Making Maps Navigable - Programming GPS and OpenStreetMap Applications with Java - page 109

8.3 Networking

The extraction of a navigable network from a map delivery restricts the
output to navigable line features. The number and complexity of links
depends on the type of navigation system, type of vehicle, etc. The number
of drivable roads is much larger for passenger cars than for large and heavy
trucks; boats can only use waterways; trains have to follow railway tracks.
Regardless of the network type (road, water, rail, etc.), a solid network
should be closed. A network is closed, if any point on the network can be
connected (routed) to any other point. The quality of the network depends
greatly on the vendor's specication. In OSM maps, the road classication
is provided as follows:

<tag k=highway v=motorway />
<tag k=highway v=primary />

<tag k=highway v=secondary />
<tag k=highway v=tertiary />

For the system, a neighborhood is a network just like the network of ma-
jor highways. It can apply the route calculation to the neighborhood (ter-
tiary) to nd the next \connector" to a higher-level network (secondary).
Most navigation systems apply two concurrent calculations: one from cur-
rent position to destination and the other vice versa.
A hierarchy simplifies the route calculation by traversing only one level
at a time. A bottom-up and a top-down connectivity of every single address
depends on the arterial classification of the field teams. With their local
knowledge, they have to decide how important each road is for routing. The
classification can be based on a number of criteria like speed categories, etc.
The network can be enhanced with hints resulting from off-line brute force
calculations.

8.3.1 The Missing <link>

On page 84, we stated that spatial data is represented in terms of points,
lines, and polygons (on the lowest level 0). Navigable networks only re-
quire \line features," not polygons. Geometrically, a line is the (shortest)
connection between two points and OSM paths are usually composed of
many lines, or in navigation jargon, edges. This is fine to collect data, but
not sucient for a network.
The OSM data format does not define an entity between edge and way:
the link. In terms of map data, a link is a polyline (sequence of nodes) with
a common set of attributes.
In practice, a eld team can initially collect a road's geometry with one
<way> . Back in the oce, the way is compared to existing map data and