# GRASS Help System

## INTRODUCTION: EXAMINING, EXTRACTING, AND ROTATING DIGITAL DATA

For data to be used by GRASS programs, it must be stored in GRASS's digital format in a GRASS database. Users can enter digital data into a GRASS database by: 1) digitizing or scanning hardcopy data (like paper maps) into GRASS's vector file format; 2) converting data already available in another digital format (like ARC-INFO data) into GRASS's digital data file format, and copying this data into a GRASS database; and 3) using GRASS analysis programs to combine and manipulate existing digital data already in GRASS format to create new GRASS data.

Many types of data already are available in a digital format. These digital data can be extracted from the media on which they are stored, converted to GRASS raster, vector, or sites data format, and placed into different kinds of digital data. Several GRASS commands extract data from magnetic tape. Discussed below are two types of data which are stored on tape and must go through the extraction process.

Elevation Tape Data

The U.S. Geological Survey (USGS) and the Defense Mapping Agency (DMA) distribute digital elevation data at several resolutions and in different tape formats. These data are referred to as digital elevation models (DEMs) and digital terrain elevation data (DTEDs). Specific GRASS programs are available to extract this data from 1/2" magnetic tape, list tape contents, rotate the data, and "flip" the data.

The various elevation data products available from these agencies are discussed in "Options for Acquiring Elevation Data" (Bradshaw and Thompson, 1988). Means by which users can convert digital elevation data produced by the DMA and the USGS into GRASS raster format are described in the paper "DTED and DEM Elevation Data Extraction" (Bradshaw, Martin and Kos, 1989). Both of these papers can be acquired from the GRASS Information Center at USACERL.

Imagery Data

Several GRASS commands are also available to extract specific forms of satellite imagery data from magnetic tape. They extract LANDSAT multi-spectral scanner (MSS), Thematic Mapper (TM), and other (e.g., SPOT) satellite data, as well as header information, from tape. Procedures for extracting these data from tape and importing them into a GRASS data base are described in the "GRASS Imagery Tutorial" (Harmon and Shapiro, 1990) distributed by the GRASS Information Center.

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Automatically created on: Wed Oct 12 14:08:40 2005