[postgis-users] st_split

Tue Oct 29 04:19:11 PDT 2013

Hey sorry for being unclear.

I put you a pseudo code and some explanation :

   - first you work with real world data (observation, measure), which are
   of a given precision (showing the uncertainty of the measure).
   If your data are real world observation ,they may be precise up to the
   centimeter, the meter , whatever. So there is no significance of keeping
   XXX digits after the comma.
   - Second about translating : for your computer manipulating a number
   like 650254,2354 is not the same as manipulating a number like 254,2354.
   Numbers are represented with double precision, which can be precise up to
   * 15 digits *thanks to postgres. Keeping the number of digits low in
   your coordinates "free" some precision for computing. You can translate by
   any number as long as it reduce the number of digits in the coordinates.
   You can also manually translate like : x := x - 859 ; y:= y - 87426.
   - Third : about ST_ClosestPoint : if you point are a result of a
   previous computing, they are already of limited precision, thus you have to
   compensate this by manually putting the points onto the lines (using
   ST_ClosestPoint will give you a new point corresponding to old point
   projected onto line).

Here is the pseudo code

--if your data are of limited precision (always the case ! )
snap to grid (your data precision)

--before computation
translate everyting (-(min_x+max_x)/2, -(min_y+max_y)/2)

--computation : do your spatial test

--if  precision issue not solved, use ST_ClosestPoint to project your point
onto lines

--after computation
translate everyting (+(min_x+max_x)/2, +(min_y+max_y)/2)

Cheers,

Rémi-C

2013/10/29 franco base <frenk.calza at gmail.com>

> Ayway
> St_split and St_snap don't work
>
> SELECT line,
> point,
> st_contains (st_snap (geom_line, geom_point,0.001), geom_point),
> st_geometrytype ((st_dump(ST_split(st_snap (geom_line, geom_point,0.001),
> geom_point))).geom),
> (st_dump(ST_split(st_snap (geom_line, geom_point,0.001),
> geom_point))).geom as splitted_geometry
>
> give
>
> "id";"line";"point";"st_contains";"st_geometrytype";"splitted_geometry"
>
> 1;7646;11764;t;"ST_LineString";"0102000020BB0B000006000000A4703D0A471F37412D211F04AF335341FAEDEBE0491F3741325530C2AE335341BD5296014F1F37418FC2F578AE3353414694F606521F3741D2DEE07BAE3353412D211F74541F37418FC2F578AE3353415713362F611F37410D996234AF335341"
>
> 2;7646;11764;t;"ST_LineString";"0102000020BB0B0000020000005713362F611F37410D996234AF33534123B9FC675020374112143FF6BC335341"
>
> 3;7646;11769;t;"ST_LineString";"0102000020BB0B000006000000A4703D0A471F37412D211F04AF335341FAEDEBE0491F3741325530C2AE335341BD5296014F1F37418FC2F578AE3353414694F606521F3741D2DEE07BAE3353412D211F74541F37418FC2F578AE3353415EF0DD584620374139BF2762BC335341"
>
> 4;7646;11769;t;"ST_LineString";"0102000020BB0B0000020000005EF0DD584620374139BF2762BC33534123B9FC675020374112143FF6BC335341"
>
> 5;7646;11762;t;"ST_LineString";"0102000020BB0B000006000000A4703D0A471F37412D211F04AF335341FAEDEBE0491F3741325530C2AE335341BD5296014F1F37418FC2F578AE3353414694F606521F3741D2DEE07BAE3353412D211F74541F37418FC2F578AE335341A5CBC785841F374156A2A33CB1335341"
>
> 6;7646;11762;t;"ST_LineString";"0102000020BB0B000002000000A5CBC785841F374156A2A33CB133534123B9FC675020374112143FF6BC335341"
>
> St_geometrytype=ST_LineString but in qgis I have point and line.
>
> point splitted geometry (id =2,4,6) = geom_point (starting gometry)
> line splitted geometry  (id =1,3,5)  = geom_line (starting gometry)
>
>
> 2013/10/29 franco base <frenk.calza at gmail.com>
>
>> Hi.
>> Yes the cause can be that every function has his own "tolerance"
>>
>> SELECT line,geom_line,point,geom_point,
>> ST_ClosestPoint(geom_line,geom_point), ----- return geometry=geom_point
>> ST_distance (geom_line,geom_point), ---- return 0
>> ST_contains(geom_line,geom_point) ---- return false
>>
>>
>> "line";"geom_line";"point";"geom_point";"st_closestpoint";"st_distance";"st_contains"
>>
>> 7646;"0102000020BB0B000006000000A4703D0A471F37412D211F04AF335341FAEDEBE0491F3741325530C2AE335341BD5296014F1F37418FC2F578AE3353414694F606521F3741D2DEE07BAE3353412D211F74541F37418FC2F578AE33534123B9FC675020374112143FF6BC335341";11764;"0101000020BB0B00005713362F611F37410D996234AF335341";"0101000020BB0B00005713362F611F37410D996234AF335341";0;f
>>
>> 7646;"0102000020BB0B000006000000A4703D0A471F37412D211F04AF335341FAEDEBE0491F3741325530C2AE335341BD5296014F1F37418FC2F578AE3353414694F606521F3741D2DEE07BAE3353412D211F74541F37418FC2F578AE33534123B9FC675020374112143FF6BC335341";11769;"0101000020BB0B00005EF0DD584620374139BF2762BC335341";"0101000020BB0B00005EF0DD584620374139BF2762BC335341";0;f
>>
>> 7646;"0102000020BB0B000006000000A4703D0A471F37412D211F04AF335341FAEDEBE0491F3741325530C2AE335341BD5296014F1F37418FC2F578AE3353414694F606521F3741D2DEE07BAE3353412D211F74541F37418FC2F578AE33534123B9FC675020374112143FF6BC335341";11762;"0101000020BB0B0000A5CBC785841F374156A2A33CB1335341";"0101000020BB0B0000A5CBC785841F374156A2A33CB1335341";0;f
>>
>> Remi I don't undesrtand very well your tip to translate date and use
>> St_SnapToGrid
>>
>> I cannot snap (using St_Snap) line to point because I'm working on a road
>> network and I cannot move the line versus point.
>>
>>
>> Thanks
>> fb
>>
>>
>> 2013/10/28 Rémi Cura <remi.cura at gmail.com>
>>
>>> I had the same problem,
>>> all function don't react the same way when hitting precision limit.
>>>
>>> You may want to translate your data to reduce digits, and/or use
>>> ST_SnapToGrid on everything before using ST_ClosestPoint.
>>>
>>> Cheers,
>>> Rémi-C
>>>
>>>
>>> 2013/10/28 Nicolas Ribot <nicolas.ribot at gmail.com>
>>>
>>>> Hmm this is strange:
>>>>
>>>> Finding the closest point (st_closestPoint) or using
>>>> st_lineLocatePoint to project point onto the line does not work either
>>>> on the provided dataset.
>>>> Though the st_distance(line, point) returns 0.
>>>>
>>>> Nicolas
>>>>
>>>>
>>>>
>>>> On 28 October 2013 18:42, Rémi Cura <remi.cura at gmail.com> wrote:
>>>> > You may want to split not with point but with the projected point one
>>>> line
>>>> > (use http://postgis.refractions.net/docs/ST_ClosestPoint.html)
>>>> > If it is not enough, you may want to translate all your geometries to
>>>> reduce
>>>> > the number of digits in coordinates.
>>>> > Another possibility is to use ST_Snap to snap line to point (won't
>>>> work the
>>>> > other way)
>>>> >
>>>> > Cheers,
>>>> > Rémi-C
>>>> >
>>>> >
>>>> > 2013/10/28 Nicolas Ribot <nicolas.ribot at gmail.com>
>>>> >>
>>>> >> Hi,
>>>> >>
>>>> >> The points are not on the line, probably due to rounding errors.
>>>> >>
>>>> >> st_contains(line, point) returns false.
>>>> >>
>>>> >> Nicolas
>>>> >>
>>>> >> On 28 October 2013 18:07, franco base <frenk.calza at gmail.com> wrote:
>>>> >> > Hi.
>>>> >> > I have a set of linestring and point on linestring.
>>>> >> > I want cut the linestring on the point.
>>>> >> > I can have n points for one linestring,
>>>> >> > so I use ST_Split
>>>> >> > (insted of ST_Line_Interpolate_Point and ST_Line_Substring)
>>>> >> > but it doesn't work.
>>>> >> >
>>>> >> > line;geom_line;point;geom_point
>>>> >> >
>>>> >> >
>>>> 7646;"0102000020BB0B000006000000A4703D0A471F37412D211F04AF335341FAEDEBE0491F3741325530C2AE335341BD5296014F1F37418FC2F578AE3353414694F606521F3741D2DEE07BAE3353412D211F74541F37418FC2F578AE33534123B9FC675020374112143FF6BC335341";11764;"0101000020BB0B00005713362F611F37410D996234AF335341"
>>>> >> >
>>>> >> >
>>>> 7646;"0102000020BB0B000006000000A4703D0A471F37412D211F04AF335341FAEDEBE0491F3741325530C2AE335341BD5296014F1F37418FC2F578AE3353414694F606521F3741D2DEE07BAE3353412D211F74541F37418FC2F578AE33534123B9FC675020374112143FF6BC335341";11769;"0101000020BB0B00005EF0DD584620374139BF2762BC335341"
>>>> >> >
>>>> >> >
>>>> 7646;"0102000020BB0B000006000000A4703D0A471F37412D211F04AF335341FAEDEBE0491F3741325530C2AE335341BD5296014F1F37418FC2F578AE3353414694F606521F3741D2DEE07BAE3353412D211F74541F37418FC2F578AE33534123B9FC675020374112143FF6BC335341";11762;"0101000020BB0B0000A5CBC785841F374156A2A33CB1335341"
>>>> >> >
>>>> >> > ST_GeometryType(St_Split(geom_line, geom_point))
>>>> >> > give "ST_GeometryCollection"
>>>> >> >
>>>> >> > but
>>>> >> >
>>>> >> > (ST_Dump(ST_Split(geom_line, geom_point))).geom
>>>> >> >
>>>> >> > return only 3 linestring (one for record) and the geometry is
>>>> egual to
>>>> >> > geom_line
>>>> >> >
>>>> >> >
>>>> >> > thanks.
>>>> >> >
>>>> >> > fb
>>>> >> >
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