Landmarks: A Thought Experiment - Landmarks: GIScience for Intelligent Services - page 32

Fig. 2.2 The Greenwich
meridian: an arbitrary choice
to anchor a geographic
coordinate system
(CC-BY-SA 3.0 Rodney
Brooks )
the location of the discovery by chalk on the ground. By doing so she defines a prime
meridian (Fig. 2.2 ) . Let us call the direction of the prime meridian North . With a
marked direction and the memory for the distance the walker has now a reproducible
characterization of the location of the discovery, one that does not change and is not
dependent on her actual pose or location. Compared to path integration, requiring
constant updating of all related locations, this is quite a relief for her memory. If she
wants to find the location again she only has to come back to the pole (a landmark),
find the prime meridian (another landmark), and memorize the distance. If she
wants to tell friends she can now text: “Go to the pole, find the prime meridian,
and walk about 20 steps”, and neither of them has to be at the pole at the time of
communication. Furthermore, future other discoveries can be linked to the pole and
prime meridian as well. The pole becomes the datum point of a global reference
coordinate system.
So, similarly to the world constructed from landmark experiences alone
(Sect. 2.1.3.1 ) , a global reference coordinate system relieves from constantly
updating internal representations, and takes over to anchor other locations.
Additionally we have gained a (polar) network structure by marking salient
locations with reference to the datum—the pole and the prime meridian. The
city of Karlsruhe in Germany, for example, shows such a radial network of lateral
circles and meridians, laid out from the palace in the center (Fig. 2.3 ) . Alternatively,
the walker could lay out a rectangular network by constructing parallels to the prime
meridian and then perpendiculars to the meridian. A rectangular network has the
advantage of allowing constant block sizes, where the radial network has constantly
increasing blocks with the distance from the pole. A rectangular grid is a street
network pattern chosen in many European settlements in the new colonies of the
eighteenth and nineteenth century, such as North America and Australia.