New perspectives for line map updating
Reference3D
Reference3D product is a world reference package, produced by Spot Image using SPOT 5 HRS stereopairs. This product has been developed in partnership with the French Mapping Agency (IGN) to guarantee the best possible accuracy. The production is partly funded by the French MoD. It contains both an accurate DEM as an altimetric reference, an HRS orthoimage as a planimetric reference and some useful metadata describing the DEM quality. The main benefit of Reference3D is to offer a good accuracy with no need for GCPs at all. Hence Reference3D is available virtually
everywhere. The characteristics of this
product have already been published by Spot Image, and can be rapidly summed up in Table 2. These official Reference3D accuracy requirements were fixed to cope with various landscape and extreme relief situations. In many operational situations, users have reported that Reference3D 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 Reference3D proved to be perfectly registered with the very accurate Serbian cadastre (see Figure 2).
On the other hand, an independent study carried out by the European Joint Research Centre (Ispra, Italy) and the FOMI (Hungarian Mapping) provided detailed scientific evaluations confirming users' feeling. The study estimates the accuracy (as shown in Table 3) for Reference3D product. These results were confirmed over other areas by:
- An other 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 Reference3D is in line with the above requirements. As we will see below, these measures are really important to evaluate the advantage of using Reference3D as a reference for SPOT 5 HRG images in a low-scale map updating context. It is an unrivalled reference database especially for military purposes (weapon systems, flight simulation), combining high quality, quality mask for risk evaluation, and total independence from local/external data. It explains why this product was defined and partly funded by the French MoD.
Fig 2: Perfect superposition between a SPOT 5 HRG image (THX) orthocorrected using Reference3D and the Serbian cadastre
Line Map updating requirements
Accuracy requirements:
We have seen that SPOT 5 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 Reference3D or the existing old maps themselves. The Table 4 shows the type of accuracy one can obtain using SPOT 5 images. 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 : very difficult or impossible
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.
Map updating method and performance
Line map updating is the key cartographic application which is now possible using SPOT 5 images with outstanding performance. The interesting scales are 1/50 000 and 1/25 000 scales which are, for most countries, the scales of the base national maps.
The method
1:25 000 scale map updating was tested several times by IGN and GDTA very successfully. The methodology, developed by IGN in the 90s for lower scales, is the following:
- Scanning of the old map master films with a very high resolution. These master plates (nowadays master films) are the printing elements of paper maps. There is one master film per colour to print the paper map, i.e. one master film for the blue containing all elements of the map printed in blue (hydrography), one for the green containing all vegetation information, one for the black (the richer master films with all buildings, roads,..), one for the bistre containing all contour lines, etc…The number of master films is variable, depending on the country and the type of map. Each master film is like a layer containing a type of information displayed in one colour, so separated from the others and so much easier to be updated
- Geocoding of the raster scanned master films
- Ortho-correction of the corresponding SPOT 5 2.5 m image using Reference3Dâ or the old map and contour lines
- Superposition of scanned master films, with a transparent background, over the SPOT orthoimage;
- Digitizing of new features, masking of features which do not exist any more
- Integration of vector corrections into each master films using the proper graphic symbol
The output of the process is a set of digital master films, ready to be combined during printing to obtain the updated paper map. The digital master films are available for the next updating a few years later.
Performance
In July 2004, IGN carried out map updating of a 1:25 000 scale map in real conditions in situ at the Serbian Survey (Vojnogeografski Institut - VGI - Belgrade) using a SPOT 5 THX image (merge between a 2.5-m Panchromatic image with a 10-m colour image). The results were the following :
- 3280 new buildings, many changes in the road network were digitized;
- Very good fitting between the Serbian map and the SPOT 5 orthoimage
- The sheet rapidly checked on the field
- Updated map ready for printing
- All the updating process and rapid field checking in two weeks
A huge benefit compared with a traditional method whirequires months for one map updating. The cost, compared with a traditional method using aerial photos, is approximately divided by five.
Use of Reference3D to extract new maps
Reference3D and SPOT 5 data are currently used as "foundation data" by several governments (including the French one) to extract vector maps at a 1:50 000 scale over areas which cannot be reached easily. The rough production principle, designed to minimize the cost of the source images, as well as the production efforts, is to divide the area to be mapped into several equal-density zones, and to affect to each zone a given source image: Reference3D orthoimage for the low density, SPOT 5 m or 2.5 m for the middle or middle+ density, and to keep the 1-m imagery (Ikonos, QuickBird) to the very minimum extent needed, over the very dense areas. This is done in the frame of multilateral production groups (MGCP, FDB).
Conclusion
As a consequence of the Satellite Revolution, map updating of 1:25 000 or 1:50 000 scale maps is now a process easy to achieve, with moderate costs and rapid results.
In this developing market, SPOT 5 data are precious because they combine and optimize the advantages of a good cartographic system (large field of view, excellent geometric properties, very high resolution) with the satellite advantages (revisit capabilities, worldwide availability). At the same time they offer global reference data packages to ensure the best accuracy. These new tools and techniques will probably definitively change the way we map our world.