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For a long time, and often until now, the updating of topographic maps was an analogue process requiring heavy aerial campaigns and time-consuming human processing. It explains why, in a lot of countries and especially the poorest, the updating of current maps cannot be done, and as a result the lack of recent topographic information is a barrier for the development of these countries. It is usually assumed that less than a third of emerged land all over the world is mapped with fairly recent topographic maps: consequently, the need in map updating is huge and urgent.
After the digital revolution and the beginning of computer-assisted map processing, geographic information entered a new era a few years ago: with the arrival on the public market of very-high-resolution digital satellite images, what I call the Satellite Revolution, which in theory allows high-scale maps of everywhere to be made while remaining seated in one’s office. This revolution in geographic information is in fact a combination of many factors such as increasing powerful computers at a cheaper price, more and more efficient processing software, the availability of very-high-resolution satellite images as mentioned previously. The result of this convergence of positive facts is a number of mature cartographic solutions to update existing maps rapidly and cheaply. Spot Image and the French Mapping Agency (IGN) are promoting, through a partnership, these outstanding cartographic solutions for map updating.
In this developing new market for satellite images, SPOT 5 images have undoubted assets: the SPOT system is the only one, which combines the essential properties for accurate and cheap mapping over a large area:
- A wide field of view (60 km by 60 km for HRG images, 600 km by 120 km for HRS) to minimize the number of images to be paid and processed;
- A very high resolution of 2.5 m, efficient for the standard scale of 1:50 000 and 1:25 000;
- A rigorous physical modelling associated with raw images and detailed satellite ephemeris available for precise geometric and photogrammetric applications; through state-of-the-art photogrammetric commercial software suites. SPOT is one of the few sensors having this rigorous modelling available for standard customers where many other sensors offer a “cheap” method much less stable in difficult operational situations. Besides, SPOT allows rigorous photogrammetric block adjustment (like aerial photographs, called in this particular case space-triangulation) with huge operational advantages (see below).
- In case of total lack of reference data for image correction (which is often the case in operational situations), Spot Image can supply one through the Reference3D package extracted from the SPOT 5 HRS sensor: this package (see below) contains a very accurate DEM (vertical accuracy: 10 m for 90% of the points) and an HRS orthoimage which can act as a GCP source (16 m horizontal accuracy for 90% of the points) Reference3D.
Fig 1: Ablock of SPOT images through space-triangulation ensures good geometry where no map is available (country B)
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