Considerations about map-updating and images resolution


Relations between map scale and image resolution
To take full benefit of images at 1, 2.5, 5 and 10m resolution, they have to be displayed at least at full resolution, that is to say respectively at 1:2 000, 1:5 000, 1:10 000 and 1:20 000 approximately with a standard screen of 800x600 pixels.

The map as then of course to be displayed at the same scale in the updating process.

The following figures (5 to 10) are trying to illustrate the complexity of map production and map updating having regard to what is pertinent to be mapped, which detail if it is mapped is still readable compared to others neighbouring details and what is the good image resolution for 1:50 000 mapping.

One can say that the greater is the scale of the images (i.e.: the greater is the resolution) the best it is for visual photo interpretation. If this is especially true for production of new maps it is not so evident for map updating, where the image scale must be closely adjust to the final map scale and to the working scale.



In figure 5, above, four ortho-images, at resolutions of 1, 2.5, 5 and 10 metres and displayed at full resolution, are presented together with a 1:50 000 scale topographic map (centre) and the corresponding enlargements.

If for updating purpose of the 1:50 000 scale map sheet if it is interesting to work at 1:25 000 or 1:20 000 or even at 1:10 000 scale, it is no more reasonable to work at 1:5 000 scale or larger which are the scales for the higher resolution images.

So very high resolution images (1 or 2.5 m) have to be sub-sampled as shown in figure 6 below, where we can notice that the visual aspect of all the images, which are displayed at the same scale of 1: 20 000, is identical whatever was the resolution of the original data set.



For the photo interpretation of the air field for example, the 1m image does not bring more information than the 10m one, for the interpretation of the building blocks in the town the 5m image which is displayed at half resolution, and even the 10m one at full resolution, are more informative than the more subsampled ones.

Geometric accuracy
The geocoding process has to reach the accuracy specifications of the topographic map. Planimetric accuracy is in general 0.2 mm at the publishing scale and elevation accuracy is suppose to be 1/3 of the contour interval. Table 4 gives these values for maps at the most common scales:

Table 4
Scale  Plani. accuracy (m)  Contour interval (m)  Elev. accuracy (m)
1/25 000  5  5 / 10  1.5 / 3
1/50 000  10  10 / 20  3 / 6
1/100 000  20  20 / 50  6 / 15
1/200 000  40  50 / 100  15 / 30


Accuracy of the geocoded image:
It means that if you want to update a 1:50 000 topographic map, you must be able to create a geocoded image with 10m accuracy.

The geometric quality of the geocoded image depends on:
  • the pixel size and the effective resolution of the sensor
  • the quality of the rectification model
  • the accuracy of the ground control points (GCP)
  • the accuracy of the DTM and the incidence angle of the satellite for the original image.
Let’s consider a 10m panchromatic SPOT image, taken with a quasi-vertical incidence angle, and corrected with sub-meter accuracy set of GCP. The best planimetric accuracy we can expect for the geocoded image is between 10 and 15 m, which is fully compatible with standards for maps between 1:50 000 and 1:100 000. In the same conditions, if the SPOT image is replaced by a one-metre resolution image, the final orthoimage will have a planimetric accuracy better than 2 to 3 m’s’, which is the standard for 1:25 000 maps. To conclude, we can say that today’s satellite images reach the geometric standards of 1:50 000 and 1:100 000 maps and tomorrow’s one-metre to five-metres resolution images will easily match the standards of 1:25 000 maps or larger.

Information extraction from satellite data
Let’s analyse for all the cartographic features (cf. table 1) their level of interpretability for four types of satellite images:
  • images with resolution around 10 m like SPOT images,
  • images with 5 and 2.5 m from satellites like IRS and the forthcoming SPOT 5 and ALOS,
  • and one meter images from IKONOS and from the future very high resolution satellites.
Communications:

Table 5
    10 m images   5 m images   2.5 m images   1m images
major roads and motorways  Identification  Identification  Identification   Identification
Secondary roads   Recognition to Identification  Identification  Identification   Identification
Major tracks   Recognition to Identification  Identification  Identification   Identification
Other tracks   Detection to Recognition  Recognition   Recognition to Identification   Recognition to Identification
Railways  Recognition  Identification  Identification   Identification
Airports and Landing areas  Recognition  Identification  Identification   Technical analysis
Footpaths  No  No   No to Recognition   No to Recognition




Various image resolution for various steps of interpretation:



Air field and sports grounds


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