SPOT 5 data for line map updating: New perspectives in mapping

Michael Tonon
Head Space cartography
IGN - Spot Image
Toulouse, France
Email: Michael.Tonon@spotimage.fr



1 Digital and satellite revolutions in map updating
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 can not 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.

2 SPOT 5: unrivalled benefits for mapping
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 Reference3Dâ 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) .
3 SPOT space-triangulation:
The rigorous SPOT physical geometric modelling allows real-life, accurate photogrammetric applications, like 3D digitizing using stereopairs, and space-triangulation.

Space-triangulation consists in modelling together long SPOT data strips linked to each other through tie points. To compute an accurate and homogeneous physical model through the whole block of images, a few Ground Control Points (GCPs) are necessary, less than ten GCPs per data strip (6 in theory, more in operational situations to reduce GCP errors). These GCPs should be distributed at least at the beginning and end of each data strip. This allows some images having no GCPs at all to be modelled (see picture n°1) which is an undoubted operational advantage. The processing time is also much shorter than a single scene physical modelling which requires the same number of GCPs…but per scene: for instance, a block of 6 data strips of 10 images each, processed using space-triangulation, requires a minimum of around 36 GCPs (6 per data strip). With the same number of images but processed per single scene, it requires around 360 GCPs spread all over the block. This space-triangulation technique allows amazing processing time compared with traditional methods: a block of 250 SPOT images, covering a country of 400,000 km², was processed in around two weeks. The numerous tie points can be acquired, depending on the software, using automatic correlation tools with little human intervention.

The space-triangulation techniques used by Spot Image increases SPOT 5 absolute accuracy: a single SPOT 5 scene has an absolute planimetric accuracy (no GCP) through physical modelling of 50 m RMS. Through space-triangulation, the SPOT 5 absolute accuracy is averaged to 12 m RMS, an accuracy compatible with 1:50000 scale mapping (if relief is correctly corrected) obtained again with no GCP (but with a good DEM).

So the space-triangulation technique with the SPOT physical modelling potential is really an unrivalled mapping tool for large areas.


Picture n°1 : a block of SPOT images through space-triangulation ensures good geometry where no map is available (country B).

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