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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004
may represent a building while another rectangle is in the place
of a railway station.
Conclusion 1: The meaning of geometric entities
strongly depends on the context in the sketch.
The lines of the sketch are aggregated to complex graphical
representations of an object, which are usually known to most
people of the same cultural background. If the representation is
too generic, annotations are used to fix the meaning.
Annotations can also be used to show that an instance of a
specific object is meant and not an arbitrary individual of a
class. For example one can annotate “House of the Jones
family” where the annotated object is a specific “of the Jones
family” instance of the class “House”.
Using specific instances makes the sketch very restrictive in its
meaning and is useful for navigation from point A to point B
where A and B are known places. On the other hand the use of
generic classes leads to unspecific parts of the sketch where
only the abstract information is of importance. If the generic
type "road" is used in the neighbourhood of the generic type
“church”, it describes every situation satisfying this constraint.
Conclusion 2: Drawn Objects can be generic classes
or specific instances of generic classes.
As sketches are drawn very quickly, they tend to be an abstract
and incomplete representation of the real world. We always
think and speak in terms that are an aggregation and
generalisation of what we see, touch, feel in our environment
and that are making communication possible. The
incompleteness is due to the way we navigate in our world. So
a sketch is not a map that shows completely everything in a
certain area but a subset of the maps contents that is needed for
a special purpose.
Conclusion 3: A sketch is an abstract and incomplete
representation of the world.
As mentioned before there are relations between maps and
sketches in some way because they refer to the same objects in
space. But there are also some clear differences. At first the
map has a reference frame with exactly measured coordinates
for the objects contained in the map. A consistent reference
frame is missing in a sketch and following from this fact there
are no measured coordinates for the objects in the sketch. In
principle the position of the objects on the sketch are an
arbitrary choice of the drawer.
We are able to match objects of a sketch to a certain situation
because usually some of the objects are annotated with their
names while other objects are assigned to a class (e.g. railroad
tracks) and their topological relations to other objects are
typical for the sketched area (Blaser, 2000).
2.2 Sketching tool
Usually pencil and paper are used to create a sketch. This way
of drawing strokes on a medium is very easy and quick and is
suitable for most purposes.
In the SPIRIT project the sketch is used as a alternative method
for generating spatial queries. But here the medium is a virtual
piece of paper with infinite boundaries and the drawing tool can
be a mouse or a pen used on a graphics tablet or better a
TabletPC. This has some disadvantages, because a computer
and a satisfying input device are not necessary at hand when
needed but on the other hand the virtual paper offers the
possibility of using editing methods like deleting and moving or
utilizing a predefined subset of symbols. Text can be typed in
contrast to drawing it which supports the interpretation of the
sketch.
The sketching tool is using a client server architecture (Figure
2) to obtain the sketch, process it and send the query to the
search engine.
Client
GUI
IM
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Processor |} Search Engine
Figure 2: Sketching tool architecture.
The user is using a java applet as a client on the search engine
homepage and draws the sketch on a virtual paper which is part
of the graphical user interface presented by the client. Then it
sends the sketch to a special processing server where the sketch
is compared to a reference data set. The result goes back to the
client where it is presented to the user. If he confirms the
correctness of the interpretation, the query is transmitted to the
search engine. Otherwise the sketch can be edited to achieve
better interpretation results.
The GUI should be easy to use and allow straightforward
editing of the sketch. This part of the sketching tool is still
under development; figure 3 shows a preliminary version,
which will change it’s appearance in the future.
| dem Frocess Query |
mr ; Railway station +
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‘Figure 3: Sketching Tool. T
3. INTERPRETING A SKETCH
3.1 Graph Representation
When the sketch is finished and transmitted to the processing
server, the main problem is to interpret the sketched situation
and tell where in space it represents reality. For this task at first
a reference is needed which provides enough information for an
identification. There is no use for information which is not
sketched in common like height information or property
borders. Very typical information is provided by the road
network and points of interest. But even invisible features like
administrative boundaries of well known administrative entities
can be important.
As stated earlier in this paper the map data of the reference has
some differences compared to the sketch data. So a comparable
representation must be found which contains the crucial
