Finding True Position of Buildings in Orthophotos
Deeboon Methakullachat
Assistant Professor, Faculty of Engineering
Kasetsart University
50 Phaholyothin Rd., Bangkok 10900
Tel: (662) 942-8555 Fax:(662) 579-4575
THAILAND
Email: fengdbm@ku.ac.th
ABSTRACT
Orthophotos of urban areas do not always provide correct information of the position of
buildings. Leaning buildings, caused by minor relief displacements, can be observed in
orthophotos generated by the traditional method. Much research has been working on producing
true orthophotos using digital surface model of a city. However the procedure is costly, time
consuming and requires special software to handle the effects of occlusion. This is why
orthophotos of urban areas generated by traditional method are still being produced in many
places in the world. We thus encourage orthophoto map users to stay with traditional
orthophotos for their good source of information. This paper describes a scheme that locates
the correct position of buildings on a traditional orthophoto, by using exterior orientation
parameters of the original photo. Building heights is another important piece of information
and is extracted as part of the output results. A mathematic model is presented together with a
numerical example of a least squares adjustment. Interestingly traditional orthophotos do store
position as well as the height of buildings in one place while true orthophotos do not.
BACKGROUND
“A picture is worth a thousand words” is perhaps the best explanation of why orthophotos are so
popular nowadays. Digital orthophotos can be displayed on a computer screen. They are lively,
easy to understand, as precise as maps and, most importantly, carry more topographic
information than any other sources.
Many people might think that to produce an orthophoto is not difficult, at least not as
complicated as that of classical photogrammetry. However orthophoto generation of urban
areas is somewhat sophisticated. In fact much research has been carried out to solve problems
in orthophoto generation in cities (Amhar et. al., 1998, Schickler and Thorpe, 1998, Skarlatos,
1999, and Rau, et. al., 2002). The word “true orthophoto” was introduced and implies that there
is a “false orthophoto” on the other side.
Regardless the methods that are used, true orthophoto generation cannot be possible without a
digital surface model (DSM). The problem is DSM data may not be available. Moreover the
procedure is costly, time consuming and requires special software. This subsequently force the
production of orthophotos in many places in the world to use only the digital terrain model
(DTM), which do not include height information of manmade objects. The resulting
orthophotos are traditional orthophotos, better not use the word false orthophotos, in which
leaning buildings maybe presented (see Figure 1).
Figure 1 An orthophoto of Kasetsart University, showing some obvious leaning buildings
We know that the true shape and position of a building on an orthophoto are at the base.
However, we cannot see all sides of the building at the base due to leaning buildings.
Fortunately this is not the case for the rooftop. In classical photogrammetry, an experience
operator knows that digitizing a rooftop of building is easier than digitizing at the base.
The big question is, if we digitize the rooftop, where exactly the position of the building is.
