M.
e
model, SAIF is more general and superior in a number
of areas compared with SDTS and DIGEST. Using an
object-oriented data model, SAIF promises a much
higher degree of extendibility than the other two. This
is crucial for the NSDI where so many divergent parties
will be involved. Claiming superiority is not that easy
concerning implementation. Nevertheless, SAIF
offers an alternative approach to traditional feature
coding by allowing different users to define features in
their own way. Thus, the formidable task of creating
and maintaining standard feature coding catalogs of
the traditional approach which is used by SDTS and
DIGEST is avoided. Finally, SAIF is supported with an
interchange environment of broader facilities.
Therefore, SAIF has been found to be the most
appropriate for the NSDI of Turkey. A number of
extensions will be needed for the adoption of SAIF.
For instance, additions for the vertical and horizontal
reference systems, and projections used in Turkey have
to be made to the SAIF Standart Schema.
Choosing SAIF will not make SDTS and DIGEST
invalid for the NSDI since these three standards are
now harmonized (O'Brien et al, 1994). On the other
hand, International standard bodies, namely European
Committee for Standardization, Open GIS Consortium,
and International Standards Organization's Technical
Committee 211 have been involved in developing
"interoperability" standards (CEN, 1993; Farley,
1994; ISO, 1995). However, when and how these
efforts will end is unclear at the moment. Even they all
succeed, there would still be three different standards in
place. Should SAIF be replaced with an internationally
accepted interoperability standard, it could still be used
within NSDI for archieving.
6. CONCLUSION
Turkey has to start building a NSDI immediately 1f she
wants to build an "information society". NSDI will
make nationwide horizontal and vertical spatial data
integration possible. One of the most important
technical requirements in building and maintaining the
NSDI is a spatial data interchange standard. To meet
this requirement, DIGEST, SAIF, and SDTS have been
evaluated by the evaluatuion criteria determined in this
study. As the result of the evaluation, SAIF has been
found to be the most appropriate for the NSDI of
Turkey. Using an object-oriented data model, SAIF
promises a much higher degree of extendibility than
both SDTS and DIGEST, which is very valuable for the
NSDI.
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International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996
