Geography Reference
In-Depth Information
(a) Buffer=0
(b) Buffer=2
Figure 10.
WMS labelling issues. (a) Requesting individual tiles yields duplicate labels between adjacent
tiles. (b) With metatiling labels are not duplicated.
The analyzed tile cache implementations (see Section 4) allow users to configure the size
of metatiles. For a given request, the cache orders a metatile of pre-configured size to the
WMS server, centered on the requested tile. Considering a scenario where the cache is neither
complete nor empty, this selection of the area to generate may not be very efficient, because it
is probable that some of the tiles contained in the metatile would already be cached.
Under the assumption that the surrounding area of the requested tile is not uniformly cached,
a novel algorithm for the optimal selection of the metatiles to generate has been developed.
This procedure, illustrated in Figure 11, seeks to obtain, based on the current state of the
cache, the metatile that contains the requested tile (but not necessarily centered in it) and that
provides the system with the maximum new information.
In order to validate the hypothesis that a performance improvement can be achieved by using
metatiles, the following experiment has been realized. A total of 2000 different tiles have been
requested to the CORINE (
CoORdination of INformation of the Environment
) service
8
proxied
by the
WMSCWrapper
tile cache. The experiment has been repeated for different metatile
sizes, always starting from an empty-cache state. The mean latencies measured for each
configuration are collected in Table 3.
8
http://www.ign.es/ign/layoutIn/corineLandCover.do
