satellite or aircraft image data. Aircraft and satellite profiling sensor
data was to be accomodated if possible, and finally it was hoped that
geographic or cartographic data in polygon form could also be handled. The
success of the designers of the Format was demonstrated by its wide
acceptance by the remote sensing community. In addition, the major Canadian
Federal Government operators of large Geographic Information Systems were
able to adapt the format to the transfer of polygon data through the Spatial
Data Transfer Format (Goodenough, et al., 1979). A variation of this format
was later adopted by the Canadian Committee on Surveys and Mapping, which
represents a wide variety of federal and provincial mapping agencies
throughout Canada.
This CCT Format Family can accommodate many extant formats without extensive
reformatting by adding a superstructure to data recorded in existing
formats. This superstructure identifies the format and provides a logical
guide to reading the tape. It has two components: a Volume Directory which
generally describes the data configuration and provides pointers to each of
the data files; and a File Descriptor in each file which describes the data
structure within the file. The Format Family does depend upon external,
written documentation for the definition of the format, the structure of the
various records and the definition of the logical formats of the data
fields.
SPACE DATA ACTIVITIES
A new committee, the Committee on Earth Observation Satellites (CEOS), was
formed in 1985 to standarize the formats and other factors of earth
observation space data for the Space Station era. This committee is
considering data from various earth-looking sensors. Initial consideration
is being given to the LANDSAT CCT family as a foundation for such a common
format.
An international body, the Consultative Committee on Space Data Systems
(CCSDS), is defining a nev, generic structure for use in the Space Station
and its data systems. It is hoped by the Committee that this structure will
also find general use. This structure is a nested Type-Length-Value set of
frames called the Standard Formatted Data Unit (SFDU).
The Type field is fixed format with pre-defined codes for various entries,
the SFDU-length field is fixed length, and the value field is the variable
length defined in the SFDU length field. The value field is the data field,
and may contain any desired data, including more SFDUs. The structure at
this definition level is generic.
The SFDU is primarily designed to be operational in an environment in which
utmost stress is placed on the ability of the receiving software to
recognize a message and parse it in an absolute minimum of time in
anticipation of further messages immediately to follow. A prime mode of
operation, therefore, is quick message recognition and local lookup of
message handling instructions. This precludes the parsing of format
definition information in the message.
However, to accommodate those situations in which self-contained data
definition is desired, the SFDU may contain an internal SFDU module which
contains the necessary structure definition. Being considered as the data
definition language is the General Data Interchange Language (GDIL)
(Billingsley, 1985), described in some detail below. The GDIL is a generic
ISO Level 6 description of the field structures of the various records plus
the logical structure definitions of the data fields.
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