[mapguide-users] Help with Spatial Contexts

[Jackie Ng]

The logical model goes like this:

A Feature Source
 - Has 1..n spatial contexts
   - Each spatial context is uniquely named
 - Has 1..n feature classes
   - Each feature class has a designated logical geometry property
     - That geometry property has a spatial context association (to an
existing name spatial context in the Feature Source)

Thus a layer's spatial context is the spatial context of the chosen geometry
property of the feature class it is referencing. You don't assign a spatial
context to a layer, you assign a feature class/geometry and the layer picks
up the referenced spatial context from that.

Now seeing you are talking about PostGIS in your other un-answered post, let
me try to have a reasonable guess at how the FDO provider translates the
PostGIS database into this logical model

A Feature Source (A connection to a PostGIS database)
 - Has 1..n spatial contexts
   - [PostGIS Provider] It most likely infers this list from the
spatial_ref_sys and geometry_columns metadata tables
 - Has 1..n feature classes
   - [PostGIS Provider] Any table that has a geometry/geography column
   - Each feature class has a designated logical geometry property
     - [PostGIS Provider] It probably either takes the first
geometry/geography column or whatever is specified in geometry_columns
     - That geometry property has a spatial context of the chosen geometry
property of the feature class it is referencing.
       - [PostGIS Provider] It probably either looks up geometry_columns or
ST_SRID()'s the geometry/geography column so it knows what spatial_ref_sys
table entry you are referring to.

Now if you're wondering what is the purpose of a spatial context, it is
primarily to determine the coordinate system of the originating layer.
MapGuide uses spatial context information to determine if layers need
on-the-fly feature re-projection when rendering maps.

Hope that helps.

- Jackie



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