Spike-Analyzer

Function for determining spikes in polygons

This function tries to determine spikes by using the buffer-function. A small negative buffer removes spikes and decreases the perimeter of the polygon significantly. It should be used either as a preselection for the spike-remover to increase performance or stand-alone for simple analyze purposes. Default values are (-0.02 [meters], 1) and lead to useful results.

Function

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-- 
-- $Id: spike_analyzer.sql 2009-12-01 08:00 Andreas Schmidt(andreas.schmidtATiz.bwl.de)  &  Nils Krüger(nils.kruegerATiz.bwl.de) $
--
-- spike_analyzer - helps to find spikes in polygons
-- http://www.izlbw.de/
-- Copyright 2009 Informatikzentrum Landesverwaltung Baden-Württemberg (IZLBW) Germany
-- Version 1.0
--
-- This is free software; you can redistribute and/or modify it under
-- the terms of the GNU General Public Licence. See the COPYING file.
-- This software is without any warrenty and you use it at your own risk
--  
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create or replace function spike_analyzer(geom geometry, testbuffer float, spikeindex float)
returns boolean as
$$

select case when st_geometrytype($1) in ('ST_Polygon', 'ST_MultiPolygon') then
	(select case when 
		(select max(index) from
			(select (case when (st_perimeter(geom) > 0 and st_area(geom) > 0 and st_perimeter(st_buffer(geom, $2)) > 0)
				then st_area(st_buffer(geom, $2)) / st_area(geom) / (st_perimeter(st_buffer(geom, $2))/st_perimeter(geom))
				else 0
			end) as index from
				(
					select (st_dumprings($1)).geom
				) as sub
			) as max
		) > $3
		then true 
		else false 
	end)
	else false 
end;

$$
language 'sql'
immutable
;

comment on function spike_analyzer(geometry, float, float) is 'Function to detect spikes in polygon-features, default values are (-0.02 [meters], 1) and lead to useful results';