Many different types of Raster Data are available from ICREST. The following definitions broadly describe the four main groups.

Satellite Imagery

Electromagnetic energy captured by sensors on board a satellite in Earth orbit provides representation of the measurement of energy emitted or reflected by the Earth in a variety of wavelengths. Earth observation imagery takes a number of forms, of which the most traditional are optical and near-infrared radiation, from about 0.4 (blue) to 2.0 (IR) micrometers. Apart from visual and near-infrared, other bands of the spectrum commonly used include thermal infrared (heat) and microwave (radar). Each of these has its own applications. There are very many satellites carrying an wide array of sensors. Traditionally, most of these were developed, launched and operated under Government control. In more recent years, commercially built and launched satellite platforms and sensors have become more common. Please see the Sensors area of this web site for more information and examples images.

Aerial Photography

Aerial Photography can be analog or digital imagery taken from an airplane. The optical axis is oriented perpendicular to the earth's surface so that the film or sensor is parallel to the surface being photographed. (also Vertical Aerial Photography). A sequence of aerial photographs may overlap so the photos can be used in stereoscopic analysis (stereo pairs). The overlap is referred to as 'endlap' (top-to-bottom area in common, same flightline) and 'sidelap' (side-to-side area in common, different flightlines). Aerial photography can be monochrome or color in the visible spectrum or can sense radiation outside the visible spectrum for example in the infrared band of the electromagnetic spectrum.

Uses
Aerial photography has many different uses for example cartographers and planners take detailed measurements from aerial photos in the preparation of maps or trained interpreters utilize aerial photos to determine land-use and environmental conditions, among other things.

Although both maps and aerial photos present a "bird's-eye" view of the earth, aerial photographs are NOT maps. Maps are orthogonal representations of the earth's surface, meaning that they are directionally and geometrically accurate (at least within the limitations imposed by projecting a 3-dimensional object onto 2 dimensions). Aerial photos, on the other hand, display a high degree of radial distortion. That is, the topography is distorted, and until corrections are made for the distortion, measurements made from a photograph are not accurate. Nevertheless, aerial photographs are a powerful tool for studying the earth's environment.

Because most GISs can correct for radial distortion, aerial photographs are an excellent data source for many types of projects, especially those that require spatial data from the same location at periodic intervals over a length of time. Typical applications include land-use surveys and habitat analysis.

Aerial photography provides many benefits and serves many applications especially when properly integrated into a Geographic Information System.

Example Product
Digital Orthophoto Quadrangle - Aerial photography of the Earth's surface, usually provided in GeoTIFF form, but also seen in other formats. A DOQ for a 7.5-min., 1:24000-scale quad is very large, thus the common use of DOQQs--digital orthophoto quarter quadrangles.

Digital Elevation Model (DEM)

A Digital Elevation Model is a "grid" of points (usually on an even spacing) that contains both horizontal and vertical data about the earth's surface. DEMs are commonly used for the creation of "ortho" photos.

1. A digital representation of a continuous variable over a two- dimensional surface by a regular array of z values referenced to a common datum. Digital elevation models are typically used to represent terrain relief. Also referred to as 'digital terrain model' (DTM).

2. An elevation database for elevation data by map sheet from the National Mapping Division of the U.S. Geological Survey (USGS).

3. The format of the USGS digital elevation data sets.

An alternative definition could be: A model of terrain relief in the form of a MATRIX. Each element of the DEM is regarded as a node of an imaginary grid. The gird is defined by identifying one of its corner (lower left usually), the distance between nodes in both the X and Y directions, the number of nodes in both the X and Y directions and the gird orientation.

Digital Raster Graphic

Digital Raster Graphics are raster images of standard quadrangle format USGS topographic maps. A DRG is a raster image of a scanned USGS topographic or planimetric map including the collar information, georeferenced to the UTM grid. A DRG is useful as a source or background layer in a GIS as a means to perform quality assurance on other digital products, and as a source for the collection and revision of vector data.

Within the 1:24,000 and 1:25,000 series are included Provisional (P) maps as well as Orthophotomaps. DRG's retain the geometric and visual qualities of the source maps in all respects with two exceptions; DRG's are referenced to the UTM grid, and the colors of the DRG may differ slightly from the original printed map. They serve a variety of purposes, from use as backgrounds in a geographic information system (GIS) to source for collecting or revising USGS base cartographic data. The DRG shows potential as a tool for validating and assessing other non-USGS digital data. DRG's can be combined with DOQ's, DEM's, and DLG's, or combinations of each to create several "hybrid" products, each with its own unique applications. DRG prototypes have been utilized as "browse images" on USGS data management systems, and may serve similar functions in production systems and sales databases. While a DRG can be made from any map, the DRG program is oriented primarily toward the production of digital raster graphics from 1:24,000 and 1:25,000 USGS topographic maps to be used in the validation and revision of digital line graph data. DRG's adhere to the USGS NMPTI Standards for Digital Raster Graphics.