INTRODUCTION
Orthophotos combine the image characteristics of a photograph with the geometric qualities of a map. They serve a variety of purposes, from interim maps to field references for Earth science investigations and analyses. The digital orthophoto is useful as a layer of a geographic information system (GIS) and as a tool for revision of digital line graphs and topographic maps.

Unlike a standard aerial photograph, relief displacement in orthophotos has been removed so that ground features are displayed in their true ground position. This allows for the direct measurement of distance, areas, angles, and positions. Also, an orthophoto displays features that may be omitted or generalized on maps.

The geographic extent of the digital orthophoto is equivalent to an orthophoto quarter-quadrangle (3.75-minutes of latitude and longitude), plus a minimum of 50 meters to to a maximum of 300 meters of overedge is included, sufficient to offer coverage to encompass the four primary and secondary horizontal datum corner points.

In order to assure that the image brightness values of the orthophoto closely portray the source imagery, very little image enhancement, other than a limited amount of analog dodging, is performed when preparing the photograph for scanning. Some deviation of brightness values may also occur during the scanning and rectification processes. Radiometric accuracy and quality are verified through visually inspecting and comparing the digital orthophoto to the original unrectified image.

The DOQQ data may be combined with other geographically referenced data to conduct automated analyses in support of various decision-making processes. These digital cartographic and geographic data may also be used as one layer in a geographic information system (GIS), as a tool for various kinds of spatial analyses, and as information for plotting base maps.

Raster data such as the DOQQs, rather than vector data, may be more effectively used for some applications. Much like the symbology on a topographic map, vector data tend to be more generalized (i.e., most ground features are distinguishable using raster data, while the vector data, only selected, cartographically significant information may be shown). Also, it may be more economical to produce raster images; however, DOQQs are not meant to be replacements for vector data.

Source: USGS EROS Data Center

INTERACTIVE MAP
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