De aling with he ight re fe rence s

64 Copyright © 2012 Op en G eos pati al Consortium

10.3.3 Tiling for W VS

Providing tiled data allows a client to fetch large amount of data more easily and allows a server to cache the data that could be requested. Thus, one major benefit of tiling mechanisms is to reduce response time. However, for image-based 3D portrayal no tiling scheme has been discussed and specified so far. Key questions to address in such effort would be, e.g., how to discretize the 3D space and how to address these tiles.

10.4 De aling with he ight re fe rence s

The exact elevation of buildings and other structures may not be captured very accurately due to the following reasons:  The model was created based on photographs.  The model was created based on ground plans and rule based reconstruction.  No accurate elevation data was available. Also, the quality of the terrain data used for the visualization varies significantly. Data available from municipalities is mostly captured using land surveying techniques or LIDAR and has a very good quality. Many research and commercial projects use globally available data sets such as SRTM, ASTER or other open products with very low accuracy. When trying to merge data from different servers within a Spatial Data Infrastructure, for instance buildings from one source and terrain from another source, vertical inconsistencies will most likely occur. Usually it is preferred to use the terrain as reference and adjust other ground objects relative to it, even if the quality of the terrain is worse. Basically two strategies for reducing vertical inconsistencies can be distinguished:  Ground objects are delivered in a format that supports a reference point per object and an attribute indicating that the elevation is always relative to the ground. This can be encoded in KMLCOLLADA combination using Placemarks. However, this leaves the logic of how to attach the object to the ground to the client. This can be done by continuous picking or by physical gravity models. The computational cost is proportional to the number of objects.  Ground objects are corrected by the service that delivers the terrain data. A separate operation can be added to the 3D portrayal service that computes elevation values for arbitrary reference points by using interpolation. The functionality is identical to that of the Elevation service described in section 6.1.5. The terrain data must be same as used for the visualization. Since the elevation data is only retrieved from a service for a set of reference points once, it does not need to be loaded by the client in advance. Copyright © 2012 Op en Geos pati al Consortium 65

10.5 Potential change s in W 3DS and W VS inte rface de finitions