GeoFilter

Geofilter is a simple geospatial filter language that is designed to work well with web applications. It's primary purpose is to help with creating readable, cacheable urls for geospatial searches but also uses an internal JSON representation that can be easily translated to other spatial filter types (e.g. OGC Filter Format).

General Syntax

The general syntax of a geofilter is a forward-slash (/) delimited filter string. The use of the forward slash means that you get readable urls for free.

For the most part, geofilter expressions are defined across two sections, with the first section specifying the type of filter to invoke and the second being comma-delimited arguments that will be passed to the filter:

http://testgeo.org/search/FILTERTYPE/FILTERARGS

When multiple filters are provided, these filters are combined only the intersection of two filters is returned with the results. For instance, the following request would return the results of filter TYPE1(ARGS1) AND TYPE2(ARGS2).

http://testgeo.org/search/TYPE1/ARGS1/TYPE2/ARGS2

At this stage, no provision for an OR operator is being considered.

Filter Types

The following filter types are being investigated / implemented for the initial release of the GeoFilter library. While some of the filter types do not include a /P:PROPNAME specifier (geospatial queries usually), others will usually require this syntax.

The geofilter initialization step, does include provision for configuring the default properties that will be targeted for both spatial and also a non-spatial filter operation. The defaults used are the_geom for a spatial operation, and name for a non-spatial operation.

In all of the filter operations specified below, you can specify a leading /P:PROPNAME url segment to override the default targeted fields.

BBOX

A bounding box filter.

/BBOX/MINLAT,MINLON,MAXLAT,MAXLON

The four arguments are all numeric floating point values, and correspond to the latitude and longitude of the bounding box sw-corner and the latitude and longitude of the bounding box ne-corner respectively.

DWITHIN

A distance within filter. The distance within filter returns all items that fall within the specified geometry. The BUFFERDIST arg is used to specify the buffer that should be applied to the geometry search. A 0 buffer indicates the exact geometry will be used, a positive buffer indicates the geometry will be expanded, and a negative buffer indicates the buffer will be shrunk (where possible).

/DWITHIN/GEOMTYPE,BUFFERDIST,GEOMETRY

EQ - Equals

/EQ/VALUE

GT - Greater Than

/GT/VALUE

GTE - Greater Than or Equal To

/GTE/VALUE

LT - Less Than

/LT/VALUE

LTE - Less Than or Equal To

/LTE/VALUE

Limitations

The Geofilter format is not designed to provide constructs for describing complicated filter operations, and has not provision for providing OR rule (all filters are combined with an AND condition).