[postgis-users] Create equally spaced lines within a polygon

[Nicolas Ribot]

Hi,

If baseline is straight and can be drawn, I was thinking about linear
referencing to identify steps for perpendicular lines, then using
translate/rotate operations to move the centerline to each point and
to rotate it to create perpendicular segments.
Then an intersection is done with original polygon to cut segments:

The below query shows the steps in separate CTE's to illustrate. they
could be grouped:
On the picture (http://imgur.com/pozhnZn), the red points are steps (70
units in my
example) computed from the centerline.

with iter as (
    -- computes the number of steps required to cut the centerline by a
fixed distance (70m here)

    select generate_series(1,ceil(st_length(geom)/70)::int) as idx,
ceil(st_length(geom)/70)::int as steps
    from centerline
), npoints as (
   -- computes the point geometries with LR functions

    select  idx, steps, st_lineInterpolatePoint(p.geom, (idx::double
precision/steps::double precision)) as pt, p.geom
    from iter n, centerline p
), rot_trans as (
   -- translate the centerline to move it to one step point (center of line
is taken for the move, then rotate it by pi/2 around
   -- the step point to create perpendicular segment

    select st_rotate(st_translate(n.geom,
        st_x(n.pt) - st_X(st_lineInterpolatePoint(n.geom, 0.5)),
--xcenter -xtarget,
        st_y(n.pt)   - st_Y(st_lineInterpolatePoint(n.geom, 0.5))),
--ycenter - ytarget
        pi()/2, n.pt) as geom
    from npoints n
)
-- finaly, cut each segment by the original polygon

select st_intersection(r.geom, t.geom) as geom
from rot_trans r, testpg t;

Nicolas


On 3 December 2013 19:04, Karl Zinglersen <karl.zinglersen at gmail.com> wrote:

> Hi, I believe I was unclear - the centerlines have to straight. But I'll
> keep in mind your reference to delauney triangles and skeletonization for
> future purposes.
> To create the centerline a manual sketch is probably OK. Could I then just
> move on with linear referencing? Which function should I use - most seems
> to be percentages of a linestring.
>
> Karl
>
> Den lørdag den 30. november 2013 13.21.46 UTC-3 skrev Nicolas Ribot:
>>
>> 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.zi... at 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
>> >
>> > _______________________________________________
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