Digital Orthophotography: A Picture Worth a Thousand Vectors By J.D. Wilson In the GIS arena it may be said, a picture is worth a thousand vectors. At least you might think so considering the explosive growth of digital orthophotography in recent years. Digital orthophotography (orthos) currently is one of the fastest growing GeoTechnologies. It emerged nearly 10 years ago, when mapping companies recognized the possibility - and potential business opportunities - of scanning aerial photos and incorporating them into the GIS as a backdrop behind the vector-based layers.
      They provide a map-accurate photographic image that is digitally corrected for systematic errors that may occur in various steps of the process, like tip and tilt of the aircraft, stretch in the film, even distortions created by the shape of the ground.
      The appeal of orthos lies in the fact that they are actual pictures of the Earth. "When you view a vector display, you're looking at an image that has been filtered through someone else's interpretation," explained Robert Fowler, marketing manager, Digital Mapping/Orthoimagery Group, for Intera Information Technologies Inc., Calgary, Alberta. "Whoever digitized the map, viewed the aerial photos and made decisions about what to digitize and what to omit. With digital orthophotos, individual users can view the source images and make their own interpretation, based on their specific needs at the time."
      These raster images are still 'dumb' images, however. The user can view them and interpret them, but the computer cannot perform any of the automated analyses for which GIS was intended. Nevertheless, they do provide a complete image of an area that could never be fully vectorized.
      Carrying non-critical data in raster form can reduce the costs associated with data acquisition. "It isn't fiscally practical to digitize everything you might need into a vector layer," Fowler said. Orthos can replace a lot of vectorized data, such as building outlines, fences, trees and shrubs.
      These types of data themes - for which no GIS analysis tool may be necessary or available - can remain in the raster image for reference and can be digitized later if a need arises. Still, it may be sufficient just to browse through a set of maps and visually pick out trees that may be encroaching on overhead powerlines.

Back