DEM Data Format

USGS standards for Digital Elevation models1. Overview USGS is the abbreviation of the U.S. Geological Survey (U. S. Geological Survey). USGS is responsible for managing the collection and distribution of Digital Map Data nationwide. 1.1 types of usgs dem data products (1) 7.5-cent DEM (generally 30 m grid spacing, UTM projection), horizontal grid spacing can be any integer between 1-30. The DEM range is approximately a standard USGS 7.5-point geographic grid without overlap. (2) The 30-minute DEM (2 × 2 second spacing) DEM ranges from 30 to 30 minutes to a standard USGS without overlap. (3) the range of 1-degree DEM (2 × 2 second spacing) DEM is approximately a standard USGS 1 degree × 1 degree geographic grid without overlap. (4) 7.5-minute Alaska DEM (1 × 2 seconds spacing, latitude, longitude ), the range is basically the same as that of the 7.5-minute DEM except that the longitude element changes from the 10-minute at the southernmost point to the 18-minute at the southernmost point. (5) 15-minute Alaska DEM (2 × 3 second spacing, latitude, longitude), covering at the southernmost tip of Alaska is 15 minutes (latitude) × 20 minutes (longitude ), the longitude range at the northern end is changed to 36. 1.2 The usgs dem file in usgs dem format consists of logical records A, B, and C. The first part is the file header record type A, which mainly records information related to DEM data; the second part is section data type B, which is divided into Section Head data and DEM data entities. The third part is the precision information type C, which can be omitted. The usgs DEM data is stored in ASCII codes. For details about the format of logical records A, B, and C, see Appendix 1, 2, and 3. Logical records A, B, and C all use 1024 bytes in length as the unit of logical records. If the length is less than 1024, spaces are used for filling. Logical record B generally contains multiple 1024-byte logical record units. To effectively use space to form a physical record unit (4096 bytes) every four logical record units ). 2. DEM Data Structure usgs dem mainly uses two types of grids, UTM projection and grid with geographical coordinates in seconds. This section describes the grid data structure in seconds. A typical DEM Data Structure in seconds is 1-2. The data coverage area is a geographical rectangle. The four coordinate points of DEM data are recorded in logical record a. For details, see Appendix 1. The coordinates of the starting point of each section are recorded in logical record B. For details, see appendix 2. These coordinates describe the rectangular shape of DEM data and the starting point coordinate of each section. The preceding Second-based DEM rules apply to all DEM data except the utm dem with 7.5 points. 3. usgs dem quality control information level 1 level 1dem data usually adopts the standard record format. The data is usually 7.5-minute DEM, and the data is usually collected by aerial photography, generally, the root mean square error RMSE of DEM data must not exceed 7 meters, the maximum error cannot exceed 15 meters, and the maximum error cannot exceed 50 meters. Level 2 level 2dem data is usually compiled. The maximum allowable root mean square error is 1/2 of the contour line spacing, and the maximum error is 1 margin spacing. Level 3 level 3 DEM data usually comes from line drawing. the maximum allowable root mean square error is 1/3 of the contour line spacing, and the maximum deviation is 2/3 of the contour line spacing. The USGS (US Geological Survey) DEM (Digital Elevation Model) format describes the grating Earth's surface (fundamentally specialized elevation scanning heightfield. It is developed by USGS's own SDTS format, but the remains of the format are generally supported by mature software because it replaces a large number of legacy files, self-contained, relatively simple domain structures and broad. The term \ 'dem \ 'is used to indicate that DEMs has many formats (similar to how \ 'image or \ 'audio/video \' has many formats ). Examples include bil, geotiff, XYZ, STM, STL, NTF, gtopo30, etc. Some image formats and "repurposing" Interpretations treat them as raster elevation data, such as the explanation of the POV light heightfield red/green RGB channel. The gray polar map bitmap regards heightfields as more common. "Usgs dem", however, as described in the USGS file "Standard for Digital Elevation shaping" tends to mean a particular format with a precisely specified structure, \ 'Format Structure the usgs dem format is independent (unique file) and is classified into three record categories: A, B, and C. there is a 1024-byte block of ASCII encoding (text) that does not have cross-platform ambiguity. Because the line-end control code is not used, and all data, including numbers, represents readable text. There is no known binary similarity in the format, although it is common practice to compress files with gzip. Floating Point Numbers are input using FORTRAN scientific notation, so the C/C ++ project needs to exchange "D" square times to indicate that the characters are parsed as "E" (and vice versa ). The A record appears once as the file header, the C record appears once as the trailer, and multiple B records (called shapes) include elevation data. A and C records each suitable within a block, but a unique B record typically requires multiple blocks. When this blocking occurs, the data is transferred to the clean place and begins at each block boundary. Record and record "old" and "new" because USGS adds several fields to a record. Fields in a record hold origin, type, summary statistics and measurement systems by shape used. One of the key items is the Quadrilateral, which is a set of four Earth coordinates that describe the areas of interest of the rectangular polygon. The B record (SHAPE) is a grating located above sea level. A variable-length vertical column starts at a specified location. They have long been a multiple of a certain 1024 bytes and a summary of the shape of the small inverted diving. The altitude is adjacent; the break or other break is expressed using "value-32767. null" the altitude is described as a six-character readable integer occupying a fixed location in the block. Reverse shape diving only appeared in the first segment, so the subsequent sections held more altitude values. When you read the DEM file from the first byte pair, you read the shape as a column from the west to the east. The altitude runs from south to north within the shape. The location where the shape changes and variable-length are essentially extracted from the use of UTM (Universal tranverse Mercator) ground reference systems. Because the measurement uses a fixed Distance Within UTM (that is, between 30 meters at the altitude of the sample), the Quadrilateral must be slightly deformed to map such a location to a spherical earth. This distortion usually occurs when a rotated square, so the elevation column starts north and stops a small number of samples near the east and west edges.