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 dissimilar computer systems with the potential for no information loss. In 1980, the U.S. Geological Survey (USGS) was designated the lead agency in developing earth science data standards for the federal government. The USGS worked with academic, industrial, and federal, state, and local government users of computer mapping and GIS to develop a standard for transfer and exchange of spatial data. In 1992, after 12 years of developing, reviewing, revising, and testing, the resulting standard--SDTS, was approved as Federal Information Processing Standard (FIPS) Publication 173, known as FIPSPUB 173-1, 1994. The FIPS version has been superseded by the current version, known as ANSI NCITS 320-1998 and was ratified by the American National Standards Institute (ANSI) June 9, 1998. Compliance with SDTS is now 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.

A single-document overview of SDTS is available at: ftp://sdts.er.usgs.gov/pub/sdts/articles/ascii/overview.doc or, ftp://sdts.er.usgs.gov/pub/sdts/articles/ps/overview.ps

DLG Data - DLG-O data are derived from standard USGS topographic quadrangles. The packaging of DLG-O data reflects this: each physical file covers the geographic area of some part of a standard quadrangle, and contains a feature set that approximates the ink colors of a standard published map.

DEM Data - DEM data are derived from standard USGS topographic quadrangles. The packaging of DEM data reflects this: each physical file covers the geographic area of some part of a standard quadrangle, Similarly, SDTS DEM data are derived directly from DEM data, and reflect the DEM data packaging. Each SDTS Data transfer has the same geographic coverage and same spatial content as the original source DEM. Links to data can be found at http://edc.usgs.gov/geodata

Example - Finding and Retrieve SDTS DLG Data

A Master Data Dictionary (MDD) must be downloaded for each scale of DLG data (100K, 2M, and Large Scale) data you intend to use. Detailed instructions are included in section 4.2.1.2, below. To download a specific data set, follow these procedures:

1. Go to the http://edc.usgs.gov/geodata

2. Click on 1:24K DLG (in blue, near the top of the page).

3. Since the name of the quadrangle is known, the data could be found by selecting either FTP via Graphics, FTP via Alphabetical List or FTP via State. However, it is common not to know exact quadrangle names: Select FTP via Graphics.

4. Select Conterminous 48 states.

5. A simple map of the U.S. appears. Click on the State of California.

6. A map of California appears, with 1-degree lines of latitude and longitude. Click near San Francisco. The next display will show approximately 30 7.5- minute quadrangles, centered on where you clicked your mouse.

7. Click on the 7.5-minute cell labeled San Francisco North, CA.

8. A "list" of one quadrangle appears. (Other paths to this page may produce lists of more quadrangles.) Select the link to San Francisco North, CA.

A list of nine directories will then appear. Each directory corresponds to one DLG overlay. San Francisco North is used in this example because all nine overlays are available. This is unusual for 1:24,000-scale data; most quadrangles have fewer than nine overlays finished.

9. Select the second overlay (in this example), hydrography.

10. Select the file listed under (not necessarily inside) the folder 1/, this should be the most recent version.

The file 1525941.HY.sdts.tar.gz contains the SDTS transfer for the hydrography layer of the San Francisco North, CA quadrangle.

11. Select the file 1525941.HY.sdts.tar.gz for downloading

12. Save it to the c:\san_francisco_north\sdts_dlg_24k folder. The file can be renamed (for example, sanfra.tar.gz). Retaining the .tar.gz extensions is strongly recommended. Some browsers will attempt to rename this file for you. In some cases, this renaming causes problems for decompression software. One common case is a rename from *.tar.gz to *_tar.gz, which confuses the WinZip decompression program.

Repeat steps 9 through 12 for all overlays of interest.

For more information on http://edc.usgs.gov

• DLGv32 Pro, a free http://mcmcweb.er.usgs.gov/drc/dlgv32pro/
• ftp://ftpmcmc.er.usgs.gov/pub/sdts/software/tools/
• http://www.cuteftp.com
• http://www.ftpx.com
• http://thor-f5.er.usgs.gov/sdts/articles/ascii/
  
  See Also, GISuser Guide to downloading Free USGS 1:100K DLGs & GISuser Guide to downloading Free 1:250K USGS DEMs & GISuser Guide to downloading Free 7.5 minute DEMs
  

Source: USGS