Digital Orthophotography in an Ortho-Rectified, Geo-Corrected Nutshell Paige Baldassaro Geospatial Applications Developer Geospatial Extension Program

First, A Word From Our Sponsor

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

Example Images Courtesy of: Computer Terrain Mapping, Inc., 2000

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

Error Caused By Tilt

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

Error Caused By Relief Displacement

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

USGS Status Graphics Map

Status Graphics of Virginia Example

Questions? Demo? Demo?