[postgis-users] extending sides of parcels to cut/intersect roads before merge

karsten karsten at terragis.net
Mon Mar 30 19:08:30 PDT 2020

Hi all,
I am working on a PostGIS polygon (parcel) layer where I need to merge
adjacent parcels of the same land use.In between the parcels there are roads
(that I would want to get rid of) by merging them to the adjacent land use
area they share the longest common boundary with. 
One complication of this is that the roads often stretch past the edges of
neighboring parcels making in difficult to merge them (in a query) to the
land use without producing a long (former road) area  'sticking' out. The
following graphs show what I am trying to accomplish :
Graph 1 (
) shows a typical parcel example with land uses 1 (yellow),  2 (blue) and 3
(orange) and a road in between in beige (light yellow). Adjacent parcels can
be merged alter for the three land uses when they are touching. However,
first I would like to split the road ( in order to later merge that area
their adjacent land use as well).
My question here is about the splitting (not the merging part of this work
Does anyone have any hints /ideas  how one can extend (all)  the parcel
boundaries (or any way to create a lines similar to a line between points 1
and 2) in order to cut roads so that I could merge them in a meaningful way
to the adjacent respective land uses ( see as an example of such a line in
red line in graph  2:
) ? 
This split is necessary as otherwise I could still merge the road to either
land use but it would 'stick' as a long shape way beyond the actual land use
area . The ideal final result (after cutting and then merging) I am after
should like in graph 3 (

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