[ZMapServer-Developers] Re: update: feature querying for PCL and ZCO

[Etienne <eti...@ul...>]

Hi,


    It seems very interesting for me. I will try it as soon as I will get it.

I have one question, I want to know if it will be possible to access the spatial
attribute of a selected feature like (ex: left, right, top, bottom coordinate)
for developping a zoom to selected feature function?

Thanks a lot and continue your great work!

Etienne

Selon Sean Gillies <sgi...@fr...>, 02.02.2005:

> Hi all,
>
> Following up on my email about ogr.py ... the primary reason why I want
> to use this module is to support feature querying in PCL (and thereby
> ZCO).
>
> I am about 1/2-way through the querying api that I would like to
> release as PCL 0.7.  Right now there is a WFS-ish means of getting
> features from disk data stores (through ogr.py).  I'll do the same for
> in-memory data stores next and then defer querying features from
> external data stores (like WFS or PostGIS) until later releases.  Chris
> Holmes has expressed interest in ZCO as a WFS client and so I might
> that up to him :)
>
> Here's how the feature querying API will work, example below comes from
> data.IFeatureStore:
>
>      def features(properties=[], filter=None, maxfeatures=0, **params):
>          """Return an iterator over filtered features
>
>          Provides a WFS-ish means to query a store for features.
>
>          Attributes
>          ----------
>          properties : []
>              list of property names to be returned with the feature.
>
>          filter : filter.Filter
>              An instance of Filter.
>
>          maxfeatures : int
>              Default value of 0 means all features.
>
>          params : mapping
>              Mapping of key=value parameters that can be added into the
> locals
>              namespace for evaluation of PythonFilters only.
>
> calling the features() method of a FeatureStore gives you an iterator
> over stored features, each item of which is a (geom, attributes) tuple.
>   The iterator will filter features depending on the "filter" parameter
> above.
>
> We're going to have two kinds of filters:
>
> 1. A very verbose and recursive filter, a fairly literal adaptation of
> the OGC's Filter Encoding spec along the lines of Geotools2
>
> 2. A filter as Python expression as exist now in the expressions of
> cartography.mapping.Rule.
>
> The second is for convenience and will only be useful for local disk
> data stores.  The more complete filter (#1) will allow us to create
> full SQL queries to PostGIS or XML requests to an external WFS service,
> two cases that can let us offload feature queries onto highly optimized
> servers.  I am also expecting that the #1 filter will provide a good
> basis for filter-generating web widgets.
>
> For you, Etienne, here is an example of the PythonFilter syntax:
>
>  >>> pyfilt = filter.PythonFilter('f.FIPS_CNTRY == "UK" and
> g.BBOX(-20,25,60,75)')
>  >>> features = store.features()
>  >>> for (geom, attributes) in features:
>  >>>     ... # do something with features
>
> Assuming you have the same world countries data as me, this gets you an
> iterator over all the U.K. countries within the specified bounding box.
>   Feature attributes are stuffed into an "f" object (as I've documented
> under Rule filters) and feature geometries are represented by the "g"
> object.  I'll be implementing the full complement of spatial operators
> to go along with BBOX, such as intersects, within, contains, etc.  The
> ogr.py module will be used to evaluate these operations.
>
> Will that work for you, Etienne?  And the rest of you?  Does anybody
> have strong feelings about features being a tuple rather than an
> object?
>
> If you want to play with this, you can check it out from CVS.  Will
> require the GDAL/OGR software and installation of the Python modules.
> Take a good close look at the rewritten PCL/setup.py file too.
>
> thanks for letting me think out loud here :)
> Sean
>
> --
> Sean Gillies
> sgillies at frii dot com
> http://users.frii.com/sgillies
>
>
>