Synergism of CIR aerial photographs and SPOT images for monitoring of project housing in Bandung, Indonesia
Recognition by the shape of the objects
For an interpreter of urban areas the most normal way to recognize objects is by looking at their shape and size. But we can only do that when the objects are big enough or near enough to be much bigger than the resolution of our eyes. They should "occupy" more than one "pixel" to have a distinct shape that can be recognized
For recognition on a SPOT image "large objects" are objects measuring, let us say, more than 100 m x 100 m. Then they can be recognized by their shape. In the colour images they are represented by at least 5 pixels by 5 pixels, each with its own colour and brightness. In PAN images or PAN-XS images they are even represented in 10 pixels by 10 pixels. This will reveal even more of the shape. We say then: the image is clearer, sharper. Let us consider some examples. An airport will be recognized by its runway which is 2-3 km long. A golf links will be recognized by the elongated lawns separated from each other by rows of trees and bushes, which are darker (red) because of more light absorption. Also sports stadium have a characteristic shape (two half circles connected by straight lines) that will be recognized.
Big housing projects, once we have studied their shape, can also be recognized by their shape every time that we look at the picture. We can also detect changes in their shape if we compare pictures from different dates. In this way change detection, or urban growth detection is made possible.
Most urban objects however are smaller than these examples and cannot be recognized by their shape. We have to use colour, tone and texture as clues to their identity.
Recognition by the colour and the texture of the object (SPOT-XS)
If we move away from an object its details cannot be seen anymore. First the small details "merge" with their environment and gradually even the larger details disappear. The details make with their environment a "texture", a surface characteristic, which depends on the composition of the surface elements.
With a SPOT image we are 800 km away from our objects. We see whole areas as a single object. We see its outlines (shape), separating it from its surroundings, but that shape is not characteristic. Residential areas for example may show an endless variety in shape. In order to determine its identity we have to rely on its colour and texture.
Unfortunately colour in itself is not too helpful. Residential areas may have many different colours, depending among other things on the density of housing, the amount and type of vegetation, the colour of the roof tiles and the width of the streets. It is more the combination of color and texture that makes identification possible. (2)
Housing areas are characterized by a high frequency texture. The colour and brightness (tone) varies from pixel to pixel. Once we become sensitive to these differences, we can recognize residential areas. With training we may even be able to distinguish between different types of housing area. A rough classification of housing areas may be possible. But this depends also on good local knowledge.
In the panchromatic (black & white) images we miss, of course, the colour. only the brightness and its variation is left as help for recognition. But it is possible to distinguish between smooth and rough textures, and therefore between types of land use.
Recognition by the tone and the texture of the object (SPOT PAN)
With SPOT PAN we have a higher resolution (10m) but without colour. Interpretation is often easier because of the higher resolution, more shape can be seen, but some urban areas are more difficult to interpret without the help of a color difference.
The resolution of PAN images is so much higher that in housing areas the texture from the colour images is often replaced by a tone pattern that can be interpreted as the "street pattern". Counting the number if darker lines gave the number of streets: the correctness of this information was confirmed by counting the streets on air photos. Also smaller objects are recognized by their shapes. Industrial buildings get a more rectangular outline, compared with the curved outlines on the color image.
Interpretation through synergism of aerial photographs and SPOT images : the Example of identifying project housing
In Banding's fringe large new residential areas have been developed for different types of houses. These developments data from the last 10 to 15 years. Some of these types are project housing (houses built in series), others real estate (individually built houses) and kampung. A further subdivision, e.g. on income level is possible.
Our research aim was however the mapping of the project housing of Greater Bandung with the help of CIR photographs of 1981 and a geo-corrected SPOT PAN-XS image 1:25,000 from 1987.
The mapping could be carried out in a short time. This was possible because in areas where interpretation was difficult, that is in the project housing from hbefore 198111, we could use the 1:30,000 false colour photographs for a first identification. The difficult areas are mainly in north Bandung where project houses have been built on sloping terrain on small sites and where considerable changes to the to the houses have been made by the people themselves. With the identified project areas we learned the typical colour and texture of this type of development.
On the aerial photographs from 1981 about 50 of these project housing areas could be identified. Many of these areas are very small (less than 10 ha) and apparently on pieces of land which do notallow further extension.
At least 30 areas did not show any growth between 1981 and August 1987 (the date of the last SPOT image used). The SPOT image was clear enough to state that with a great certainty.
In about 40 new areas projects started in the period 1981-1987, while 10 areas have expanded.
All areas with project housing (152) have been plotted on an overlay map on the PAN-XS image scale 1:25,000 and have been measured separately. The aggregated data have been given in table 1. It may be interesting to note that the interpretation of the image and the measurement of the delineated areas took only 2 Mondays, which proves the efficiency of the SPOT PAN-XS in combination with small scale older photographs for this kind of study.
The housing areas do not look all the same in the false color image. They show a pattern of lighter and darker spots. And their overall color varies from whitish, through bluish to greenish. These color differences reflect among other things stages of development. Some areas are still building sites. Other project housing areas are covered by houses, which either are not yet inhabited, or have just recently been occupied by renters: again others have been inhabited some time shown by abundant vegetation in the gardens and new additions made to the houses.
Verification in the field indeed revealed that areas with a different colour and texture differed as to the stage of development. But also the roof material, the spacing of the houses and the garden size proved to be different.
The SPOT XS image seems to show main roads in the area, but only when a strip of green vegetation makes them appear wider. The normal streets which divide the residential areas in "blocks" are not visible. They are too narrow to influence the colour of pixels enough to make a line discernable.
