3D mapping from space?

Prof Dr Armin Gruen, Dr Kirsten Wolff

Lately we observe an amazing increase in earth-observation platforms equipped with ultra high-resolution imagers. With the recent deployment of WorldView-1 we have reached the 0.5 m footprint level.
This raises the issue of 3D topo-mapping from space, in a more pressing way than ever before. Topo-maps of medium and larger scales (1:50,000 and better) are still missing in some parts of the world, in others they are hopelessly outdated. Upto- date aerial images, as a traditional data source for mapping, are not always and everywhere available. In contrast, highresolution satellite images with stereo capabilities constitute an interesting tool for mapping and the image providers advertise their use quite extensively.

Topo-mapping is worldwide controlled by specifications, which may differ from country to country. Therefore it is difficult to give general recommendations with respect to the question which sensor would be feasible for which map scale. In addition, digital mapping is largely scale-free, which makes the issue even more controversial.

In the literature we find many predictions and recommendations on 3D mapping from space, but mostly without substantial empirical evidence. 3D mapping is very often reduced either to the generation of ortho-images or/and to the georeferencing accuracy and DTM generation accuracy. But mapping is much more, as we all know. 3D mapping from satellite imagery is still a topic which causes many misconceptions. We hope we can contribute with this paper to a clarification of some of the issues.

What is 3D mapping?

A consistent definition of 3D mapping is missing. We are well used to conventional 2D and 2.5D mapping, resulting in an analogue map as final product. 3D mapping however gives us many more options, but also raises more questions.

With such new technology of digital mapping we have to address a number of problems, which are not necessarily all new, but so far only sparsely treated in R&D. Among those are:
+ 3D mapping – how does this differ from traditional 2D and 2.5D mapping?
+ Which objects have to be mapped and at which resolution and accuracy?
+ How should truly 3D objects be modelled in terms of geometry, topology and possibly also texture?
+ How should these objects be represented in the database?
+ Digital mapping – how much automation is currently possible?
+ Image interpretation – which pixel size do we need in order to bee able to extract features and objects that are required for topomapping
at a certain scale?
+ Image quality – what are the differences in image quality (and thus interpretability) between aerial and satellite images of the same Ground Sampling Distance (GSD)?
+ Orientation/geo-referencing – how accurately can we georeference the new satellite images (with and without GCPs) in planimetry and height?
+ DSM generation – what are the expected accuracies in automated DSM generation, which parameters determine the accuracies of the DSMs and what is the reliability of the estimated surface models?
+ DSM-DTM reduction – what are the most successful approaches for
DSM to DTM reduction and what are the main problems to be solved?

3D mapping requires totally new approaches to modelling. Most of the
traditional procedures and commercial software packages, which have been developed under 2.5D assumptions will inevitably fail under strict 3D requirements.