Interview: CityGML - Modeling the City for the Future
Written by Jeff Thurston
Thursday, 12 June 2008
CityGML is emerging as a model for defining the structures for organising urban information. It aims to identify the meaning and function of objects in a city. Thomas Kolbe became initially involved in CityGML from the beginning. Vector1 Media editor Jeff Thurston met up with him at the recently held Open Geospatial Consortium (OGC) Interoperability Day in Potsdam, Germany for this interview on the topic of CityGML.
V1 Magazine: How did you get
involved in CityGML?
Kolbe: It would probably not be wrong
to say that I was one of the initiators back in 2002. We had
assembled a group of interested people, the Special Interest Group
3D, back then and many people were beginning to realise that there
was no way of exchanging 3D city models without incurring a loss of
information. The members of the SIG 3D wanted to do more than
visualise information alone, they wanted to exchange the underlying
information. They were also interested in more than individual
buildings, they were interested in looking at whole cities.
It
was apparent that the envisaged model quality could not be exchanged
easily, so we started working on a data model and an exchange format.
But at that time it was not foreseeable that that we would be where
we are today. Around 2004 the Ordnance Survey, UK became involved and
we started to realise that a German standard alone would not be
enough. We changed direction toward a more international approach.
Since some of the people were already active in the Open Geospatial
Consortium (OGC) then it was possible to begin exploring this work
through the OGC.
V1 Magazine: What caused you to
appreciate that an international direction would be beneficial?
Kolbe: We were drawn to the usefulness
of taking this approach. By then we had a demo that some OGC members
had seen, like Bentley and Oracle. They appreciated where this work
could lead to and approved our efforts as they expanded within OGC.
Then other key players saw the usefulness and began to rally around
the concept and that put the work into an international perspective
quickly.
This gave us greater confidence in our
work. It is rewarding today to watch so many people explore
possibilities for using CityGML. It is interesting to see them
investigating and realizing the differences between CityGML and KML,
for example.
CityGML is intended as a means to share
the information contents of virtual 3D city models. KML is more about
visualising the information. CityGML is not meant as an end user
format within a browser per se, but instead, it is designed to
provide information about the objects themselves.
V1 Magazine: What is the difference
between CityGML and KML?
Kolbe: CityGML is intended as a
means to share the information contents of virtual 3D city models.
KML is more about visualising the information. CityGML is not meant
as an end user format within a browser per se, but instead, it is
designed to provide information about the objects themselves. A
CityGML dataset may have different visualizations where objects may
be selected or highlighted according to different thematic criteria
resulting in multiple KML files.
V1 Magazine: Then CityGML is about
encoding information?
Kolbe: Yes, but in the second place. In
the first place, CityGML defines structures for the organization of
urban information which can be used in a broad range of applications.
CityGML is not just interested in the shapes and graphical
appearances of objects. Instead, it seeks to represent the meaning
and functions of objects. It is more about the semantic quality of
the information. For example, this might include information about
building facades and their specific surface material and noise
absorption. That information could then be shared with different
applications like noise dispersion simulation.
V1 Magazine: That would suggest that
it could be useful in building information modeling (BIM) perhaps?
Kolbe: Yes. In 2005 there was the OGC
testbed (OWS-4) and we had a meeting with the people from the
International Alliance for Interoperability. At the time we discussed
the relationship of CityGML to Industry Foundation Classes (IFC) and
it was clear that IFC would be a wonderful data source for CityGML.
Also some of parts of the building model in CityGML are inspired by
IFC.
V1 Magazine: Why would CityGML and
IFC work well together?
Kolbe: It's important to keep in mind
that both models were not only modelling shapes but the underlying
purpose of the objects. However, they are both working at different
scales. While CityGML is representing objects at the scale of a city,
the IFC model is modeling at the building level with a great amount
of detail and richness.
For planning and construction, IFC
would be more approprate. However, when those objects need to be
integrated across the city with others, then it would be better if
they were transformed to CityGML and then can be used together with
other geographic data. This would then allow for combined spatial and
thematic queries on buildings across cities within a GIS or a spatial
database. The feasibility of this approach was demonstrated within
the OGC testbed OWS-4 in a homeland security scenario.
V1 Magazine: The situation you
describe could be likened to CAD for use in designing the buildings
and GIS in use for the wider geography.
Kolbe: Yes, that's a good analogy. The
tools change dependent upon the scale and scope. CityGML and IFC are
not just different at scale, but they also follow 2 different
modelling paradigms. Industrial and engineering professionals tend to
have a generative approach. For example, they construct a number of
objects and then bring them together to form a building shape. The
modelling employs a constructive geometric model which depends upon
combinations of parametric geometries.
Alternatively, CityGML is modelled from
observation and tailored to what can be seen. In CityGML one might
have different information for the inside and outside surfaces of a
wall. But IFC models would model the wall as one object. Using
CityGML we have to account for the fact that from a single wall
actually two ore more different wall surfaces are observable. As you
can imagine, IFC allows for much quicker manual construction, but
CityGML is much closer to surveying or photogrammetric registration
methods.
CityGML and IFC are not just different
at scale, but they also follow 2 different modelling paradigms.
Industrial and engineering professionals tend to have a generative
approach.
V1 Magazine: What do you see as the
interesting direction both CityGML and IFC will take in the future?
Kolbe: One of the questions being asked
is, how do we go about deriving IFC models from already existing
buildings? It will be interesting in the future to take a CityGML
model and try to reconstruct an IFC model from it. CityGML can be
easier reconstructed from observations than IFC. I'm interested in
using photogrammetry or laser scanning as a means to automatically
derive CityGML models. So this could be a promising processing path
to come to IFC models for the built-up world with CityGML as
intermediate step. Generally, it is much easier to move from an IFC
model to a CityGML model but hard to go in the reverse.
BIM is booming in North America and
many people are concentrating on IFC. Semantic 3D city models have
been much more investigated in Europe so far. I expect we will see
BIM in Europe within the near future but also semantic city models
coming to the US.
V1 Magazine: What other questions do
you find interesting going forward?
Kolbe: I think about those usages of 3D city
models where you have a 3D database, but do not employ a graphics
card to work with the model. Taking a homeland security example, we
might want to have an application where an important politician moves
along a particular route. In the application we are interested to
find all the windows and buildings which have good view on that route
and where possibly a sniper can hide. Instead of virtually visiting
all building models with a 3D viewer along the route, we might rather
want to query the city model to create a report of all corresponding
windows, rooms, and buildings in order to check these. Thus we would
exploit the semantic information of a city model along the route, and
especially the details that come with a highly detailed CityGML or
IFC model so we can locate and identify the windows.
Guided visualisation is another example
where CityGML is useful because we might to take a virtual tour. But
soon we may get stuck with our 3D viewer in some corner of the 3D
model or become over loaded with information and miss the points from
the things we want to see. A CityGML model would be searchable, and
provide those points or types of information to the visitor,
automatically guiding them along the way so they do not miss the
types of things they are interested in. We could even ask the model
where the best locations are for making a video of the things we
like.
V1 Magazine: What sorts of things
are you interested in today in your work?
Kolbe: My main interests lie in the
semantic modeling of city models and their automatic extraction using
photogrammetry or laser scanning. I am interested in developing
flexible models that can extract objects from imagery or point clouds
and classify them. I like the work which involves the deconstruction
of models and the reconstruction of them. Here, the whole field of
object recognition still faces the problem that while more flexible
and generic models allow to represent more complex real world
objects, they are often less specific and thus may not reject wrong
hypothesis more often. Another field of interest is the ad-hoc
integration of 3D city models, where different model parts from
different providers or data sources should be merged automatically.
CityGML is emerging as a model for defining the structures for organising urban information. It aims to identify the meaning and function of objects in a city. Thomas Kolbe became initially involved in CityGML from the beginning. Vector1 Media editor Jeff Thurston met up with him at the recently held Open Geospatial Consortium (OGC) Interoperability Day in Potsdam, Germany for this interview on the topic of CityGML.