Uses for Digital Orthophotos

A digital orthophoto can be used for various applications. The DOQ serves as an excellent
cartographic base upon which associated digital layers can be overlaid. The accuracy and
extraordinary detail provided by the DOQ allow users to evaluate their data for accuracy and
completeness, make real-time modifications to their data, and even generate new files. Other
applications include vegetation and timber management, routing and habitat analysis,
environmental impact assessments, emergency evacuation planning, flood analysis, soil erosion
assessment, facility management, and groundwater and watershed analysis.

Producing a Digital Orthophoto

The following items are required to produce a digital orthophoto:

   1. a minimum of three ground positions that can be identified on the photograph to be rectified, 
   2. camera calibration parameters, such as the calibrated focal length and the coordinates of the
camera fiducials, 
   3. a digital elevation model (DEM), and 
   4. a digital image produced by scanning an aerial photograph with a precise, high-resolution
scanner. 

The digital image is rectified to generate an orthophoto by processing items 1 through 3 for each
image picture element (pixel) using rigorous photogrammetric equations on a high-speed
computer. The finished product is a spatially accurate image with planimetric features
represented in their true geographic positions.

A DOQ can be incorporated into any geographic information system (GIS), which can
manipulate raster images. It functions exceptionally well as a cartographic base for displaying,
generating, and modifying digital planimetric data.

Characteristics of a Standard Digital Orthophoto 

The standard digital orthophoto produced by the U.S. Geological Survey (USGS) is a
black-and-white or color-infrared digital image map. Each DOQ covers 3.75-minutes of
longitude and 3.75-minutes of latitude. It is referenced to the North American Datum of 1983
and is cast upon the Universal Transverse Mercator projection. Each pixel represents 1 square
meter on the ground. Every DOQ has between 50 and 300 meters of overedge image beyond the
primary and secondary quadrangle corner ticks. This overedge facilitates tonal matching and
mosaicking of adjacent images.

Image mosaicking techniques can be used to combine two or more rectified images into one. For
example, a 7.5-minute DOQ can be produced by mosaicking four 3.75-minute DOQ's.

The standard DOQ is formatted with an ASCII keyword header, followed by a series of 8-bit
binary image data records. The radiometric image brightness values are stored as 256 gray levels.
The file size of a black-and-white digital orthophoto can approach 55 megabytes. 

The header record provided in the DOQ contains a wide range of descriptive information about
the image data, including photographic source type, date, software systems used to create the
DOQ, and the production date of the DEM metadata used in the orthophoto rectification process.
Primary and secondary datum coordinates for the upper left pixel are included in the header
record to allow users to spatially reference other digital data with the DOQ.

The accuracy and quality of USGS digital orthophotos must meet National Map Accuracy
Standards at 1:12,000 scale for 3.75-minute quarter quadrangles and at 1:24,000 scale for
7.5-minute quadrangles. Accuracy and quality depend on the following: 

     photographs that meet National Aerial Photography Program standards, which are
quarter-quadrangle centered. The photographs are exposed at a flying height of 20,000 feet above
mean terrain and are taken using a wide-angle camera; 
     a DEM with the same area coverage as the digital orthophoto that is equal to or better than a
level-1 DEM having a root-mean-square error in elevation no greater than 7 meters; 
     a highly accurate image scanning process that employs a scanning aperture between 7.5 and
30 micrometers (µm). A 1:40,000-scale image scanned with a 25-µm aperture produces a ground
sample distance of 1 meter; 
     and identifiable ground control positions with coordinates acquired from ground surveys or
aerotriangulation.