[postgis-users] Order of linestrings when using st_linemerge

[Ben Madin]

G'day all,

I have an example where I have a large number of multilinestrings
representing the seaway track  from one port to another. In order to
interpolate the position of  a vessel (this is historical data) along these
tracks, we used a combination of st_linemerge and st_interpolate.

The multilinestring respects the direction of travel (from the origin to
the destination), so we thought this approach would work.

However, to use st_interpolate, we need linestrings, so we used linemerge
(looks perfect) but the line now goes from the destination to the origin.
(of course, this is not predictable - sometimes it is in the right order).

Is there any way to 'force' the linestring to have the same ordering as the
multi-line string? we are using linestringm, can we take advantage of the M
value?

example:

SELECT
st_asewkt(st_linemerge(st_geomfromewkt('SRID=4326;MULTILINESTRINGM((115.723248216579
-32.0454874969596 1,115.356718360803 -31.2649393614626
1.47593019400197,114.996206563468 -30.4833681123784
1.94793716099846,114.641442213985 -29.70082100447
2.41616662431401,114.292166370259 -28.9173431382814
2.88076010193012,113.948131063039 -28.1329775961666
3.34185510703625,113.609098642971 -27.347765569158
3.79958533994624,113.274841167352 -26.5617464753925
4.25408087178885,112.945139823841 -25.7749580707562
4.70546832037234),(112.945139823841 -25.7749580707562
4.70546832037234,112.569944851044 -24.955984154436
5.17822970085566,112.199711533849 -24.1360828251298
5.64785480680645,111.834198632833 -23.3153001472376
6.11447655756696,111.473173947045 -22.493680351942
6.57822465026799,111.116413762924 -21.6712659546459
7.03922572419879,110.763702333863 -20.8480978652059
7.49760351897373,110.414831388289 -20.0242154915183
7.95347902680775,110.069599664243 -19.1996568369764
8.40697063920594,109.727812468601 -18.3744585922724
8.85819428836509,109.389281259168 -17.5486562219864
9.30726358357774,109.053823248005 -16.7222840463694
9.75428994291997,108.721261024435 -15.8953753186967
10.1993827204972,108.391422196296 -15.0679622985443
10.6426493295126,108.06413904806 -14.2400763213105
11.0841953614157,107.73924821455 -13.411747864288
11.524124701379,107.416590369051 -12.5830066095668
11.9625396403441,107.096009924666 -11.7538815040321
12.3995409838705,106.777354747869 -10.9244008167035
12.8352281580124,106.46047588321 -10.0945921936453
13.2696993124424,106.145227288244 -9.2644827106659
13.7030514210349,105.831465577744 -8.43409892401077
14.1353803801144,105.519049776353 -7.60346691924052
14.5667811045696,105.207841078825 -6.77261235847855
14.997347622028),(105.990650130672 -5.91167003050344
14.997347622028,105.599572459125 -6.34228797455069
15.2785767014246,105.207841078825 -6.77261235847855
15.5600407259128),(106.663050248418 -5.99905605734687
15.5600407259128,105.990650130672 -5.91167003050344
15.8868531270388),(106.881437549142 -6.08393762734487
15.8868531270388,106.709016406789 -6.01184678072196
15.9769961743295),(106.709016406789 -6.01184678072196
15.9769961743295,106.663050248418 -5.99905605734687 16))')));


cheers

Ben




-- 

Ben Madin
BVMS PhD



m : +61 448 887 220

e : ben at ausvet.com.au

5 Shuffrey Street, Fremantle
Western Australia

on the web: www.ausvet.com.au


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