The USGS DEM standard is a

geospatial Geographic data and information is defined in the ISO/TC 211 series of standards as data and information having an implicit or explicit association with a location relative to Earth (a geographic location or geographic position). It is also call ...

file format A file format is a Computer standard, standard way that information is encoded for storage in a computer file. It specifies how bits are used to encode information in a digital storage medium. File formats may be either proprietary format, pr ...

developed by the

United States Geological Survey The United States Geological Survey (USGS), founded as the Geological Survey, is an agency of the U.S. Department of the Interior whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on Mar ...

for storing a

raster file:Rgb-raster-image.svg, upright=1, The Smiley, smiley face in the top left corner is a raster image. When enlarged, individual pixels appear as squares. Enlarging further, each pixel can be analyzed, with their colors constructed through comb ...

-based

digital elevation model A digital elevation model (DEM) or digital surface model (DSM) is a 3D computer graphics representation of elevation data to represent terrain or overlaying objects, commonly of a planet, Natural satellite, moon, or asteroid. A "global DEM" refer ...

. It is an

open standard An open standard is a standard that is openly accessible and usable by anyone. It is also a common prerequisite that open standards use an open license that provides for extensibility. Typically, anybody can participate in their development due to ...

, and is used throughout the world. It has been superseded by the USGS's own SDTS format but the format remains popular due to large numbers of legacy files, self-containment, relatively simple field structure and broad, mature software support.

DEM Level

A USGS DEM can be classified into one of four levels of quality. This is due to the multiple methods of data collection, and certainty in the data.

Format Structure

The USGS DEM format is a self-contained (single file) set of ASCII-encoded (text) 1024-byte (1024 ASCII chars) blocks that fall into three record categories called A, B, and C. There is no cross-platform ambiguity since line ending control codes are not used, and all data including numbers is represented in readable text form. There is no known binary analogue of the format, although it is common practice to compress the files with

gzip gzip is a file format and a software application used for file compression and decompression. The program was created by Jean-loup Gailly and Mark Adler as a free software replacement for the compress program used in early Unix systems, and ...

. Floating-point numbers are encoded using Fortran scientific notation, so C/ C++ programs need to swap the "D" exponent-indicating character with "E" when parsing (and vice versa when writing). A record fields hold the origin, type, summary statistics and the measurement systems used by the profiles. The A record appears once as the file's header, the C record also appears once as the trailer, and multiple B records (called profiles) comprise the elevation data. A and C records each fit within one block, but a single B record typically requires multiple blocks. When such block-spanning occurs, data are shifted to start cleanly on each block boundary. A records also come in "old" and "new" flavors, because the USGS added several fields to the A record. One of the key items is the quadrangle, which is a set of four terrestrial coordinates describing the four-sided polygon enclosing the area of interest. {, class="wikitable" , +A Record Layout:^{Incomplete – please contribute—full details are in the Appendix of http://nationalmap.gov/standards/pdf/2DEM0198.PDF} , - ! Starting Col ! Ending Col ! Description , - , 0 , 134 , Descriptive Name of the represented area , - , 150 , 155 , ? , - , 156 , 161 , ? , - , 162 , 167 , UTM Zone number , - , 529 , 534 , Unit of resolution of ground grid (0=radian;1=feet;2=metre;3=arc-second) , - , 535 , 540 , Unit of resolution Elevation (1=feet;2=metre) , - , 546 , 569 , Easting of the South West corner , - , 570 , 593 , Northing of the South West corner , - , 594 , 617 , Easting of the North West corner , - , 618 , 641 , Northing of the North West corner , - , 642 , 665 , Easting of the North East corner , - , 666 , 689 , Northing of the North East corner , - , 690 , 713 , Easting of the South East corner , - , 714 , 737 , Northing of the South East corner , - , 738 , 761 , Minimum elevation found in this file , - , 762 , 786 , Maximum elevation found in this file , - , 816 , 827 , Resolution per grid cell East – West , - , 828 , 839 , Resolution per grid cell North – South , - , 858 , 863 , Number of columns B records (profiles) are a variable-length longitudinal column of raster elevations that start at a specified location. They are some multiple of 1024 bytes long and contain a small header summarizing the profile. The elevations are contiguous; breaks or other discontinuities are expressed using "void" elevations of value -32767. Each elevation is described as a six-character readable integer occupying a fixed location in a block. The profile header only appears in the first block, so subsequent blocks hold more elevation values. When reading the DEM file from first byte to last, one reads the profiles as columns from west to east. The elevations within a profile run from south to north. The variable-location and variable-length nature of profiles stems mainly from the use of the UTM (

Universal Transverse Mercator The Universal Transverse Mercator (UTM) is a map projection system for assigning coordinates to locations on the surface of the Earth. Like the traditional method of latitude and longitude, it is a horizontal position representation, which mean ...

) ground reference system. Since measurements within UTM employ fixed distances (e.g., 30 meters between elevation samples), the quadrangle must slightly distort to map such locations onto the spherical Earth. This distortion usually manifests as a rotated square, hence the elevation columns near the east and west edges start more northward and contain fewer samples. C records contain root-mean squared error (RMSE) quality control data, using ten six-character integer fields.

DEM Level

Format Structure

External links