[postgis-users] Spurious Results using ST_Split

[Jeff Adams - NOAA Affiliate]

Thanks again Remi-C. I have been playing around with the buffer function. I
ended up clipping with the original boundary and then "pushed" the polygon
boundary out 0.001 meter using the st_buffer function before running the
st_within function. Seemed to do the trick, crossing my fingers that I do
not have any split geometries that originated 0.001 meters outside of the
polygon!


On Fri, May 23, 2014 at 9:41 AM, Rémi Cura <remi.cura at gmail.com> wrote:

> The idea is ST_Within(A,B) is true if no point (mathematical sense) of A
> is outside of B.
>
> Now, when splitting a line with another one (the boundary of the polygon
> in your case), postgis creates a point just on the boundary of this polygon
> to end the line.
> The problem is, this point could be in either side of the line depending
> of the rounding error of the computing.
>
> It is not "reported" because it is the limit of the current postGis
> approach regarding numerical precision.
>
> The solution either involve enforcing the knowledge you have of what you
> want
> (for instance, force the point that was created by the split to be on the
> boundary of the polygon)
> or use error threshold (effectively becoming fuzzy predicates).
>
> In your case and if you want fast processing, you could change your
> predicate to avoid using ST_Within but rather predicates involving a
> threshold like ST_Dwithin.
> Here you could use ST_Max_Distance to filter out outside geometries.
>
> Now if you really want correct answer I'm affraid you would have to use
> the buffer function (example : bufferize your lines with tha amount of
> uncertainity on your data and compute the shared area with the polygon. If
> the shared area is big relatovely to the buffer area, you are inside, else
> you are outside.
> )
>
>
> A more pragmatic approach would be to cut your lines not with the polygon
> but with a buffer of the boundary of the polygon, so that your cut lines a
> clearly inside or outside the polygons.
>
> Cheers;
> Rémi-C
>
>
>
>
>
>
> 2014-05-23 14:28 GMT+02:00 Jeff Adams - NOAA Affiliate <
> jeff.adams at noaa.gov>:
>
> Thanks for the response Remi-C. Could you please give me a brief overview
>> of what the workaround would involve? Is this a reported issue?
>>
>>
>> On Fri, May 23, 2014 at 8:08 AM, Rémi Cura <remi.cura at gmail.com> wrote:
>>
>>> Hey,
>>> it looks like a classical precision issue.
>>> There are some workaround but they are a little bit annoying.
>>>
>>> Cheers,
>>> Rémi-C
>>>
>>>
>>> 2014-05-23 13:46 GMT+02:00 Jeff Adams - NOAA Affiliate <
>>> jeff.adams at noaa.gov>:
>>>
>>>>  Greetings,
>>>>
>>>> I have been working with the st_split function and have been receiving
>>>> strange results. Specifically, I have been using a polygon geometry to
>>>> split a linestring geometry. After the split I have been using st_within to
>>>> identify which of the resulting split linestring geometries are inside and
>>>> which are outside. Sometimes the st_within results are as expected, other
>>>> times, split linestring geometries that are clearly within the polygon
>>>> feature (they were created using the polygon as the blade) are not
>>>> identified as so. When checking in ArcGIS, the features that returned false
>>>> using the st_within function in PostGIS (but by the nature of how they were
>>>> created should be within the polygon) are being identified by ArcGIS as
>>>> being within the polygon. Does anybody know why PostGIS' st_within is
>>>> returning false with these features? Any workarounds? I am performing this
>>>> analysis on a fairly large dataset.
>>>>
>>>> Thanks in advance...
>>>> Jeff
>>>>
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>>>
>>>
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>>
>>
>>
>> --
>>  Jeffrey D. Adams
>> Contractor
>> OAI, Inc.
>> In support of:
>> National Marine Fisheries Service
>> Office of Protected Resources
>> 1315 East West Hwy, Building SSMC3
>> Silver Spring, MD 20910
>> phone: (301) 427-8434
>> fax: (301) 713-0376
>>
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>
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-- 
Jeffrey D. Adams
Contractor
OAI, Inc.
In support of:
National Marine Fisheries Service
Office of Protected Resources
1315 East West Hwy, Building SSMC3
Silver Spring, MD 20910
phone: (301) 427-8434
fax: (301) 713-0376
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