracy.  Further
discussion and a foundation for geographic referencing requirements
can be found in the report of a NASA sponsored workshop held in 1977
(Simonett, 1978).
The raster profile will not be used for raw sensor (non-
georeferenced) data.  Because many existing image file formats do not
require these data, the raster profile will be limited to data sets
that have received this additional work.  The extra effort of the
data provider will be more than balanced by the reduced effort for
many users.
A major new source of remotely sensed data is expected to be encoded
using NASA Earth Observing Systems specifications that are currently
under development.  NASA probably will adopt the Hierarchical Data
Format as a transfer standard (Suresh, written commun., 1992).  The
raster profile could provide a future option for these data, but only
if they have been geographically registered.
            Summary of USGS Raster Profile Requirements
In the interest of file size and processing efficiency, the raster
profile will require binary encoding schemes that are more efficient
than character codes.  Software encoding and decoding routines will
need to be added to the existing SDTS support routines and ISO 8211
(FIPS 123) tools.
The raster profile should transfer unsigned byte data for DOQ and
most remotely sensed data.  For DEM's, 2-byte integer data will be
necessary.  The raster profile will not require the inclusion of
binary encoding of real-valued (i.e. floating point) data sets.  For
nonimage portions of the transfer (e.g. headers and data quality),
character encoding of floating point values will be needed, as
allowed by the standard.
The raster profile will transfer of complete raster images and will
not employ tiling techniques that could speed up access and allow
windowing operations, but increase data structure complexity.
Image compression techniques are efficient and are becoming
standardized.  The raster profile should transfer data using a
"lossless" compression method (e.g. LZW) as described by Welch
(1984), or a higher compression "lossy" method (e.g. JPEG), as
described by Wallace (1991).
                Future Plans for the Raster Profile
To promote the prototype profile and provide well-documented examples
to the user community, several data sets will be generated and
distributed using the raster profile.  Planned inclusions from the
USGS are: a DOQ sampler of data in compressed and uncompressed form,
remotely sensed AVHRR biweekly composites on CD-ROM, and (or) an
AVHRR companion disk that contains data in both raster (DEM,
remotely-sensed, and ancillary data) and vector (DLG format ancillary
data) form.  Sample data sets from other sources of raster data will
also be generated for the raster profile test and demonstration
period.
For the raster data transfer process to be both complete and simple,
it is  necessary to provide users with software, documentation, and
some assistance.  To date, software subroutine libraries have been
developed and are being tested for the vector profile.  It is planned
that additional routines for the raster profile will be written.
In a recent session at an SDTS Implementation Workshop in Reston,
Virginia, the plans for a raster profile were discussed among a group
of about 20 people.  A mailing list was established for a group of
about 65 users, system vendors, and data providers.  If you would
like to be added to this list or would like to participate in the
development of the raster profile, contact Janette Cascio, USGS, 526
National Center, Reston, VA 22092.
Planned milestones for developing an SDTS raster profile are as
follows:
        Raster profile interest group technical exchange meeting
           Winter 1992
        Design of a prototype raster profile
           Spring 1993
        Creation of sample data sets and software
           Spring/Summer 1993
        Test and Demonstration period    (six months)
           ends Fall 1993
        Completion of raster profile
           Winter 1993
        Submission to NIST for FIPS approval
           Winter 1993
REFERENCES
Fegeas, R., 1992, "A Prototype SDTS Vector Profile for Geographic
Vector Data with Topology", Draft:02-14-92, U.S. Geological Survey,
521 National Center, Reston, VA 22092, 17 p.
Simonett, D.S., ed., 1978, "Geobase Information System Impacts on
Space Image Formats", Report by the UCSB Remote Sensing Unit,
Workshop supported by NASA contract NASW-3118, Santa Barbara, CA, 128
p.
Suresh, R., 1992, written communication, "Version 0 Standard Data
Format Selection and Hierarchical Data Format Overview", NASA GSFC,
38 p.
U. S. Geological Survey, 1990, "Digital Elevation Models - Data Users
Guide 5, USGS National Mapping Division, 510 National Center, Reston,
VA 22092 , 51 p.
U. S. Geological Survey, 1992, "Standards for Digital Orthophotos",
Version 7/92, National Mapping Program Technical Instructions, USGS
National Mapping Division, 510 National Center, Reston, VA 22092,
[variously paged]
Wallace, G.K., 1991, "The JPEG Still Picture Compression Standard",
Communications of the ACM, Vol. 34, no. 4, April 1991, p. 31-44
Welch, T.A., 1984, "A Technique for High Performance Data
Compression, IEEE Computer, Vol 17, no. 6, June 1984
Biographical Sketch
David D. Greenlee is a Staff Scientist in the Data Services Branch
of the USGS's EROS Data Center, Sioux Falls, SD 57198.
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