[postgis-users] splitting 4D (XYZM) linestrings and interpolating/retaining M

[Phil Hurvitz]

Hi all, I'm trying to split 4D linestrings and interpolate the M value.

Here is a sample data set:

with --make a simple xyzm line l as (select 
st_makeline(st_makepoint(0,0,0,0), st_makepoint(0,1,0,20)) as g0) --show 
as text, clearly shows M value select st_asewkt(g0) from l;

           st_asewkt
------------------------------
  LINESTRING(0 0 0 0,0 1 0 20)

Splitting with a point as the blade interpolates the M value. If I split 
the line at 25% along its length, the M value is interpolated appropriately.

--split by point
with
--make a simple xyzm line
l as (select st_makeline(st_makepoint(0,0,0,0), st_makepoint(0,1,0,20)) as g0)
--an xy point
, p as (select st_makepoint(0, 0.25) as g1)
--split
, s as (select st_split(l.g0, p.g1) as g2 from l, p)
--show as text, clearly shows first element ends and second element starts at M = 5, which is 25% along
select st_asewkt((st_dump(g2)).geom) from s;

             st_asewkt
---------------------------------
  LINESTRING(0 0 0 0,0 0.25 0 5)
  LINESTRING(0 0.25 0 5,0 1 0 20)

However, splitting with a line seems to cause the M value to disappear 
(set to zero):

--split by line
with
--make a simple xyzm line
l as (select st_makeline(st_makepoint(0,0,0,0), st_makepoint(0,1,0,20)) as g0)
--an xy line for splitting
, c as (select st_makeline(st_makepoint(-1, 0.25), st_makepoint(1, 0.25)) as g1)
--split
, s as (select st_split(l.g0, c.g1) as g2 from l, c)
--show as text, M is now zero
select st_asewkt((st_dump(g2)).geom) from s;

          st_asewkt
----------------------------
  LINESTRING(0 0 0,0 0.25 0)
  LINESTRING(0 0.25 0,0 1 0)

Also the same happens splitting with a polygon, in this case, a buffer:

--split with buffer
with
--make a simple xyzm line
l as (select st_makeline(st_makepoint(0,0,0,0), st_makepoint(0,2,0,20)) as g0)
--a buffer for splitting
, b as (select st_force4d(st_buffer(st_makepoint(0,1,0,5), 0.5, 10)) as g1)
--split with buffer
, x as (select st_split(l.g0, b.g1) as g2 from l, b)
--M goes to zero
select st_asewkt((st_dump(g2)).geom) from x;

           st_asewkt
-----------------------------
  LINESTRING(0 0 0,0 0.5 0)
  LINESTRING(0 0.5 0,0 1.5 0)
  LINESTRING(0 1.5 0,0 2 0)

Can anyone provide advice on how to split the 4D line with a non-point 
geometry and calculate interpolated M values?

Thanks for any guidance.

-- 
-P.

**************************************************************
Philip M. Hurvitz, PhD | Research Assistant Professor | UW-CBE
Urban Form Lab  | 1107 NE 45th Street, Suite 535  | Box 354802
University of Washington, Seattle, Washington  98195-4802, USA
phurvitz at u.washington.edu | http://gis.washington.edu/phurvitz
"What is essential is invisible to the eye." -de Saint-Exupéry
**************************************************************