WSDOT Photogrammetry makes two principal products which are derived from aerial photography, ground-based laser scanning, and accurate field control surveying. Those two products are 3-D computer aided drafting & design (CAD) files and georeferenced orthophotos. Both are mainly for use in computer software environments.
A. CAD files are similar to traditional maps, consisting of linear and point features which have been precisely measured from the aerial photos or laser scans, then represented by stylized lines and symbols in a digital graphics file, or plotted on hardcopy media.
B. Orthophotos are made by combining the 3-D CAD data with very precise digital scanned aerial photographic images. Powerful software applications are used to correct the image, pixel by pixel so that each pixel and feature is in it’s correct geographic position.
Either of these products can be made according to a wide range of specifications for accuracy, precision, and resolution. Both CAD files and Orthophotos are usable in GIS and a variety of other applications. Both can be used for accurate measurement of location, distances, and areas. CAD data can also be used to measure elevations. There are three common approaches to making these products, although many variations are possible.
1. 3-D CAD files for design base maps are made from large scale (low altitude) aerial photos, laser scanning, and very accurate ground control surveys.
2. The type of mapping described in (1) can be enhanced by adding orthophotos. Since the 3-D CAD data and the aerial photos are already done, there is only about a 5-10% additional cost to make the orthos.
3. Rather than doing either (1) or (2) we can do orthophotos only. This requires less labor than is needed to collect the very detailed CAD data for design use, so the cost is less than the cost of CAD mapping. However, the photos and survey used to do a mapping project which is planned to be "orthos only" cannot be used to go back and make precise CAD files for design work. One would have to start over with new photos and ground control as described in item (1).
Planning Photogrammetric Projects:
There are some more factors that should be considered when planning for photogrammetric products. First, in spite of major technological gains in productivity, it is still labor intensive. Urban transportation corridor mapping can take many labor hours per mile depending on corridor width. Second, available resources are always a factor. WSDOT has a few people doing production work in this area, and 3 Pacific Northwest private mapping firms currently under contract to help us do WSDOT mapping work. Finally, weather and time of year determine when and whether we can acquire the images needed for a given product. Orthophotos, for example, are best when the source images are taken with the sun high in the sky during summer, or may need to be done from photos where the foliage is in "leaf-off" condition. Photography for CAD mapping can be acquired on any clear day at any time of the year but accuracy improves with good image quality. Laser scanning can be done all year, day or night, as long as the ground surface is dry.