Among the products created and distributed by the USGS and shown on TerraFly are Digital Orthophoto Quadrangles (DOQs), Landsat, and Topographical Maps.

DOQ Description

A DOQ is a computer-generated image of an aerial photograph. It has been orthorectified--altered so that it has the geometric properties of a map.

DOQ's meet National Map Accuracy Standards, thus the user can measure distances accurately on a DOQ. The standard DOQ from the U.S. Geological Survey is a black-and-white (gray-scale) or color-infrared image covering 3.75 minutes of latitude by 3.75 minutes of longitude. Thus, four such photos can be combined, or mosaicked, to cover the area represented by a standard USGS 7.5-minute, 1:24,000-scale topographic map. Mosaicking is facilitated by the fact that the images overlap. The DOQ's are referenced to the North American Datum of 1983 and use the Universal Transverse Mercator projection. Their resolution is such that each pixel represents a square meter.

The standard gray-scale DOQ includes an ASCII keyword header followed by a series of 8-bit binary image lines. The average file size for a black-and-white DOQ is 55 megabytes . Header information includes photographic source type, date, production software, and date of the digital elevation model metadata used in orthorectification. Also, primary and secondary datum coordinates for the upper left pixel are included to assist in referencing other digital geospatial data. To view or manipulate one of these images, the user must extract the header information from the image and be equipped with editing software designed to handle large image files. A minority of the DOQ's are not gray scale, but color-infrared, a false-color scheme used for certain scientific and analytic purposes. These files average about 150 megabytes and are stored in band-interleaved-by-pixel, or BIP, format.

DOQ Applications and Uses

DOQ applications include land and timber management, routing and habitat analysis, environmental impact assessment, evacuation planning, flood analysis, soil erosion assessment, facility management, and groundwater and watershed analysis. The accuracy and extraordinary detail of DOQ's allow users to evaluate their data for accuracy and thoroughness, to modify their data promptly, and even to generate new files.

A DOQ can be used in a wide variety of geographic information systems and is an excellent cartographic base on which to overlay any number of associated thematic layers for displaying, generating, and modifiying planimetric data or associated data files.

Landsat Description

The ETM+ instrument is an eight-band multispectral scanning radiometer capable of providing high-resolution image information of the Earths surface. It detects spectrally-filtered radiation at visible, near-infrared, short-wave, and thermal infrared frequency bands from the sun-lit Earth. Nominal ground sample distances or "pixel" sizes are 49 feet (15 meters) in the panchromatic band; 98 feet (30 meters) in the 6 visible, near and short-wave infrared bands; and 197 feet (60 meters) in the thermal infrared band.

Landsat Applications and Uses

The Landsat 7 applications include Agriculture, Forestry, Range Resources, Land Use and Mapping, Geology, Water Resources, Coastal Resources, and Environment.

Topographical Map Description

One of the most widely used of all maps is the topographic map. The characteristic that distinguishes topographic maps from other maps is the use of contour lines to portray the shape and elevation of the land. Topographic maps render the three-dimensional ups and downs of the terrain on a two-dimensional surface.

Topographic maps usually portray both natural and constructed features. They show and name natural features, including mountains, valleys, plains, lakes, rivers, and vegetation. They also identify the constructed features, such as roads, boundaries, transmission lines, and major buildings.

The wide range of information provided by topographic maps makes them extremely useful to professional and recreational map users alike. Topographic maps are used for engineering, energy exploration, natural resource conservation, environmental management, public works design, commercial and residential planning, and outdoor activities like hiking, camping, and fishing. The U.S. Geological Survey (USGS) produces topographic maps at several different scales to meet these various needs.