1. Digital Orthophotography in an Ortho-Rectified, Geo-Corrected Nutshell Paige Baldassaro Geospatial Applications Developer Geospatial Extension Program pmb@vt.edu http://www.cnr.vt.edu/gep

2. First, A Word From Our Sponsor http://www.cnr.vt.edu/gep

3. 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

4. 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

5. 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

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

7. 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

8. Error Caused By Tilt

12. 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

13. Error Caused By Relief Displacement

14. 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

15. 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

16. USGS Status Graphics Map http://statgraph.cr.usgs.gov/viewer.htm

17. Status Graphics of Virginia Example

18. Questions? Demo? Demo?