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Digital Orthophotography in an Ortho-Rectified, Geo-Corrected
NutshellPaige Baldassaro
Geospatial Applications DeveloperGeospatial Extension Program
[email protected]://www.cnr.vt.edu/gep
A Camera Does Not An OrthoPhoto Make
• An orthophotograph is an aerial photograph that can be used as a map– Requires post
processing by computer
– Planimetric– Measure distances
To Be An Aerial photograph Is Just Not Good Enough
• A conventional aerial photograph contains image distortions caused by– Poor camera optics– Tilting of the camera– Tilting of the terrain (topography)– Distance from the central perspective– Vertical relief
• It does not have a uniform scale• Contains errors in location, distance, and area
To Be An Aerial photograph Is Just Not Good Enough
• A conventional aerial photograph contains image distortions caused by– Poor camera optics
– Tilting of the camera– Tilting of the terrain (topography)– Distance from the central perspective– Vertical relief
• It does not have a uniform scale.
Tilt
Relief Displacement
Tilt
• Cause: the camera and/or terrain are not perpendicular to the photographic plane
• Result: scale is not preserved throughout the image
• Relevance: distances measured on the map are not representative of the true ground distance
Relief Displacement
• Cause: objects and/or terrain with vertical relief are not imaged from the central perspective (nadir)
• Result: objects appear to lean outward from the center
• Relevance: correct planimetric position is not represented on map
DOQ/DOQQ
• DOQ – Digital Orthophoto Quadrangle– Orthophotos prepared in a digital format,
designed to correspond to the 7.5-minute quadrangles of the USGS.
• DOQQ – Digital Orthophoto Quarter Quadrangle– ¼ of a DOQ
Digital Orthophotography Uses
• Effects of tilt and relief are removed from the aerial photograph by a mathematical process called rectification.
• An orthophoto may serve as a base map onto which other map information may be overlain.
• A DOQ can be used on-screen to collect, review, and revise other digital data, especially digital line graphs (DLG) and topographic maps.
• When the DOQ is combined with other digital products, such as digital raster graphics (DRG) or digital elevation models (DEM), the resulting image provides additional visual information for the extraction and revision of base cartographic information.
Courtesy of: USGS