3D mapping from space?

Prof Dr Armin Gruen, Dr Kirsten Wolff

From 2d maps to 3d landscape models

Worldwide databases like Google Earth have given a wider audience access to georeferenced data. This data is useful for all kind of planning and inspection purposes, but usually does not satisfy the demands of a professional map user. However, it has sharpened the mind for what can be done with geo-data, in terms of visualization and interactivity. And it has set a certain standard in perception of terrain and other geo-related data. Has Google Earth even changed irreversibly our perception of a map? If yes, then what do we, as experts or occasional users, expect form modern maps after Google Earth?

Mapping has grown way beyond the traditional domain of topo-mapping and thematic mapping. Markets for geospatial technologies include nowadays applications in insurance and risk management, natural and man-made hazards, real estate, Location-based Services, environmental monitoring, car navigation, oil and gas, homeland security and many more. However, the data contained in topo-maps (or “landscape models”) can in the future serve as the backbone of these diverse applications.

The flexibility in handling of and in modifying digital data has brought up the issues of “real-time mapping” (see recent disasters in Myanmar and China), “mapping on demand” (individualized data collection), “personal mapping” (representations for a particular purpose or person). Mobile Mapping platforms on cars, trains and UAVs allow for real-time raw data collection at an unprecedented speed and flexibility. New devices like mobile phones and PDAs in LBS-related applications bring up the notion of “ubiquitous mapping”.

how does this all relate to the task of topo-mapping?

There is a large amount of data already now available which would qualify for “3D maps”: 3D city models, forestry models, Cultural Heritage models, etc. Should this data form a joint database with the more conventional content of a traditional topographic map?

This all requires us to reconsider radically our mapping goals, tasks, procedures and products.

First steps in this direction have been taken. The Swiss Department of Lands (“swisstopo”) has defined a countrywide map system on the basis of a Topographical Landscape Model (TLM). This Landscape Model includes all objects that are currently represented in topo-maps, but in truly 3D form. This includes terrain, buildings, water objects, public transportation, public spaces and facilities, landcover, administrative borders and “points of interest”. Compared to the situation before this model features some novelties, as for instance – It serves as the basemodel for the whole country. Subsequent level models for cartography and for representation (at varying scales) are defined, which are derived from this unique dataset of the basemodel. Therefore all the objects of the basemodel are geometrically correctly modelled. There are no displacements of elements and no generalizations.
– The data is always actual. It is continuously updated, and not only at certain fixed intervals.
– The accuracy is very high. The object accuracy is specified to 1 m.
– The data is truly 3D. Therefore all objects can be correctly modelled and no information has to be suppressed because of lack of modelling tools.
– The data model is set up such that it is ready for extensions.

At this point the data is still acquired from aerial images, as of January 2008 only from digital cameras like the Leica ADS40.

This Landscape Model is interesting from a conceptional point of view. It remains to be seen how it performs in practice.

3d landscape Models from aerials or satellite images?

With the increased availability of ultra high-resolution satellite images and (partially) dropping prices this becomes a burning question. Both data sources do have distinct advantages and disadvantages, which are briefly listed here:
Pro satellite images:
• The satellite platform is operational 365 days of the year
• Frequent re-visit times (e.g. every 4 days or even more)
• Imagery is post-processed relatively quickly
• There are no Air Traffic Control restrictions
• Large area footprints decrease the need for block adjustment and creation of image mosaics
• The satellite can easily access remote or restricted areas
• No aircraft, cameras or otherwise expensive equipments are required (by the end user)

Contra satellite images:
• The image acquisition geometry is not flexible
• The image resolution is fixed for a particular sensor and low compared to most aerial imagery
• The radiometric resolution is often too low (problems in shadows and saturation areas)
• The image quality is often impaired by different factors and artifacts
• The typical off-nadir viewing angle of up to 25˚ is problematic in image matching
• The reliability of capture and delivery of imagery can be poor at times
• Strong possibility of cloud cover and thus occlusions
• The cost of the imagery may be too high (when compared to aerials)
The selection of any one of the data sources depends on many factors. The decision can only be made efficiently when all the project parameters are available. We have reported about an extreme case in Bhutan (Fraser et al.,2008), where access to aerial images is impaired and where pilot projects are underway to use satellite imagery for the
generation of a new topo-map 1:25,000.

In our following pilot test for topomapping we compared map objects derived from IKONOS 1m GSD stereo images with map data from the Swiss topo-map 1:25,000, which is usually derived form aerial images at scale 1:30,000 (which in turn corresponds to an image pixel size of about 0.5 m).