[postgis-users] Use of tolerances in topology

[Jose Carlos Martinez]

Hi Sandro, Im learning from topology.sql how the topology functions are 
working with more detail.

I have a question about how the tolerances are managed, for example:

I see (ST_AddIsoEdge) that you have used some times spatial predicates 
as st_contains or st_intersects. As you know these predicates (and 
others too) just work in a good way if the geometries are snapped to 
each other previously,  so my question is why you didnt use st_Dwithin 
with a tolerance? or maybe these functions expect the geometries to be 
already snapped by totopogeom for example and they shouldnt be used 
directly?

regards,
Jose


CREATE OR REPLACE FUNCTION topology.ST_AddIsoEdge(atopology varchar, 
anode integer, anothernode integer, acurve geometry)
....

   --
   -- n) Check if curve crosses (contains) any node
   -- I used _contains_ here to leave endpoints out
   --
   FOR rec IN EXECUTE 'SELECT node_id FROM '
     || quote_ident(atopology) || '.node '
     || ' WHERE geom && ' || quote_literal(acurve::text)
     || ' AND ST_Contains(' || quote_literal(acurve::text)
     || ',geom)'
   LOOP
     RAISE EXCEPTION
       'SQL/MM Spatial exception - geometry crosses a node';
   END LOOP;

   --
   -- o) Check if curve intersects any other edge
   --
   FOR rec IN EXECUTE 'SELECT * FROM '
     || quote_ident(atopology) || '.edge_data
     WHERE ST_Intersects(geom, ' || quote_literal(acurve::text) || 
'::geometry)'
   LOOP
     RAISE EXCEPTION 'SQL/MM Spatial exception - geometry intersects an 
edge';
   END LOOP;