Methodology
Geographic objects of a topographic map
As it has been written in the introduction, there are three types of maps produced from satellite images: traditional line maps, new imagemaps and thematic land-cover / land-use maps. With or without image background, all these products shows updated geographic features like villages, roads or forests. Table 1 reviews all the geographic objects present on topographical maps at the most common scale.
Table 1 - geographic objects of a topographic map
| Type |
1/25 000 – 1/50 000 |
1/100 000 –1/200 000 |
| Communications |
major roads and motorways
secondary roads
tracks
footpaths
railways
Landing areas |
major roads and motorways
secondary roads
major tracksrailways
Landing areas |
| Equipments |
Power lines
Tunnels
Bridges
Sport fields |
Power lines
Major tunnels
Major bridges |
| Settlements |
High-density urban areas
Low-density urban areas
Villages
Isolated buildings |
High-density urban areas
Low-density urban areas
Villages
Major churches |
| Relief |
Contours at 5 to 20 m vertical interval
Height spots |
Contours at 20 to 100 m vertical interval
Height spots |
| Hydrology |
Rivers and channels
Streams
Lakes and dams
Springs
Wells |
Rivers and channels
Lakes and dams |
| Vegetation / land cover |
Cultivated area
Orchard, plantation
Grassland
Bush
Several types of forest
Rocky areas |
Cultivated area
Bush
Forest
Rocky areas |
| Artificial limits |
Administrative boundaries
Cadastral boundaries |
Administrative boundaries |
| Writing |
Toponymy |
Toponymy |
| Tourist information |
Sometime present |
Frequently present |
The main difference between each scale map is the density of details shown on the map. The purpose of this physical limitation is to leave the final paper product enough synthetic to be understandable at a glance.
Effective Resolution of images
This aspect is very important for the interpretability of the images.
The sampling interval is the most frequent parameter used to characterise remote sensing images: in a SPOT panchromatic image for example, one CCD element records the reflected light each 10m along the scan line (1,3 micron on the CCD) and a line each 1,504 nano-second along the orbit of the spacecraft. This parameter is usually given as the ‘pixel size’ (10x10m for SPOT P), and it is often misused as an indication of the resolution.
When a photographic film is scanned, the sampling interval is determined by the increments of the sensing element along a line of scan, and by the rotation of the drum between successive lines.
Unfortunately, this doesn’t mean that the signal recorded by the CCD element integrates only the reflected light of the objects located inside the pixel size area on the ground: the signal is also influenced by objects located outside this area. The area ‘seen’ by an individual sensing element is called the instantaneous field of view (IFOV). The objects located in the centre of the area have a stronger influence than those located near the edges. The reduction of influence on the signal of the objects according to their distance from the centre is called the modulation and is described by the MTF (modulation transfer function).
So the concept of “effective ground resolution” is more realistic: it indicates the diameter of a circular area where the modulation of signal is always higher than 50%. The corresponding area is called the effective instantaneous field of view (EIFOV). We have to note that if a digital image has an effective ground resolution much larger than the sampling interval, it looks blurred.
The MTF of a sensing device depends on many factors: the optical quality of the lens and the electronic characteristics (or film properties) are more or less stable in time, for a given device, but for remote sensing systems, the optical quality of the atmosphere is usually the most limiting factor. Two images of the same area, acquired with the same device at different dates can have very different MTF, and thereafter, very different potential for interpretation. Haze is an obvious explanation for images having a poor MTF, but urban pollution and forest fire smokes are not negligible, even when the latter are located very far away from the site (high altitude layers of smoke are frequent in tropical areas).
Measuring the effective resolution on an image is not an easy task if calibrated targets are not available, and it should be done separately on each image. Differences of MTF in different parts of an image can even be observed. The most commonly used method to evaluate MTF is based on the comparison of recorded standardised features (airfields, buildings, etc.) with reference images, having a known MTF (usually generated by a controlled degradation of a higher resolution image).
Based on the compilation of multiple scientific references, the effective resolution of SPOT panchromatic images is often put to between 15 and 25m and between 30 and 50m for multispectral mode, for Landsat 5 the effective resolution is in between 45 and 75m.
For aerial photographs, the resolution is usually expressed in line pairs/mm. Common values are 40 lp/mm on film, and 15 lp/mm for paper copies, and we can say that the resulting resolution of optical aerial film systems, including film and camera, is actually anticipated to be around 20 microns at the film scale. Like with satellite imagery, the principal cause of the degradation of the MTF is related to atmospheric conditions. The MTF of the film scanner is usually adapted to the sampling interval, in order to produce sharp images, that is to say with 20 microns resolution for the image the good scanning interval is 30 microns. For good 1:80 000 photos, this can be translated to an effective resolution (EIFOV) of approx. 2 to 2.4m, for 1:30 000 scale photos this gives a resolution a little bit better than 1m (90cm)!
It is worth noticing that the processing of the images to produce orthoimages damages to some extent the MTF, because of resampling.