Truly Useful Orthophotos

True orthophotos are frequently proposed as a means to overcome the obstruction of ground details caused by leaning buildings, which severely curtail the usefulness of orthophotos in built-up areas. However, the required procedures are not only very expensive, but also seem to be a roundabout way to create a map of a building roof, instead of exploiting the new possibilities that technology offers. This article questions the usefulness of true orthophotos and points to a new technology that overcomes the current restriction of orthophotos at a fraction of the cost of true orthophotos, while opening up the usage of image data to a new range of applications.

Rolf Becker

An orthophoto is an image that has been transformed from a central to an ortho­gonal projection. In aerial photography this rectification, based on a digital terrain model (DTM), reverses the image displacement caused by terrain height variations. Provided that an accurate DTM, as well as the elements of the absolute orientation have been used, orthophotos provide a terrain presentation with a geometric accuracy comparable to that of conventional line maps, but with a potentially higher visual information content. Due to their short production time and low cost, orthophotos are in­creasingly replacing conventional mapping for many applications, in particular as important background information in GIS.
However, orthophotos have severe limitations in built-up areas. Buildings (if they are not part of the terrain model) lean away from the projection center of the image (or more accurately the nadir point). The higher the building and the further it is located from the nadir point, the more pronounced the lean. As such, some sides of the buildings are shown while others are hidden, together with much of the detail on the ground. Similarly, this applies to other objects that are not part of the terrain model, for example, trees, towers, sign boards, etc.
The effect of image displacement can be minimized by having the photography flow with a longer focal length, by increasing the overlap, and by using only the central part of overlapping photographs where the relief displacement is at its minimum. Using a longer focal length reduces the height accuracy of photogrammetric mapping, while flying with a higher overlap increases the number of photographs proportionally. Even so, both solutions still result in large areas of the terrain being obstructed.

Removes Building Lean
So called “true” orthophotos are heralded as a comprehensive solution to the abovementioned problem. In principle they are the same as conventional orthophotos, except that in addition to the terrain model, an accurate digital surface model (DSM) is required that models objects like buildings with sufficient accuracy. This removes the building lean but leaves blank areas that need to be filled-in from corresponding sections of the overlapping images. The extent to which this is possible depends on the height and spacing of the buildings. What applies for conventional orthophotos also applies here: the longer the focal length of the camera and the larger the overlap of the photography the more successful is this procedure.
Yet, to produce the DSM of a townscape with the required accuracy is very expensive and time consuming. Even where it does exist, it is unlikely to be up-to-date.
Much more importantly, the results do not meet expectations. The more rigorous any of the proposed procedures are applied, the more map-like the orthophoto is. But as can be seen in Figure 1, this can hardly be the objective. The image content is restricted to roads and roof-tops, with a skyscraper hardly distinguishable from the roof of a parking lot. The town literally loses its face. Much of the information content of aerial photography is erased. The perspective is lost! In conclusion, turning an image into a town map achieves little—at a great expense.
Approximately 500 years ago, cartographers realized the drawback of presenting a town in a plan view as seen from directly above. They devised the techniques for presenting the third dimension of a town on a two-dimensional surface. Instead of drawing only the footprints they showed the buildings in perspective. Museums and arch­ives hold many of their works. An outstanding example is the birdseye perspective of Prague by Josef Daniel von Huber (opposite). Produced in the so-called military or isometric perspective, the buildings are shown in oblique view, with the vanishing point at infinity, but placed in their correct position on an undistorted, true to scale map. It shows a town alive.
Today, aerial photography provides a simpler, but more rigorous solution. Taking advantage of the fact that aerial photography for mapping purposes is flown with considerable overlap, each object can be looked at from different viewpoints, in much the same way as we change our position to see what is behind a building. Only now, we have access from vantage points in the sky.
This approach is implemented in PromptServer, an image processing server that displays geo-referenced images in the majority of the current GIS and CAD software. PromptServer’s ViewPoint functionality analyzes the overlap of the available images and displays by default this image with its center of projection nearest to the area of interest, i.e., where the minimum displacement occurs. This is similar to conventional orthophotos when it displays only the central part of a photograph, but with the subtle difference that PromptServer avoids cutlines.
Of much more importance, with PromptServer’s Viewpoint functionality, a building, or any other object that leans over details of interest, can—with a single command—be pushed to lean in a different direction in order to uncover the obstructed details (see Figure 2).
Flipping the views in a fraction of a second looks like magic, and provides us with a completely new insight on our towns. Not only are the problems of ground visibility solved, but details on the faces of the objects become visible. This reveals information such as type of construction, number of floors, number of windows and doors, emergency accessibility, color of building, and much more. Municipal services, utility companies of all kinds, police, fire brigades, security services, insurance companies, etc., can all profit from this information that is otherwise discarded.
This is achieved at a fraction of the cost of true orthophotos, and—instead of suppressing information—makes the full information content of aerial photography available to the user.
In case of new photography, the Viewpoint functionality is available almost immediately after new photography has been flown, since no DSM needs to be established or updated. In aerial photography flown with a standard overlap (60% longitudinal and 30% lateral), the whole survey area can be viewed from at least two directions and more than half the area, near the edges where the largest displacements occur, from three to six directions.
If each object must be viewable from four cardinal directions as in Figure 3, aerial photography needs to be flown with 60% overlap, both longitudinally and laterally; this is similar to what is required for true orthophotos.

How Accurate Is It?
Another advantage of the Viewpoint technology it that it allows the assessment of the accuracy of orthomaps, which depends largely on the quality of the terrain model that has been used in their production. Except for checking how well the adjoining images match, there is little possibility for the user to assess the accuracy of the orthophotos being worked on. Even these criteria are no longer applicable since today’s efficient mosaicking and feathering procedures camouflage most discrepancies.
However, PromptServer provides an accuracy assessment that is available to the producer as well as the data user. When flipping display between overlapping images, there should be no apparent shift of image details that have been correctly presented in the terrain model. If there is a shift, the user can make his own assessment as to whether or not and to what extent this interferes with his objectives.

Conclusion
Orthophotos have a tremendous appeal for a wide range of applications. By portraying towns in a plan view, we disregard their primary attribute, the fact that they are built-up. This causes us miss important aspects that should be taken into account in the development, management, and maintenance of our habitat.
The new imaging technology introduced in this article overcomes the shortcomings of conventional as well as true orthophotos. By providing an insight into our town with an ease that so far has not been possible within GIS or CAD environments, Viewpoint opens up the usage of image data to a new range of applications in the fields of GIS, AM/FM as well as homeland security.

About the Author
Rolf Becker is the Technical Director of the MAPS Group located in Munich, Germany.

Reference:
Jan Mokre: Kartographische Zimelien, Oesterreichische Nationalbibliotek, Wien 1995.

Back