[postgis-devel] Postgis "Split" with tolerance

[Sandro Santilli]

On Fri, Nov 29, 2013 at 10:48:54AM +0100, Rémi Cura wrote:
> ST_Split(line,point) is currently broken.
> This is quite a neutral statement, because given a random point on a random
> line, split won't do anything (p_correct_answer<1/10k )!
> 
> The only work around I can think about are
> _ snap the line to the point : it implies moving the line, which is not an
> option if you do it several times with different points, and is more
> abstractly a very bad idea

This is what you'd be doing anyway by implementing a tolerance.
That is, the line would be moved anyway.

> _ change the point to a very small line in the perpendicular direction of
> the line we want to split, then use split(line,line). Again not a very good
> option because you would possibiliy split the line in more than one point
> (and anyway split(line,line) has also precision issues).

Still this is a legit options.

> I understand PostGIS is a big project and we have to be conservative,
> that is "we don't break a fine function with changes bringing unknown
> consequences for unproven benefits",
> but this function is broken, and we could change it to a function working
> most of the time   !

Yes, we could change it by accepting a tolerance value.
What's wrong with that ?

> *Pros of this change :*
> 
> _a line will be split by a point correctly (p_error < 10^6)
> _no heavy changes in code or methods
>
> *Cons of this change :*
> 
> _the line would be split by any points close enough (lengthofline/10^12).
> (that is all points less than 10 micro meters away from a line 1000 km long
> !).

PostGIS doesn't deal with unit, so you don't know if those are
micro meters or micro peta meters.
You might be splitting a milli meter line by a point... 

All I'm saying is I'd like precision/tolerance to be _EXPLICIT_.
Particularely important when you're about to run a _set_ of operations
and you want them to be _consistent_ as per precision.

> *I would also like adding ST_Split(geom,geom,tolerance).*
> In fact it could be an easy wrapper around ST_Split(geom,geom) if this
> function was working. (something like: if the point is DWithin, split the
> closest part of the line)

Please go ahead. Such a function would be accepted.

NOTE: TopoGeo_addPoint does take a tolerane, and does use ST_Split after
      snapping the line to a DWithin point to implement it.
      Haing it available natively in C would speed that up.

--strk;

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