• 7.5-Minute DEMs correspond to the USGS 1:24,000- and 1:25,000-scale topographic quadrangle maps, and are available for all of the U.S. and its territories. They are cast to the Universal Transverse Mercator (UTM) projection system and are referenced to either the North American Datum (NAD) of 1927 (NAD27) or the NAD of 1983 (NAD83). Most files will have a grid spacing of 30 meters, but 10-meter grids are also available for some locations. The average file size of a 30-meter DEM is 1.1 megabytes and 9.9 megabytes for a 10-meter DEM. They are available in Native and Spatial Data Transfer Standard (SDTS) format.
• 30-Minute DEMs correspond to the east half or west half of the USGS 30-minute by 60-minute topographic quadrangle map, and are available for the conterminous U.S. and Hawaii. They are expressed in geographic coordinates (latitude/longitude) and are referenced to either NAD27 or NAD83. Grid spacing is 2 arc-seconds.
• 1-Degree DEMs correspond to the 3 arc-second (or 1:250,000-scale) USGS topographic map series, and are available for all of the contiguous U.S. and most of Alaska. They are expressed in geographic coordinates (latitude/longitude). Most are referenced to the World Geodetic Survey (WGS) system of 1972 (WGS72), but some are referenced to the WGS system of 1984 (WGS84). Grid spacing is 3 arc-seconds.

National Mapping Standards - Standards set the criteria and specifications to ensure that all products prepared by the USGS Geography Discipline (formerly National Mapping) under the National Mapping Program (NMP) reflect current mapping and data policies and are accurate and consistent in style and content.

DEM Standards - The USGS has provided the National Mapping Program Standards for digital elevation data in these publications.

SDTS Support Software for Programmers - The USGS and some individuals have developed several software tools to assist programmers with implementing SDTS.

The Spatial Data Transfer Standard - Guide for Technical Managers : a concise, comprehensive guide to the SDTS for technical managers in spatial data user organizations and software development groups involved in SDTS use or implementation

Sample data from USGS - If you're unsure about a data format we suggest you grab a sample dataset before you purchase or download gigabytes of data.

dlgv32 Pro is a limited-feature version of commercial software called Global Mapper which was created from the original dlgv32 source code. Enables Auto-clip collar for DRG's and Opens SDTS DLG and SDTS DEM files directly from .tar.gz archives

SDTS DEM to ArcView Grid Conversion Utility - This ArcView extension, which predates the ArcView 3.2 utility, enables the user to import SDTS format grid digital elevation data directly into ArcView with Spatial Analyst.

sdts2dem: a Translator for SDTS DEM Files - sdts2dem translates USGS DEM files from SDTS format to "native" [.DEM] format. It is in the public domain and may be freely redistributed. So what do SDTS format DEM data look like? Well, if you've downloaded data and you now have a bunch of files with the .DDF extension these are SDTS data.

BLM SDTS Utilities - thsi ftp site hosted by the BLM provides access to a number of utilities and scripts to help you work with SDTS format data.

SDTS++ - SDTS++ is a C++ toolkit that programmers can use to write applications that can read or write SDTS datasets. Application developers can use SDTS++ library classes to work with the logical structure of these datasets without having to worry about the physical details of each dataset.

Glenn