Hi, You should look at this french article for skeletonization of irregular polygons. The st_delaunayTriangles method can also help. Note that the middle line of a polygon is not always a straight line. Then, using linear referencing, it should be easy to walk the middle line to create perpendicular segments.
Nicolas On 29 November 2013 18:25, Karl Zinglersen <karl.zingler...@gmail.com> wrote: > I need to create transect lines across polygons for planning aerial surveys > for musk oxen and caribous in Greenland. > 1. In QGIS I have made polygons of the polygon zones for each survey. Each > polygon hold an attribute of spacing value, e.g. 5, 10 and 20 km. > 2. Lines must be equally spaced and parallel. > 3. Lines must be perpendicular to the polygon centerline. > 4. Centerline must run at the longest distance across the polygon - and > consequently the transects run as the shorter distances. > I've attached a hand made sketch, which display the concept. > > I've tried this query > > SELECT GENERATE_SERIES(FLOOR(ST_YMin(the_polygon))::int , > CEILING(ST_YMax(the_polygon))::int,200) y_value, ST_XMin(the_polygon) x_min, > ST_XMax(the_polygon) x_max from > (SELECT the_geom AS the_polygon FROM lakes) l > > > SELECT ST_Intersection(the_geom, the_polygon) AS the_geom FROM > (SELECT the_polygon, ST_Setsrid(ST_MakeLine(ST_MakePoint(x_min, > y_value),ST_MakePoint(x_max, y_value) ), ST_Srid(the_polygon)) AS the_geom > FROM > (SELECT the_polygon, > GENERATE_SERIES(FLOOR(ST_YMin(the_polygon))::int , > CEILING(ST_YMax(the_polygon))::int,200) y_value, ST_XMin(the_polygon) x_min, > ST_XMax(the_polygon) x_max from > (SELECT the_geom AS the_polygon FROM lakes) l > )c > > ) lines > > from gis stackechange > http://gis.stackexchange.com/questions/24064/filling-a-polygon-with-lines-using-postgis > > however these transects runs horisontally and don't seem to take into > account the shape and rotation of the polygon centerline. > > Should I be able to change the query or should take a different approach? > > Karl Zinglersen > > _______________________________________________ > postgis-users mailing list > postgis-users@lists.osgeo.org > http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users _______________________________________________ postgis-users mailing list postgis-users@lists.osgeo.org http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users