Orthophotography is the technique whereby aerial photographs are first "stretched" and then mosaiced together to make a true map.
The process is complicated because a number of factors that are inherent in all raw photography have to be removed if an accurate map is to be produced. The main problems to be overcome are :-
Height distortion - Anyone who has looked at an aerial photograph showing a tall building will have noticed that on the side closest to the centre you can see the whole wall of the building and not just the roof. For a map (which is a vertical view) you want just the base (or footprint) of the building. Well, what goes for buildings, goes for hills and mountains in spades. Positioning errors due to height distortion can easily be 20%.
Photograph geometry - A photograph is a radial projection. That is a fancy way of saying that the only things you can reliably measure on a photograph are angles from the centre and even that is only true if the photograph is taken perfectly parallel to the ground. Of course that happy state never exists and every photograph is rotated, tipped and tilted to a greater or lesser extent and the effect of that has to be removed.
Camera characteristics - To make a map one is simply measuring visible things on an aerial photograph. Using digital techniques one can reliably make those measurements with an accuracy of about 0.02 mm. However, if the exposure was taken over 3 km up in the air and we want the map accurate within 1 meter, we have to know the details of the camera (such as the focal length and symmetry) ten times better than we can measure on the photograph. In other words to 0.002 of a mm!
Miscellaneous errors - It will be appreciated that with these sort of accuracies there are a host of other nasties that creep in. Lens distortion is a bad one but it is constant and once it is known it can be eliminated. Film instability due to temperature change and stretches from bad handling is not a constant and causes problems. There are other lesser contributors such as atmospheric refraction and earth curvature but the effects of these are normally minimal.
The advent of high speed computing and digital image handling has made so called Soft Copy Photogrammetry (the process to produce a digital orthophoto) quicker, simpler and more automated than it used to be. However the key to the overwhelming advantage of digital orthophotography lies in the ability of the computer to take in a pair of stereo images and automatically derive the critical ground heights of that terrain. Although the mathematics to solve this problem has been known for more than fifty years, the technology to actually do it has only become available in the last four or five years.
Aerial Photography is commissioned to physically photograph the area of concern. Strips of photography are flown with 60% overlap and 25% sidelap. This 'stereo coverage' provides the information required to produce an orthophoto. Image Capture is the next step in the process. This involves scanning of the photographs into a computer readable format. For mapping purposes one requires a scanner that precisely translates what is on the film to the same position on the digital image. Normal scanners cannot do this but our Vexcel scanner is one of the three types in the world that is designed for this purpose. We are also the South African agents for these scanners. Ground Control is then conducted by sending our survey team to the area. Depending on the area and the size of the job they may travel by 4x4 vehicle or helicopter (whichever is the more economic). Their job is to identify a set pattern of features on both the photograph and on the ground and then survey them using precision GPS. This is essential in order to provide a frame of reference to a known geographic coordinate system and to make sure the heights are related to mean sea level. Once these data are collected and correlated, a mathematical model of the surface of the ground is created in the computer. The process used to perform this is called Image Correlation and the result is an accurate grid of heighted points called a Digital Terrain Model (or DTM). The spacing of the points varies from job to job but typically might be in the 5 to 20 meters range. The final rectification of the original images is conducted one picture element (pixel) at a time by using the height model to work out each correct position and little by little a new (now orthogonal) photograph is built. To extract contours from the DTM and add that information to the image is a fairly trivial additional process. Digital images create huge data sets and in our office we now have nearly a Terrabyte (1000 Giga-Bytes) of hard drive storage capacity. These sets also require some serious computing horse power. The workstations we use are produced by Silicon Graphics and of a similar type to those used in the production of movies like Jurassic Park . Although they are at the bottom of the Silicon Graphics range these machines have about twice the processing power of the very best dual Pentium III. It must be emphasized that the result you get from this process is completely different from the result from what is known as a 'rubber sheet warp'. In that process the photograph is physically stretched to some known reference points like an old map, in an attempt to achieve some sort of scale. Unless the ground is dead flat the accuracy of this process is obviously poor. Despite these drawbacks a warped image does also have its place for quick information and map detail updates. Look at the Digital Imaging page to see how we use image warping to produce composite mosaics with our digital camera.
Proper photogrammetric quality products available from systems are :-
Digital Elevation and Digital Terrain Models in virtually any format. Digital Orthophotos at any scale deliverable as paper plots or as TIF images on CDROM (which can be loaded as backdrops into most CAD/GIS packages). Elevation contours as separate TIF images and DXF files (which can be plotted as an overlay on the orthoimage). Slope and aspect maps with any defined bands of interest. Perspective 3 dimensional views and "Cyber City fly through" virtual reality scenes of built up areas. 
Orthophotography