An interesting feature of GML is to consider it as a database, but only in the strictest sense of the term. That is, as a collection of data. As a database format, it can be queried. In order to do this, we need a query language with spatial operators. In addition, in order to use any query language over GML, it is necessary to find an implementation that allows to exploit all its features, i.e., an efficient storage of GML documents is necessary. The general aim of this chapter is to discuss different approaches for storing and querying GML documents. In order to achieve our aim we discuss well-known approaches to the storage of XML documents (with only alphanumeric data) and their application to GML documents. Although there are many approaches to storing and retrieving XML documents with only alphanumeric features, few approaches are applicable to query GML documents.
The general aim of this chapter is to discuss different approaches for storing and querying GML documents. In order to achieve our aim we discuss well-known approaches to the storage of XML documents (with only alphanumeric data) and their application to GML documents.
In the following sections we show that this is not a trivial problem because, due to the resources required to query and store spatial elements, appropriate XML-based approaches with alphanumeric operators do not obtain good results when combined with spatial operators. Furthermore, some XML-based approaches are not applicable to GML documents.
Key Terms in this Chapter
Joins Operator: Join is a dyadic operator that is written as RS in relational algebra where R and S are relations. The result of the join is the set of all combinations of tuples in R and S that are equal on their common attribute names. This operation is the most expensive in an implementation.
GML: Geography Markup Language - GML is an XML grammar written in XML Schema for the modelling, transport, and storage of geographic information.
SQL: Structured Query Language – SQL is the most popular computer language used to create, modify and retrieve data from relational database management systems.
XML: eXtensible Markup Language - XML is a W3C-recommended general-purpose markup language for creating special-purpose languages.