Communication Aspects: How Landmarks Enrich the Communication Between Human and Machine - Landmarks: GIScience for Intelligent Services - page 190

a

b

B

End

A

Start

c

Fig. 6.7 Different geometric operations in processing a sketch, after [ 7 ] . ( a ) Buffer zones used to
determine whether a stroke forms part of an object; ( b ) closing gaps within an object; ( c )removing
overshoots for polygonal objects

to close gaps, for lines interruptions in their flow may be closed (see Fig. 6.7 b ).
If objects form a closed loop, i.e., represent a region, any overshooting lines are
removed (see Fig. 6.7 c ).
Furthermore, visual understanding has to determine how the various objects
relate to each other, i.e., to understand spatial relationships in the sketch. It is widely
accepted that qualitative spatial relations are best suited to this end [ 13 , 23 , 29 , 65 ] .
Egenhofer [ 23 ] suggested to use five types of binary spatial relations: coarse
topological relations (using the nine-intersection model [ 24 ] ), detailed topological
relations, metrical refinements (in line with [ 25 ] ), coarse cardinal directions [ 30 ] ,
and detailed cardinal directions. Using these relations results in fairly complex,
comprehensive sketch descriptions that are also flexible enough to allow for relaxing
relations in an ordered manner. This is important for querying spatial data based on
sketches where users ask for configurations that reflect the provided sketch in a
qualitative way, i.e., describe scenes that are similar to the one sketched.
After establishing what objects there are in a sketch and how they relate to each
other, the next step is to understand what these objects represent. This conceptual
understanding is much harder than visual understanding , which should not be hard
to guess given the previous discussions around geometric and semantic information
in this topic.
The simplest way of handling conceptual understanding is to offer a fixed
vocabulary. Such a fixed set of objects and relationships between these objects
restrict what users may express in their sketch. In general, it helps understanding