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



4 SPOT 5 accuracy
All satellite image providers announce impressive planimetric accuracy for their satellite orthoimages. The problem of this purely commercial approach is that it is not specified that such figures are reached only in some specific conditions, never experienced in operational situations. These specific but not revealed conditions are usually:
  • A smooth test area with low relief;
  • A place where a huge number of extraordinary GCPs (metric accuracy) and an amazingly good DEM are available;
  • Using raw data and satellite ephemeris which are often not provided to normal users;
  • Using an exceptional but not marketed software package.
In other words, as soon as the user works with "normal" existing reference data, in mountainous areas for instance, using a commercial software, the geometric nightmare starts.

What one can say about the SPOT system, is that SPOT modelling can reach metric accuracy . The difference from a lot of other sensors is that this photogrammetric potential can be approached much more easily with SPOT in operational situations than with other systems, thanks to what we have seen above: rigorous physical modelling, well known by users and commercial software packages, raw image data and detailed ephemeris provided to customers.

In real situation, the orthoimage accuracy depends on the type of modelling but also on the accuracy of the reference data (a general case for any sensor). The main difference between the SPOT system and other ones is that the SPOT rigorous modelling allows to leverage the best possible benefit from poor existing reference data when other sensors' modelling geometry diverge completely using the same GCPs and show their geometric modelling instability.

In practice, what is the accuracy of SPOT 5 orthoimages? The following chart gives the accuracy we obtain regularly in relation with the type of reference data used :


Situation n°1 and 2 are reassuring: with no existing ground data but "on-the-shelf" SPOT reference data, it is possible to get reasonable accuracy for our mapping purposes almost everywhere.

5 The Spot Image Reference3Dâ product
Reference3Dâ product is a world reference package, produced from SPOT 5 HRS stereopairs. This product has been designed and is currently produced in partnership with the French Mapping Agency (IGN) to guarantee the best possible accuracy. It contains an accurate DEM, an HRS orthoimage and some useful metadata describing the DEM quality. The main benefit of Reference3Dâ is to offer a good accuracy with no need for GCPs at all. Hence Reference3Dâ is available virtually everywhere .

The characteristics of this product have already been published by Spot Image, and can be rapidly summed up in the following chart:


These official Reference3Dâ accuracy requirements were fixed to cope with various landscape and extreme relief situations. In many operational situations, users have reported that Reference3Dâ accuracy is better than the above values, and that the "official" requirements give an under-estimated accuracy for a standard case. For instance, a test performed in July 2004 in Serbia shows impressive results: an HRG SPOT 5 image corrected using Reference3Dâ proved to be perfectly registered with the very accurate Serbian cadastre (see picture n°2).


ăCNES, Spot Image, IGN 2004
Picture n°2: perfect superposition between a SPOT 5 HRG image (THX) orthocorrected using Reference3Dâ and the Serbian cadastre.

On the other hand, an independent study carried out by the European Joint Research Centre (Ispra, Italy) and the FÖMI (Hungarian Mapping) provided detailed scientific evaluations confirming users' feeling. The study estimates the following accuracy for Reference3Dâ product:


3"An independent evaluation of the Spot Image / IGN Reference3Dâ digital elevation model and HRS orthoimage products" Simon Kay, Rafal Zieliński, Peter Spruyt (European Joint Research Centre - Italy) Szabolcs Mihály, Peter Winkler, Gyula Ivan (Institute of Geodesy, Cartography and Remote Sensing - FÖMI, Budapest), 2004

These results were confirmed over other areas by:
  • another independent study carried out by a Belgian private company ,
  • and an in-depth evaluation by a Chinese mapping agency
Both studies give the same type of results, stating that Reference3Dâ is in line with the above requirements.

As we will see below, these measures are really important to evaluate the advantage of using Reference3Dâ as a reference for SPOT 5 HRG images in a low-scale map updating context.

6 Line Map updating requirements
Accuracy requirements :
Geometrically, the line map updating requirements are the following :


  Planimetric accuracy Altimetric accuracy
1:50 000 scale map Around 10 m Around 7 m
1:25 000 scale map Around 5 m Around 3 m

We have seen that SPOT5 HRG images have the geometric potential to reach easily the required planimetric accuracy. It depends now on the reference data. In any case, it is possible to reach these specifications using Reference3Dâ or the existing old maps themselves. The chart below shows the type of accuracy easy one can obtain using SPOT 5 images :


The altimetric specifications for mapping are required if one needs to generate missing contour lines. Although it is not exactly our purpose here - because the contour lines are supposed to be in the old existing map - it is a very important matter as soon as one wants to generate a new map. In that context, Reference3Dâ can be a solution :


Altimetric accuracy Reference3Dâ altimetric accuracy
1:50 000 scale map Around 7 m (20 m interval) Around 5 m (“normal” situation)
1:25 000 scale map Around 4 m (10 m interval) Around 5 m (“normal” situation)

Reference3Dâ DEM is fully compatible with the 20 m interval contour lines required for 1:50 000 scale maps. It is also very close to the accuracy required by the 10 m interval contour lines of 1:25 000 scale maps, which means that Reference3Dâ can be an acceptable substitution altimetric reference even for 1:25 000 scale contour lines generation.

The previous chart shows that one can easily find an operational technical solution to obtain a better accuracy than required for 1:25 000 or 1:50 000 mapping, or very close, using SPOT 5 images.

Requirements about the map content :
SPOT 5 2.5-m images potential for photo-interpretation and mapping at the scale 1:25 000 were tested by a Japanese company, PASCO, in 2003: during the test, operators tried to photo-interpret the main keys of a standard 1:25 000 map, to see if it is possible to identify them. The operators in this study put a mark according to the level of interpretation they were able to achieve for each theme:
  • A: easy;
  • B: possible;
  • C: needed help by field identification;
  • D: difficult;
  • E:verydifficultorimpossible.
The result of PASCO evaluation is the following :


The conclusion of this study is that with a minimum of help, almost all features can be interpreted for a 1:25 000 scale map. In the PASCO test, the photo-interpretation was done without any additional help, only the SPOT image, but in an updating process, the photo-interpretation is made easier by the old map: in that context, the photo-interpretation problem is no more to identify everything but to identify only the changes between the old map and the image, which is much easier.

In any case, like with aerial photos, some complementary field survey is necessary, especially for place name updating and for some specific data collection and control.

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