Digital Orthophotography

Ortho, or rectified photography, has the geometric accuracy of a map but contains the immense detail of a photograph. Orthophotos, orthophotomaps and topographic orthophotomaps are all produced at prices often comparable or less than standard topographic mapping, due to reduction in labor component. The popularity of digital orthophotography is growing fast with the advent of more powerful computers and software.

The applications for digital orthos are widespread. One of the most common uses is a backdrop for GIS applications. Contours can also be overlaid on the digital ortho if desired. If planimetric features are desired, the level of detail captured is left up to the client. Some features that are less important may be left out of the planimetric capture, but are still visible on the digital image. Another advantage over conventional orthophotos is the ability to alter the scale of the image and the ability to plot at different scales. A conventional orthophotograph is made by producing a rectified negative from which the hardcopy prints are made. There is no digital image produced with this method. In addition, a very high quality plot may be produced from the digital image using a quality inkjet plotter with high quality paper. These plots are comparable in quality to a photographic enlargement and sometimes less expensive.

The accuracies obtained are approximately the same as conventional mapping using a stereoplotter, which will meet or exceed National Map Accuracy Standards, however the overall accuracy is determined by the scan resolution and the accuracy of the DEM or DTM. For example, a horizontal or vertical measurement of less than one meter can't be obtained from a digital image scanned with a two-meter ground pixel resolution.

In conclusion, digital orthophotography has become a very useful tool that has many advantages over conventional mapping and conventional orthophotography. The main advantage is the ability to manipulate the output and viewing scales and the ability to plot hardcopy prints without the need for expensive photo lab equipment. Also, the introduction of higher-powered computers with huge storage capabilities has placed this technology in the hands of smaller companies and government agencies.

3Di LLC, 28969 Information Lane, Easton, MD 21601. Tel: 800-546-4437; Fax: 410-820-5613.