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SPATIAL DATA TRANSFER STANDARD (SDTS) FREQUENTLY ASKED QUESTIONS

This document was last updated. March 17, 1998

1.0 What is the SDTS?

1.1 What is an SDTS transfer?
1.2 What is an SDTS encoder?
1.3 What is an SDTS decoder?
1.4 Where can I get SDTS information on the Internet?
2.0 What is a Profile?

2.1 What is the Topological Vector Profile (TVP)?
2.2 What is the Raster Profile?
2.3 What is the Point Profile?
2.4 What is the Transportation Network Profile (TNP)?
2.5 What is the Computer-Aided Design and Drafting (CADD) Profile?
3.0 What data is available in SDTS?

3.1 United States Geological Survey (USGS)
3.2 National Oceanic and Atmospheric Administration (NOAA)
3.3 United States Census Bureau
4.0 Is there software available for translating and using SDTS data?

4.1 Commercial SDTS software
4.1.1 Applied Software Technologies
4.1.2 Avenza Software Marketing Inc
4.1.3 Environmental Systems Research Institute (ESRI)
4.1.4 ERDAS
4.1.5 Intergraph
4.1.6 UNISYS
4.2 Public Domain Software
4.2.1 USGS Support Software
4.2.1.1 SDTS++ Programmers Library
4.2.1.2 FIPS 123 Function Library
4.2.1.3 Master Data Dictionary Application
4.2.1.4 USGS Data Viewers
4.2.2 Department of Transportations Bureau of Transportation Statistics
4.2.3 Bureau of Land Management (BLM)
4.2.4 Oceanography Department, U.S. Naval Academy (Peter Guth)
4.2.5 U. S. Army Corps of Engineers (USACE)
5.0 Answers to SDTS technical questions

5.1 What is the terminology SDTS uses for its structures?
5.2 What are the spatial identifiers (or foreign identifiers) in SDTS?
5.3 Does SDTS support spatial object sharing among aggregate spatial objects?
5.4 Does SDTS support transactional updates?
5.5 Does SDTS support only its own internal Feature Coding Scheme?
5.6 How does a SDTS decoder application deal with nonstandard Feature Coding Schemes?
5.8 Can encoding schemes other than ISO 8211 be used with SDTS?
6.0 What is Conformance Testing?

6.1 What is the first profile to be tested?
6.2 What is a Conformance Test Suite
6.3 Who conducts conformance testing?
7.0 What is going on???? (Miscellaneous Questions)

7.1 Why has the USGS stopped making DLG's in the optional format and why are they forcing me to use this new SDTS?
7.2 I've downloaded your data and uncompressed it but what are all these .ddf files?
7.3 I receive an error message when I try to download the data from your site. What is going on?
7.4 I'm getting a Segment Violation Error when I use SDTSIMPORT and SDTSLIST on the ????le??.ddf file. What is going on?
7.5 When I display my coverages to the screen my line coverages have gaps and holes in them. What am I doing wrong?
7.6 What is this .gz extension and what is a tar file? Can I use them on my DOS machine?
7.7 When I try to bring SDTS DEMs into ARC/INFO using the SDTSIMPORT command I keep getting a "Line Segmentation" error. Why is this?
7.8 How can I use the DEM data in the SDTS format?

1.0 What is SDTS?

SDTS is the acronym for the Spatial Data Transfer Standard. SDTS is a FIPS(173) and FGDC (STD-002) standard that defines a non-proprietary format for packaging vector or raster spatial data with attributes, metadata, a data quality report and usually a data dictionary. SDTS is primarily intended to be used for spatial data product distribution and archiving. The format and structure of a SDTS file set is designed to enable blind transfer of information between different hardware/software environments without loss of contextual information. The SDTS is not intended to replace the internal processing structures of any geographic information system. SDTS is not a database or a product specification or a software package.

The SDTS was designed by an ACSM-sponsored committee representing government agencies, universities, and private companies that all saw a requirement for a better way to exchange GIS data. The National Mapping Division of the U.S. Geological Survey (USGS) is the maintenance agency for the standard (FIPSPUB 173-1) (FGDC-STD-002).

The SDTS home page is located at:

http://mcmcweb.er.usgs.gov/sdts

1.1 What is an SDTS Transfer?

SDTS transfers are the data sets used to exchange spatial data between different computer systems. SDTS transfers consist of a series of modules (the SDTS's logical grouping of related data) encoded into files using ISO 8211 standard, "Specification for a Data Descriptive File for Information Interchange" (ANSI 1986).

1.2 What is an SDTS encoder?

An SDTS encoder translates data from a particular user data format and system into an SDTS transfer.

1.3 What is an SDTS decoder?

An SDTS decoder is an application that takes data sets from SDTS structures into native structures.

1.4 Where can I get SDTS information on the Internet?

The U.S. Geological Survey's SDTS web site is at:

http://mcmcweb.er.usgs.gov/sdts

Documents and sample data sets are at:

http://thor-f5.er.usgs.gov/sdts

The USGS Geospatial Data Standards page is at:

http://mapping.usgs.gov/www/html/1stand.html

SDTS data set can be obtained at:

http://edcwww.cr.usgs.gov/doc/edchome/ndcdb/ndcdb.html

2.0 What is a Profile?

SDTS is designed to support all types of spatial data. However, developing a single translator that could support all of the SDTS options and all the varieties of spatial data models is not practical. What is practical is implementing SDTS through the use of profiles.

A profile is a well-defined subset of SDTS created for translating a specific type or model of spatial data with as few SDTS options as possible. Because a profile is meant to transfer a specific type of spatial data, it identifies only the portion of the SDTS that applies to that specific data model. All the other SDTS options are excluded. This reduces the complexity of the SDTS, eliminating unnecessary and invalid choices for that particular data type. For example, if a topologically structured vector data file is to be transferred, SDTS modules dealing with raster data would not be included in the profile. For more detailed information on profiles, see:

http://mcmcweb.er.usgs.gov/sdts/profile.html

2.1 What is the Topological Vector Profile (TVP)?

The first SDTS profile developed was the Topological Vector Profile or TVP. The TVP received FIPS approval and is Part 4 of the SDTS. It has also received FGDC approval as FGDC-STD-002.4. The TVP applies to geographic vector data with planar graph topology. TVP currently accommodates the USGS DLG-3 and DLG-F data, along with the Census Bureau's TIGER data. For more detailed information of the TVP, see:

http://mcmcweb.er.usgs.gov/sdts/SDTS_standard_oct91/index_4.html

2.2 What is the Raster Profile?

The raster profile is titled SDTS Raster Profile and Extensions (SRPE). It will be approved by FGDC in 1998 as Part 5 of the SDTS. The SRPE applies to two-dimensional raster grid or image data that may optionally be georeferenced. The raster profile supports the transfer and archive of image data, digital terrain models, gridded GIS layers, and other gridded data. The extensions allow alternate file formats to be used to encode image data. The extensions permitted by this profile are the Basic Image Interchange Format (BIIF), TIFF, JFIF, and JPEG. SRPE will accommodate USGS DEM's and DOQ's. For more detailed information of the Raster Profile, see:

http://thor-f5.er.usgs.gov/sdts/standard/latest_draft/pdf/srpe11s.pdf
This file is in Portable Document Format (pdf) and requires the Adobe Acrobat Reader. This free pdf reader is available for download at:
http://www.adobe.com/prodindex/acrobat/readstep.html

2.3 What is the Point Profile?

The Point Profile is a subset of SDTS used for high precision geodetic control and survey points. It was developed by the National Oceanic and Atmospheric Administration (NOAA) and the Federal Geodetic Control Subcommittee (FGDC). It is endorsed by FGDC as FGDC-STD-002.6 For more detailed information of the Point Profile see:

http://www.ngdc.noaa.gov/seg/tools/sdts/main.html

http://www.ngs.noaa.gov/FGCS/sdtsp.html

2.4 What is the Transportation Network Profile (TNP)?

The TNP is a subset of SDTS for use with geographic vector data with non-planar network topology. The TNP document is available from the FGDC Ground Transportation web site at:

http://www.bts.gov/gis/reference/tnp_11.html

2.5 What is the Computer-Aided Design and Drafting (CADD) Profile?

The draft CADD profile is a subset of SDTS that supports the non-proprietary exchange of CADD type spatial data. It is sponsored by the FGDC Facilities Working Group and the Army Corps of Engineers. For more detailed information on the CADD Profile please go to the following: (Look under #4 on that page)

http://corpsgeo1.usace.army.mil/FGDC/data_standards.html

3.0 What data is available in SDTS?

SDTS has been mandated as a federal standard, so most current data sources will be federal agencies. Below you will find a list of some federal agencies who have or will be offering SDTS data. (data sources will be updated as they come to our attention)

3.1 United States Geological Survey (USGS)

The USGS is making much of its digital map data available in SDTS as an additional distribution format. By producing data in SDTS format, the USGS hopes to create a demand for the data and software to read it. This will make it profitable for commercial software developers to write viewers and decoders for using the data. All USGS map data continues to be available in native formats. Sales policies for native-format data have not changed. The USGS has no plans to abandon any of the older formats. To promote SDTS as a GIS format, USGS SDTS data is available over the Internet at no charge. USGS data and products are not generally free; SDTS data is an exception.

USGS data that is available in SDTS includes:

1:24,000 (Large Scale) DLG
1:100,000 DLG
1:2,000,000 DLG
1:24,000 DEM

This data may be found on the USGS GeoData web page at:

http://edcwww.cr.usgs.gov/doc/edchome/ndcdb/ndcdb.html

3.2 National Oceanic and Atmospheric Administration (NOAA)

NOAA is making its National Geodetic Survey high precision geodetic control and survey point data available in the SDTS format using the Point Profile. This data is available at:

http://www.ngs.noaa.gov/datasheet.html

3.3 United States Census Bureau

The Census Bureau was a primary developer of the first SDTS profile and initially made some of their Tiger data available as samples in SDTS. These sample datasets are available at the following:

http://thor-f5.er.usgs.gov/sdts/datasets/tvp/tiger

However, there are no plans at this time for converting any more of their Tiger data into SDTS.

4.0 Is there software available for translating and using SDTS data?

This software is divided into two categories: Commercial (Section 4.1) and Public Domain (Section 4.2).

4.1 Commercial SDTS software

This section contains information provided by companies working on SDTS translators. The listing provided below does not constitute an endorsement of these companies or their products by the USGS. Companies are listed in alphabetical order.

4.1.1 Applied Software Technologies

Applications Software Technologies, Incorporated's GIS Solutions group has developed SDTS translators for the TVP and TNP profiles. These translators are part of the GeoMorph product. GeoMorph can preview and translate GIS data between fifteen (15) different desktop mapping formats, including MapInfo MIFF (MIF/MID), AutoCAD r12 DWG and DXF, Microstation IDGS (DGN), ESRI Shape (SHP), and SDTS. The GeoMorph system will allow you to view free government SDTS data and import it into your favorite Windows mapping package, or to convert your mapping files into SDTS format.

Companion products currently in development will allow software developers to add file formats to the growing list of formats GeoMorph supports, and to put GeoMorph's Import/Export technology directly into their applications, to support all of the supported file formats as if they were the application's native format. For more information about GeoMorph or Applications Software Technologies, Inc. Please visit their web site at:

http://www.aztechcon.com/geomorph
or call them at (920)499-0533.

4.1.2 Avenza Software Marketing Inc.

Avenza Software Marketing Inc. supports SDTS in its MAPublisher 2.0, which imports popular GIS formats into high end graphics programs for cartographic and graphics management, and final EPS output. At this time, SDTS import is vector only. All other MAPublisher imports -- USGS DLG, AutoCAD DXF, MapInfo mid/mif, ArcView shape, ARC/INFO generate, and georeferenced TIFF and JPEG -- are imported with all database attributes intact and editable. Completed files can be saved as PDF for cross-platform or Internet upload.

MAPublisher is available for Adobe Illustrator (Macintosh) and Macromedia FreeHand (Windows 95/NT). Avenza is a third-party developer for Adobe and Macromedia. Visit:

http://www.avenza.com
or email info@avenza.com

4.1.3 Environmental Systems Research Institute (ESRI)

Environmental Systems Research Institute (ESRI) has included in Version 7 of the ARC/INFO product an SDTS translator. In addition to the SDTSIMPORT and SDTSEXPORT commands, there are two commands, SDTSINFO and SDTSLIST, to obtain general information about the transfer and list out individual modules within a transfer. Only Version 7.04 and later are capable of transferring multiple files in a single transfer. ESRI's white paper on SDTS is available via anonymous ftp at:

ftp://ftp.esri.com/pub/marketing/white_papers/sdts.pdf

This file is in pdf format, and gives good step by step instructions on importing SDTS files. It requires the use of the Adobe Acrobat Reader. This free reader is available at:

http://www.adobe.com/prodindex/acrobat/readstep.html
ESRI is working to have support for the Raster Profile in their next version.

4.1.4 ERDAS

ERDAS developed a prototype encoder and decoder for the Raster Profile and has sample data sets available for review. The availability of the SDTS translators will make it possible to bring data from a number of raster formats, such as DOQ or DEM, into the IMG format, and then produce SDTS datasets or vice versa. The SDTS Raster encoder and decoder will be included with ERDAS Imagine Version 8.3, which is expected to be released August 1996. ERDAS Imagine Version 8.2, which is available now, contains a TVP translator. The following environments are supported:

SUN OS - SUN Solaria
HPUX - Dec Alpha
Silicon Graphics SGI - IBM RS/6000
Windows NT - Windows 95

For more information contact Larry Warnick, phone: 404-248-9000, ext. 2257, or e-mail: warnick@erdas.com Visit the ERDAS Home Page at:

http://www.erdas.com

4.1.5 Intergraph

Intergraph released an SDTS translator in April 1996. The software translates SDTS data to and from the MGE environment on Windows NT. The translator and a metadata editor are available free of charge through Intergraph's ftp node. For more information see:

http://www.intergraph.com/iss/products/mapping/translation/sdts.htm
Or contact Lacey Sharpe at ltsharpe@ingr.com

4.1.6 UNISYS

UNISYS is developing SDTS translators using the TVP format. UNISYS provides services to support or implement SDTS in the TVP. Also there are services available for project management which can help in defining feature classes which conform to the metadata standards. The status of development is as follows:

SDTS to SYSTEM 9 - Complete and available
SYSTEM 9 to SDTS - Complete and available
GINA to SDTS - Complete and available
OSNTF to SDTS - Complete and available
SDTS to SpatialWare - In development

4.2 Public Domain Software

Public Domain Software is available to support programmers and to support data consumers.

4.2.1 USGS Support Software

The USGS has developed several software tools to assist programmers with implementing SDTS and to provide SDTS viewing capabilities.

4.2.1.1 SDTS++ Programmers Library

The SDTS++ is a C++ class library developed by the USGS to be used in SDTS application development. It was released in February 1998 with both read and write support for SDTS files. It currently runs under Windows 95/NT, and was built using Microsoft Visual C++. This library is located at:

http://mcmcweb.er.usgs.gov/sdts/sdtsxx/index.html

4.2.1.2 FIPS 123 Function Library

The FIPS 123 Function Library is a library of C Language functions which read and write the ISO 8211 (FIPS 123) format used by SDTS. This was the first support library and is the basis for many SDTS utilities. The FTP distribution includes the source for some sample applications. The ones useful for viewing and debugging are show173, sum173, and show8211. It can be found at:

http://mcmcweb.er.usgs.gov/sdts/source.html

Note: This is older code written for research purposes. It is no longer supported by the USGS. There are no plans to fix any bugs or to enhance this library. Developers are encouraged to use SDTS++ (see 4.2.1.1).

4.2.1.3 Master Data Dictionary Application

This is software which creates SDTS master data dictionary transfers from ASCII input files. The software utilizes the FIPS 123 Function Library. This software includes:

msddbat/ - Script files for compiling code
msdddat/ - Example data input files
msdddoc/ - Software User Guide
msddinc/ - Include Files
msddsrc/ - C Source Code

It can be found at:

http://mcmcweb.er.usgs.gov/sdts/source.html

4.2.1.4 USGS Data Viewers

There are data viewing applications for the USGS DEM and DLG-3 in SDTS format. These applications run under Windows 95/NT, and graphically show the spatial content of the SDTS transfer, as well as the textual content of metadata reports and attributes. For more information:

http://mcmcweb.er.usgs.gov/drc/dlgv32pro/index.html

4.2.2 U.S. Department of Transportation's Bureau of Transportation Statistics

This agency has developed the SDTS Encoding Program (EP). This software, developed by the Volpe National Transportation Systems Center, provides interactive conversions of spatial data into SDTS format as described by FIPS 123. This software has been made available for the benefit of government and private companies that are able to apply SDTS-EP to their spatial data requirements. Runs in a PC Windows environment. For details see:

http://mcmcweb.er.usgs.gov/sdts/source.html

4.2.3 Bureau of Land Management (BLM)

This site contains numerous shareware programs. Included are some for translating SDTS data. Some of these include:

SDTS to mif (MapInfo) (sdts2mif.exe - 04/15/97)
SDTS to dxf (AutoCad) (sdts2dxf.exe - 04/15/97)
SDTS to DLG (sdts2dlg.exe - 07/15/97)
SDTS2ARC (ARC/INFO) (sdts2arc.exe - 03/25/98)
For downloading and general information see:
ftp://ftp.blm.gov/pub/gis/sdts

4.2.4 Oceanography Department, U.S. Naval Academy (Peter L. Guth)

MICRODEM freeware manipulates DEM's on Windows 95 and Windows NT platforms. It will accept the following formats: DTED, USGS ASCII for 1:24K and 1:250K DEM's and SDTS for 1:24K DEM's. The program also is capable of merging imagery and vector data sets (Tiger or DLG formats) with the DEM's. More information about MICRODEM and a downloadable version is located at:

http://www.usna.edu/Users/oceano/pguth/website/microdem.htm
http://mcmcweb.er.usgs.gov/sdts/public_domain.html

4.2.5 U. S. Army Corps of Engineers

The Corps has included SDTS translation in their GRASS software. The GRASS-SDTS software can be found at:

ftp://129.229.103.1/pub/grass/outgoing/SDTS

For more information see also:

http:mcmcweb.er.usgs.gov/sdts/source.html

5.0 Answers to SDTS technical questions

5.1 What is the terminology SDTS uses for its structures?

An SDTS transfer is a set of modules that are related by the information they contain. A module is composed of records. A record is composed of fields. A field is composed of subfields. The subfields contain the actual data being transferred. The higher level structures convey the meaning and context of the information. All concepts in SDTS are represented with this set of structures: transfer, module, record, field, and subfield.

For example, an SDTS Transfer must contain an Identification Module. This module contains records which identify the overall content and origin of the spatial data set. The records contain fields like Identification Field, Conformance Field, and Attribute Field. The Identification Field contains subfields like Standard Version, Profile Identification, Title, Map Date, and Scale. The SDTS Line Module which is for representation of one-dimensional spatial objects has fields for Spatial Addresses, Attributes, Polygon Left/Right, etc.

5.2 What are spatial object identifiers (or foreign identifiers) in SDTS?

Every spatial object in an SDTS transfer has a unique identifier. The identifier is formed by combining a module name with a record identifier separated with a "#", for example "LE12#35." The module records are the structures that represent the spatial objects in an SDTS transfer. Every module must have a unique name in an SDTS transfer and every record must have a unique record identifier within its containing module. Combining the module name and record identifier yields a unique identifier for every module record within a single SDTS transfer. SDTS calls this a "foreign identifier." Foreign identifier fields permit one module record to reference another module record (in the same or another module) in the transfer. This permits a Polygon Module record to reference a Line Module record as part of its boundary.

5.3 Does SDTS support spatial object sharing among aggregate spatial objects?

Yes it does. If two polygons share the same boundary, then this boundary can be represented by SDTS structures only once, and then referenced via "id" by each polygon that uses it. In the general case, each simple spatial object (point, line, polygon) is represented once, and they can be referenced by any number of composite objects.

5.4 Does SDTS support transactional updates?

"I have already received an SDTS transfer and loaded the data into my GIS. Now the original data producer has updated the data. How do I get just the updates into my system?"

This task is not directly in the scope of SDTS. However, SDTS can be used to achieve such updates if the consumer and producer agree on a protocol. A data producer could issue an SDTS transfer that contains only those spatial objects that have changed. The data consumer could run a special decoder that added/modified/deleted spatial objects depending on the contents of the SDTS transfer. The SDTS Task Force is not aware of any group using SDTS in this manner. However, USGS is looking for participants to work on a Feature based Profile which could likely include a transactional update method.

5.5 Does SDTS support only its own internal Feature Coding Scheme?

SDTS Part 2 defines a standard set of entity and attribute terms. However, a transfer does not need to use the SDTS Entity terms to be compliant. Any set of terms is acceptable, as long as the definitions of the terms is encoded in the transfer or is available in external documentation.

5.6 How does a SDTS decoder application deal with nonstandard Feature Coding Schemes?

This depends heavily on the robustness of the decoder application. Some have proposed that the software user would set up certain configuration files that control the behavior of the decoder software. You would be responsible for cross walk tables that describe how each term in the transfer would translate into your system.

5.7 How does ISO 8211 relate to SDTS?

ISO 8211 and SDTS are independent standards. ISO 8211 is a "general purpose" data exchange standard that can be used to transfer data and its description (labels and formats) between different computers and software packages. SDTS is an "application-specific" exchange standard intended for geographic and cartographic spatial data. SDTS uses ISO 8211 as its format level. SDTS Part 3 describes how an SDTS transfer is created using ISO 8211 files and structures. The files of an SDTS Transfer have the extension ".DDF" which stands for a "Data Descriptive File". This term comes from the title of the ISO 8211 standard.

5.8 Can encoding schemes other than ISO 8211 be used with SDTS?

SDTS was structured in such a way to permit alternate encoding schemes. Currently, the Standard only includes an ISO 8211 encoding scheme. If necessary, alternate encoding schemes could be added to the base standard or by Profiles.

The first profile to permit alternate coding formats is the SDTS Raster Profile and Extensions (SRPE). The SRPE permits BIFF, TIFF, JFIF, and JPEG to be used to encode an image rather than using ISO 8211. (See FAQ question 2.2 for more information.)

6.0 What is Conformance testing?

Conformance testing verifies the degree to which a product complies with a standard. Conformance testing provides relevant parties such as developers, buyers, and users added confidence in product quality, increases levels of confidence in product quality, and maximizes the probability of successful interoperability.

The method used for the SDTS is called falsification testing. This method tests as many requirements of the standard as are feasible. The testing attempts to find errors that indicate non-conformance to the standard. If no errors are found, the product is considered conforming.

Falsification testing cannot ensure complete conformance unless every requirement of the standard is tested. Because of this, conformance testing is an excellent quality assurance/quality control tool to insure that a product is SDTS compliant.

Three types of products can be tested in an SDTS conformance test: SDTS transfers, SDTS encoders, and SDTS decoders.

6.1 What is the first profile to be tested?

Initial conformance testing will be done on the Topological Vector Profile (TVP). The TVP supports the transfer of vector data with geometry and topology.

6.2 What is a Conformancei Test Suite?

The components of a conformance test, collectively called a test suite, consist of software, procedures, data sets, and documentation.

6.3 Who conducts conformance testing?

Conformance testing is conducted by an accredited testing laboratory. Testing laboratories for various standards include the NIST, as well as other federal agencies, universities, and commercial laboratories accredited by a recognized Certificate Issuing Organization (CIO).

7.0 What is going on???? (Miscellaneous Questions)

Answers to miscellaneous and common questions when dealing with SDTS data.

7.1 Why has the USGS stopped making DLG's in the optional format and why are they forcing me to use this new SDTS?

The USGS has not stopped producing its data in native formats. All USGS data is available in native format including: DLG-O, DEM, DRG and DOQ at the following location:

http://edcwww.cr.usgs.gov/webglis

SDTS data is also being produced as an additional format and was not meant to replace DLG's or any of the others. For more information on what data is produced and why please see section 3.1. of the FAQ.

7.2 I have downloaded your data and uncompressed it but what are all these .ddf files?

The .ddf (Data Descriptive File) extension is the format used for the ISO 8211 compliant files and SDTS files. ISO 8211 and SDTS are two different standards, however, SDTS utilizes the ISO 8211 .ddf file format. These files contain two types of records:

A .ddf always contains one DDR and one or more DRs. For more information on the ISO 8211 or the SDTS file formats, see the "Guide for Technical Managers" located at the following:

http://mcmcweb.er.usgs.gov/sdts/training.html

This file is in a .pdf format and requires the use of the Adobe Acrobat Reader. A downloadable version of this program is located at:

http://www.adobe.com/prodindex/acrobat/readstep.html

7.3 I receive an error message when I try to download the data from your site. What is going on?

The following error message is received from time to time: "The downloaded file is not available. This could be due to your security or language settings or because the server was unable to retrieve the requested files."

This error can be caused by two different scenarios:

1. The server on the other end of the connection is down for some reason. Your only option then is to keep trying until the server is back on line.

2. Your web browser is not setup correctly to access ftp servers. The quickest solution to this would be to change the URL address to an FTP address. For example if you are accessing your data from:

http://edcftp.cr.usgs.gov/pub/data/DLG/2M
you would change it to:
ftp://edcftp.cr.usgs.gov/pub/data/DLG/2M

7.4 I am getting a "Segment Violation Error" when I use SDTSIMPORT and SDTSLIST on the ????le??.ddf file. What is going on?

This problem in most cases is caused by the use of a commercial decompression software package that may have certain default settings checked.

SDTS data are archived in a non-proprietary zip format (GNU Zip). If you are planning to use an "off the shelf" decompression software like Winzip TM to unzip and untar these files be aware that many have a default conversion setting that MUST NOT be used!

For example, within WinZipTM, under "Options/Configuration", there is a checkbox that reads "TAR file smart CR/LF translation" - insure that this box is NOT checked. This will disable the default setting.

If you have received this error and have not unchecked the default options mentioned above, then your only recourse is to delete the .ddf files and download the compressed data again. Now decompress the data again with the default setting unchecked and you should have no problem.

7.5 When I display my coverages to the screen my line coverages have gaps and holes in them. What am I doing wrong?

There are two possible answers to the above question:

1. If you are downloading and using 100K data and what you are getting is only one quarter of the sheet then you will need to import all the other layers of the 100K. The SDTSIMPORT defaults to using the 01 layer and you have to specifically import each of the other layers. (for example: 02-?)

This can be a problem not only with ARC/INFO but also with several of the desktop formats as well.

2. This can also be caused by the use of commercial decompression programs with the wrong settings. Please see section 7.4 above.

7.6 What is the .gz extension and what is a tar file? Can I use them on my DOS machine?

GNU Zip (gz) and tar are Unix compression and bundling packages. Tar will bundle a number of files into one file and gzip will compress that file. The SDTS files (.ddf) for a particular layer are bundled together using the tar command and they are then compressed using the gzip command.

There are DOS equivalents for both of these commands, they can be found at:

http://thor-f5.er.usgs.gov/sdts/software/tools

You will need to pick up the gunzip.exe and the tar.exe commands. They are used to decompress SDTS files in the following way:

1. Download an SDTS file (????.????.sdts.tar.gz)

2. Go to a DOS prompt

3. Type: gunzip "sdts file name".gz

4. Type: tar xvf "sdts file name"

This will produce the SDTS .ddf files.

7.7 When I try to bring SDTS DEMs into ARC/INFO using the SDTSIMPORT command I keep getting a "Line Segmentation" error. Why is this?

It is important to note that the ARC/INFO SDTSIMPORT command only imports SDTS DLG files--not DEM files. ESRI is aware of this and is developing translation software. This should not preclude you from expressing your need (to your GIS vendor) for such translators. Some commercial and public domain translation software currently exists, and others are forthcoming. See Item 7.8 below.

7.8 How can I use the DEM data in the SDTS format?

Currently, the following viewers and translators exist for SDTS DEMs:

1. SDTS2ARC--Freeware that translates SDTS DEMs into an ASCII grid format that can be brought into a number of software packages including ARC/INFO. Please note that the resulting data is no longer georeferenced or registered and no metadata will be included in the transformation. This software is available for download at:

ftp://ftp.blm.gov/pub/gis/sdts

2. MICRODEM--Freeware that allows viewing and some data manipulation. For additional information visit:

http://mcmcweb.er.usgs.gov/sdts/public_domain.html

3. dlgv32 Pro--Windows 95/NT based freeware viewer. For more information visit:

http://mcmcweb.er.usgs.gov/drc/dlgv32pro/index.html

4. ERDAS Imagine--Commercial GIS package that allows data editing and manipulation. More information may be found at:

http://mcmcweb.er.usgs.gov/sdts/implement-priv.html


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