What is Digital Imagery?
A Digital or raster imagery file is comprised of an array of pixels
having varying shades or tones which together define shapes. Television
and computer monitors use raster technology to project shapes.
An orthophoto image is a plan view (vertical view) image where distortions caused by vertical relief, atmospheric refraction, camera lens distortion and aircraft movements have been corrected. Image displacement due to variance in terrain relief is also removed by differentially rectifying the image to the terrain surface model or (DEM). The image can be produced to a defined scale allowing for accurate measurement of distances and angles in a given projection system.
With digital ortho-imagery, both hardcopy and softcopy data is available. The image can be enhanced to accentuate contrast and brightness as well as to bring out minor variances in shaded areas which may otherwise be imperceptible.
Raster image data is usually derived from one of two sources.- The scanning of black and white or colour aerial photography.
- Remote multispectral sensors mounted on either a satellite or aircraft.
The black and white digital image is defined as 8 bit or 256 grey tone levels where "0" is black and "255" is white. One pixel is assigned one grey level. A colour image can be produced with red, green and blue (RGB) each having 256 possible tones. One pixel is defined by one set of RGB tones. The resulting file is three times larger than a black and white file.
Orthophoto mosaics can be produced by merging several images with image tone and geometry maintained over the entire project thus producing seamless images and data.
The pixel resolution of the final image is an important factor to consider when ordering an orthomap. Increasing image resolution will increase the information content of the image, however the file size of the image is also increased. A typical requirement where information such as street painted lines are required would be as follows:
- 1:10,000 scale air photo scanned at 24 micro-metre pixel resolution
- 1:2,500 scale final ortho-map scale having a 25 cm ground scale pixel size
- map area of 1700 m x 1700 m.
- Data file size approximately 50 Megabytes
Perspective views and fly throughs can readily be produced as a by-product of digital imagery. Parameters such as viewpoint position, view angle and field of view are incorporated with the Digital Terrain Model (DTM) and digital image to render a single scene or succession of scenes.
Hardcopy products can either be on photographic print material or reproducible mylar for blue prints. The hardcopy offers portability, familiarity and ease of reproduction.
Quick prints can be produced on standard laser-jet or ink-jet printers and plotters via desktop publishing software.
Softcopy includes digital imagery on tapes or CD ROM disks. Softcopy is an inexpensive component to incorporate into a G.I.S database. The imagery can be used as a base for existing vector data and also as a base for extraction of new data. Using this imagery for database update can improve the efficiency and accuracy of land use management processes.
Hardware upgrades for handling raster data require a minimum one Gigabyte hard drive on 486 PC platform with at least 16 Mb of RAM memory. Most graphics and GIS packages have raster data manipulation modules which are useful for clipping areas, adding vector data and producing printable files.