Processing Digital Maps - Programming GPS and OpenStreetMap Applications with Java - page 90

For this one project, the choice of a path doesn't make a big dierence.
For many cities in Germany, it does make a difference, since the processing
of different products can be parallelized and clipping from unpacked files
is obviously faster.
In the end, all the processing time and disk space should be logged with
every map compilation and for every change of the setup; even for every
option applied to osmosis. A map compiler does not scale linearly with
changes and can always be fine-tuned. Also, the storage management and
moving of files over the network takes time. In the case of the OSM com-
piler, it would be helpful to abstract the deliveries folder from the hardware
(symbolic links). This way the compiler can work with one structure, re-
gardless of the underlying hard disks.
Extracting Map Layers
When downloading via JOSM, or clipping with osmosis, the result set in-
cludes all map data available in the given bounding box. A map compiler
processes this given map data to dedicated maps for a concrete applica-
tion. Therefore, it is vital to separate the data for different purposes. For
example a navigation system usually requires one map for rendering and
another for navigation. Osmosis is capable of extracting different layers of
a map. These layers are basically defined by the attribution of the map
primitives. With the command-line options below, osmosis extracts the
primary highways from the input map to a separate file:
--rx RGB-NW.osm
--wkv keyValueList="highway.primary"
--un
--wx RGB-NW_hw.1.osm
Table 7.3 lists the extraction times and resulting file sizes, when splitting
the RGB-NW.osm file into different layers. The whole process only takes 30
seconds. Note that the sum of the layer maps is about two and one-
half times as big as the source file. Consequently, this compilation step
requires about four times the size of the source to avoid hardware
failures.
In case of RGB-NW each road layer can be visually compared to the
GPX traces 7 with JOSM and adopted accordingly to match the Moving
Objects with the map. For the simple project RGB-NW, every layer was
compared to the traces and manually cleaned and then merged back to-
gether to one OSM file with JOSM.
7 I.e., convert RGB-BUELL-NW-88-080523.gpx into a layer for a better visualization.