A common concept of a Filter function is provided by the OGC Filter Encoding specification which is a named function with any number of arguments. And it can be used in a filter expression to perform specific calculations and it has richer expressiveness for definition of filters. Filter functions with the syntax appropriate to the language can be used in both the XML Filter Encoding language and the textual ECQL language. Filter element provides many different kinds of functions with a wide range of functionality including mathematics, string formatting, and geometric operations. It can also be used for filter element functions.
The following examples show how to use filter function or filter element functions. The first example shows the use of geometry type function for filter element. The second example shows how to use functions in SLD to get improved label rendering. For WFS filtering, we can assume that we have a feature type. And its geometry field can contain any kind of geometry. For a certain application, what we need to do is to extract only the features whose geometry is a simple point or a multipoint which can be done by using a Geo Server-specific filter function named geometry type. WFS 2.0 namespaces do not rely on any one GML version, which needs a distinct namespace and mode location for GML. This POST example uses a spatial query to select features. Note the complete announcement of namespace prefixes. In a GET request, a Filter Element can place namespaces.
For SLD format, we need to show elevation labels in a contour map. The raw figure values may display with unexpected decimal places (such as “150.0” or “149.999999”), as the elevations are stored as floating point values. We want to make sure the numbers are rounded appropriately (i.e. to display “150”).
Filter functions in SLD symbolizer expressions are used without important overhead, unless the function is performing very huge calculation. But using functions in WFS filtering or SLD rule expressions may cause performance issues in certain cases. This is usually due to a native data store filter encoder doesn't recognize specific filter functions, and thus filter must carry out the functions in memory instead. For instance, given a filter like BBOX (geom,-10, 30, 20, 45) and geometry type= 'Point' most data stores will split the filter into two separate parts. The bounding box filter will be encoded as a primary filter and carried out in SQL. At the same time the geometry type function will be executed in memory on the results coming from the primary filter. Since I have introduced some information about Air Filter Element functions, can you understand totally? Hope you can understand and use filter element properly. For more information, please visit the website through the link www.hefilter.com.
