[WTS.RFC] Comments on WTS RFC

[Thomas H. Kolbe]

Dear WTS RFC Submitters,

due to the fact that the WTS will be the first 2,5D resp. 3D portrayal
service of the OGC, I would like to make a comment regarding
missing 3D and interoperability functionalities. The proposed extension
results from practical experiences that 23 members of the Special Interest
Group "3D" of the initiative Geodata Infrastructure North Rhine-Westphalia
GDI NRW, Germany, made this year in three pilot projects on interoperable
3D visualization of 3D city models.

Speaking for the SIG 3D, I see the enclosed comment not for inclusion
in the upcoming version 1.0 but as an important discussion point for
future revisions.

Best regards,
                Thomas H. Kolbe

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PART A
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1. Evaluator:

    Dr. Thomas H. Kolbe
    Speaker of the SIG 3D of the GDI NRW, Germany

    Institute for Cartography and Geoinformation
    University of Bonn
    Meckenheimer Allee 172
    53115 Bonn
    Germany

    EMail: kolbe at ikg.uni-bonn.de
    Phone: +49 228 731760
    WWW:   http://www.ikg.uni-bonn.de/kolbe and http://www.gdi-nrw.org


2. Submission:

    OGC Request 18: Web Terrain Service (WTS) RFC


PART B
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1. Specification Section number: GENERAL

2. Criticality: MINOR (MAJOR with respect to future revisions)

3. Comments/justifications for changes:

In the SIG 3D of the Geodata Infrastructure North Rhine-Westphalia,
Germany, we are currently running three pilot projects on interoperable
3D geovisualization of 3D city models (involved cities among others are:
Cologne, Düsseldorf, and Leverkusen). Before the projects started in
May 2003, we have evaluated the WTS Specification and came to the
conclusion that for 3D application scenarios the WTS lacks two
important functionalities:

1) Explicit support of the third dimension - especially concerning the
     output format. The only suggested output of the WTS are (static)
     2D images. Whereas this is helpful in limiting the needed complexity
     of the client, it does not allow to interactively view and navigate
     a 3D scene. Interviews with more than 60 companies and municipalities
     indicated however, that for most applications in the domain of
     3D city models 3D interaction and navigation is crucial.

2) (This point is also a consequence of focusing on a 2D output format)
     In contrast to the Web Map Service (WMS), output images from
     different WTS cannot be merged into one scene image within the
     client in general. A scene view from a WTS may contain (3D) objects
     that stand nearer to the viewer than some other objects in the view
     of another WTS with the same perspective. Thus, nearer objects might
     get occluded, if the second image is simply stacked on top of the
     first. This problem generally restricts the WTS currently to some
     sort of "front-end" portrayal service, that cannot be cascaded in
     the same flexible way as Web Map Services.

As a solution, we suggest to add a new operation "GetScene" in future
versions of the WTS. This operation should return a 3D scene graph
(3D graphic elements) of a bounded geographic volume in an appropriate
format. In the previously mentioned pilot projects we proposed VRML97
as the mandatory output format for "GetScene", because VRML97 is a
widespread 3D graphic modeling and exchange format that can
be read and written by most 3D GIS, CAD, and graphic systems
(other 3D graphic formats like GeoVRML, X3D, and MPEG4-BIFS are
possible, too). Switching to 3D output solves both problems explained
above, because 3D elements from different WTS could easily be
integrated within one client. Since the rendering step would
be done by the client, 3D navigation can be implemented on the client
side (VRML browsers offer already free navigation).

If needed, more details about the stated problems and the pilot
projects can be found in the paper "Applications and Solutions for
Interoperable 3d Geo-Visualization" that we have presented on the
Photogrammetric Week 2003 in Stuttgart. It can be downloaded from:
http://www.ikg.uni-bonn.de/kolbe/publications/Altmaier_und_Kolbe_PhoWo2003.pdf