Geography Reference
In-Depth Information
Web Map Tile Services for Spatial Data
Infrastructures: Management and Optimization
Ricardo García, Juan Pablo de Castro, Elena Verdú,
María Jesús Verdú and Luisa María Regueras
Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/46129
1. Introduction
Web mapping has become a popular way of distributing online mapping through the Internet.
Multiple services, like the popular Google Maps or Microsoft Bing Maps, allow users to
visualize cartography by using a simple Web browser and an Internet connection. However,
geographic information is an expensive resource, and for this reason standardization is
needed to promote its availability and reuse. In order to standardize this kind of map
services, the Open Geospatial Consortium (OGC) developed the Web Map Service (WMS)
recommendation [1]. This standard provides a simple HTTP interface for requesting
geo-referenced map images from one or more distributed geospatial databases. It was
designed for custom maps rendering, enabling clients to request exactly the desired map
image. This way, clients can request arbitrary sized map images to the server, superposing
multiple layers, covering an arbitrary geographic bounding box, in any supported coordinate
reference system or even applying specific styles and background colors.
However, this flexibility reduces the potential to cache map images, because the probability
of receiving two exact map requests is very low. Therefore, it forces images to be dynamically
generated on the fly each time a request is received. This involves a very time-consuming
and computationally-expensive process that negatively affects service scalability and users'
Quality of Service (QoS).
A common approach to improve the cachability of requests is to divide the map into a discrete
set of images, called tiles, and restrict user requests to that set [2]. Several specifications have
been developed to address how cacheable image tiles are advertised from server-side and
how a client requests cached image tiles. The Open Source Geospatial Foundation (OSGeo)
developed the WMS Tile Caching (usually known as WMS-C) proposal [3]. Later, the OGC
released the Web Map Tile Service Standard (WMTS) [4] inspired by the former and other
similar initiatives.
