Editing USGS DEM (section)

==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]].

Floating-point numbers are encoded using [[Fortran]] scientific notation, so [[C (programming language)|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 (geography)|quadrangle]], which is a set of four terrestrial coordinates describing the four-sided polygon enclosing the area of interest.

{| class="wikitable"
|+A Record Layout:<sup>Incomplete – please contribute—full details are in the Appendix of http://nationalmap.gov/standards/pdf/2DEM0198.PDF </sup>
|-
! 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) <!-- added resolution --~~~~ -->
|-
| 535
| 540
| Unit of resolution Elevation (1=feet;2=metre) <!-- added resolution -->
|-
| 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 <!-- Changed meters per grid to resolution per grid -->
|-
| 828
| 839
| Resolution per grid cell North – South <!-- Changed meters per grid to resolution per grid -->
|-
| 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]]) 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.