Synergism of CIR aerial photographs and SPOT images for monitoring of project housing in Bandung, Indonesia
Drs. Victor F.L. Polle
ITC, P.O. Box 6, Enschede, 7500 AA Enschede, The Netherlands
Aim and method of the research.
Until 1986 only aerial photographs or map made by extensive ground surveys could provide an overview of the urban development. Although satellite remote sensing had already started in 1972 with the LANDSAT MSS from the USA, for a long time we could only order images with a resolution of 70 meters. These images were too vague to be of any use for the planning of high density cities in developing countries.
With the arrival of Landsat TM the resolution dropped to 30 meters, which was a considerable improvement. And in February 1986 the /French arrived on the market with the Spot images of still higher resolution. Two types of images could be ordered:
colour resolution 20 meters or Panchromatic black & white resolution 10 meters.Suddenly satellite remote sensing also became interesting for urban planners.Urban planners usually order aerial photographs of a large scale: 1:5,000- 1:20,000, enlarged sometimes to 1:2,000 or 1:1,000. Occasionally smaller scales, up to 1:50,000 were used for large metropolitan areas (e.g. Colombo 1981). The resolution of SPOT images is comparable to aerial photographs of scale 1:200,000 or smaller. Town planners will have to become familiar with these smaller. Town planners will have to become familiar with these smaller scales, which require a different interpretation technique. Since 1986 we have been doing research with SPOT images of Bandung in Indonesia. (1) The planning of Greater Bandung is the responsibility of two ministries (Public Works and Interior Affairs) and of two authorities (the Municipality and the Region). Aerial photographs of the area have been taken recently to make orthophotomaps at scale 1:2,000.
A part of Java was covered in 1981 by CIR photography at scale 1:30,000. But these photographs have probably not rully been used, although they covered the Greater Bandung area, and even the whole Region Bandung, because it is not well defined which organization is responsible for the mapping of Greater Bandung. These images were, however, of great help during our research, especially because their false colour is comparable to the SPOT false colour.The aim was to assess the usefulness of SPOT PAN-XS images in combination with already available CIR aerial photography for the monitoring of the housing projects, which are spread over a large area on many big small sites, and which until now have not been properly mapped in their spatial distribution. See fiigure1. What is the efficiency and the accuracy of this method?
Figure 1: The spread of the Bandung project housing over an area of 19 km by 14 km, as identified on a SPOT PAN-XS image scale 1:25,000. The areas north of the generalized 700 m line are for 75% from before 1981 and have been identified on CIR aerial photography 1:30:000 from 1981 (See table 1).Note that most project housing is found outside the old municipal boundary and therefore not built under the planning jurisdiction of the municipality. Only in 1989 the area of jurisdiction of the municipality. Only in 1989 the area of jurisdiction of the municipality has been enlarged. Still some major developments are taking place outside the 2989 boundary.
The photographic products of SPOT
A city like Bandung, population 1.5 m., is not recorded by SPOT regularly. Recording is only done if a "programming request" has been placed with SPOT IMAGE in France. An extra problem for Indonesian cities is the almost constant cloud cover. It may take months before a successful recording can be made. But for our research, which started in 1986, we could now use an Xs recording from 6 July 1986 and a stereo pair in PAN from 29 June 1987 and 21 August 1987. For ground truth we obtained colour small format aerial photographs 1:18,000 from August 1987.
For the urban planner the photographic products from spot (based on pre-processing level IB) are the most practical until now. On a photograph the size of a map, a whole city can be seen, together with its environment in which it is likely to expand in, let us say, the coming fifteen years.
For the colour images the maximum useful scale for the enlargement is 1:50,000 although these enlargements do not look sharp anymore. The usual colour "translation" is the false colour. The vegetation becomes red, while manmade objects and bare soil become yellow, blue or green in a variety of shades.
Black & white images can be enlarged to 1:25,000. It is also possible to combine the XS and PAN images (with image processing equipment like ERDAS or ILWIS) to coloured images (PAN-XS) with a resolution of 10 m x 10 m which can also be enlarged to 1"25,000.
The enlargements should be interpreted with the naked eye. Use of a magnifying glass is meaningless because the picture elements (pixels) become so much larger than the resolution of the eye (approximately 0.111 mm) that our brains cannot perceive the terrain objects anymore.
Resolution and the size of the urban objects
The SPOT photographs 1:50,000 or 1:25,000 are much less sharp than aerial photographs of the same scale. If we loo8k at air photos with a magnifying glass we can see the individual houses even in the most densely built-up kampung. We can see details like the pitched roof, the flat roof extension overt a terrace and the occasional trees in the gardens.
The scene elements of SPOT form a regular grid over the city. The position of the grid is randomly selected. Also the pixels are therefore representing a random part of the terrain. In the city the terrain characteristics change over short distances, often less than 10 meters. The largest part of the pixel set will be mixed pixels. That means that the reflected light from a scene element comes partly from buildings, partly from vegetation in gardens and partly from the roads or railroads. We cannot see small objects separated from their environment.
Large objects however, like an airport, a golf links, a sports stadium, are represented by more than one pixel. We can see their shape. Also the outline of objects like "a residential area" or "an industrial area" can be seen in this way.
