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I am using post-gis to create a polyline that describes the flow direction of water in a landscape/catchment, to use in a hydrological model. To do that I start with a regular grid (as polygon) and connect the centre of each cell to the centre each of the 8 cells around it, using ST_Intersect. (afterwards I delete the irrelevant lines, based on the elevation of the grid). When I do this, most connections are made as I want, except that only half of the diagonal connections are made:

enter image description here

In the figure the green squares is my grid, and the white lines are the output polyline

here is a code sample I use:

INSERT INTO test_olaf.olafmain(id,id_in,id_out,ngf_in,ngf_out,the_geom,geom_in,geom_out SELECT DISTINCT ON (ST_Intersection(parcel_a.the_geom,parcel_b.the_geom) CAST(CAST(parcel_a.id as TEXT)||CAST(parcel_b.id as TEXT) as INTEGER), 
nextval('olaf_s2s_id'),
parcel_a.id, 
parcel_b.id,
parcel_a.elevation,
parcel_b.elevation,
ST_MakeLine(ST_centroid(parcel_a.the_geom),ST_centroid(parcel_b.the_geom)),
parcel_a.the_geom,
parcel_b.the_geom 

FROM test_olaf.grid as parcel_a, test_olaf.grid as parcel_b -- test_olaf.grid is my grid (based on a DEM, converted into a polygon)
WHERE ST_Intersects(parcel_a.the_geom,parcel_b.the_geom)

)
;

Does anyone know how I can connect my cell-centre to all the cells around it, or why the ST_intersect function does not work for all the cells?

thanks! Annette

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1 Answer 1

up vote 0 down vote accepted

Try adding a small buffer to one of the polygons to avoid floating-point errors.

If the units are degrees, 0.00001 will buffer by a meter or two. If the units are meters, use 1.0 or 0.1.

ST_Intersection(ST_Buffer(parcel_a.the_geom, 0.00001),parcel_b.the_geom)

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that solved it, thanks a lot! –  Annette Jun 26 '13 at 13:37
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