Spatial Data Transfer Standard

Definition
The Spatial Data Transfer Standard (SDTS) is a robust way of transferring earth-referenced spatial data between different computer systems with the potential of no information loss. It is a general mechanism for the transfer of geographically referenced spatial data and its supporting metadata (i.e., attributes, data quality reports, coordinate reference systems, security information).

About
The SDTS was designed by a group of people representing government agencies, universities, and private companies that saw a requirement for a robust way of transferring earth-referenced spatial data between various computer systems with the potential for no  information loss. After 12 years of discussion, development, review, and testing, the resulting SDTS was approved as Federal Information Processing Standard (FIPS) 173, and as FGDC-STD 002. Compliance with SDTS, also known as FIPS 173, is mandatory for all federal agencies. SDTS is a transfer standard that embraces the idea of self-contained transfers; spatial data, attributes, georeferencing, a data quality report, a data dictionary, and other supporting metadata are all included in the transfer.

The SDTS is a standard. Standards are documents that specify rules; they are neither software nor databases. The SDTS is a standard for data transfer, as opposed to a standard for data processing. SDTS does not replace existing Geographic Information System (GIS) processing formats. The SDTS is designed specifically for spatial data. Other standards exist for other kinds of data (digital images, documents, electronic signals)

Purpose of SDTS
The purpose of the SDTS is to promote and facilitate the transfer of digital spatial data between dissimilar computer systems, while preserving information meaning and minimizing the need for information external to the transfer. Implementation of SDTS is of significant interest to users and producers of digital spatial data because of the potential for increased access to and sharing of spatial data, the reduction of information loss in data exchange, the elimination of the duplication of data acquisition, and the increase in the quality and integrity of spatial data. SDTS is neutral, modular, growth-oriented, extensible, and flexible--all characteristics of an "open systems" standard.

SDTS Standards
PART 1--Logical Specifications consists of three main sections, which explain the SDTS conceptual model and SDTS spatial object types, components of a data quality report, and the layout of all SDTS modules.

PART 2--Spatial Features contains a catalogue of spatial features and associated attributes. This part addresses a need for definition of common spatial feature terms to ensure greater compatibility in data transfers. The current version of Part 2 is limited to small- and medium-scale spatial features commomly used on topographic quadrangle maps and hydrographic charts.

PART 3--ISO 8211 Encoding This part explains the use of a general purpose file exchange standard, ISO 8211, to create SDTS filesets (i.e. transfers).

PART 4--Topological Vector Profile The Topological Vector Profile (TVP) is the first of a potential series of SDTS profiles, each of which defines how the SDTS base specification (Parts 1, 2, and 3) must be implemented for a particular type of data. The TVP limits options and identifies specific requirements for SDTS transfers of data sets consisting of topologically structured area and linear spatial features.

PART 5--Raster Profile and Extensions The Raster Profile is for 2-dimensional image and gridded raster data. It permits alternate image file formats using the ISO Basic Image Interchange Format (BIIF) or Georeferenced Tagged Information File Format (GeoTIFF).

PART 6--Point Profile The Point Profile contains specifications for use with geographic point data only, with the option to carry high precision coordinates such as those required for geodetic network control points. This profile is a modification of Part 4, the Topological Vector Profile, and follows many of the conventions of that profile.

Part 7--Computer Aided Design and Drafting Profile The Computer Aided Design and Drafting Profile (CADD) contains specifications for an SDTS profile for use with vector-based geographic data as represented in CADD software. The purpose of this profile is to facilitate the translation of this data between CADD packages without loss of data, and support the translation of this data between CADD and mainstream GIS packages. CADD software makes up a large portion of the Geographic Information Systems (GIS) marketplace. CADD software allows for several types of elements, in particular, the use of three-dimensional elements and complex curves that are not commonly used by GIS. This profile allows the representation of two- and three-dimensional geographic vector data from CADD packages to be transferred via the SDTS standard, and supports two-dimensional vector data and three-dimensional vector data, where the third dimension is the "height" of the object. These data may or may not have topology. This profile does not support raster data or two-dimensional transfers already represented by another profile.

Conversion of USGS Data to SDTS
In 1995, USGS began converting digital cartographic data holdings to SDTS. 1:24,000-scale (or 7.5 minute) DEM data were converted in early 1998. The 1:2,000,000-scale DLG data were revised in 1995 and are now available in SDTS. This includes coverage of the conterminous United States for the following categories of data: hydrography, transportation, boundaries (county names were included with the transfer), United States Public Land Survey System, and man-made features.

All 1:100,000-scale DLG files have been converted to SDTS format. This includes national coverage of the hydrography and transportation overlays, with partial coverage of other layers. All 1:24,000-scale DLG files have been converted to SDTS format. Coverage varies between overlays at this scale, but no overlay has complete coverage.("All files" means all files for which DLG data have been produced. Coverage of 1:24,000 and 1:100,000- scale DLG data is not complete).

Advantages of SDTS
Advantages of the SDTS include data and cost sharing, flexibility, and improved quality information and metadata, all with the potential for no loss of information during transfer. The SDTS is not vendor specific or proprietary.