[postgis-users] Left of/Right of test

[Denis Rouzaud]

I was thinking of the case where you have something like his:


**********************
*                    *
*                    *
*         X          *
*                    *
*                    *
*                    *
* *
*******     **********

This might be a particular case, but I don't know the answer (as a human 
;) )

Or did I miss your point ?

Cheers,

Denis




On 07/23/2013 05:03 PM, Andreas Neumann wrote:
> Hi Denis,
>
> Thank you for your reply.
>
> Actually I really want to test a LINESTRING against POLYGON centroids
> (determined with ST_PointOnSurface), not just a line against a point.
>
> Is this question not answerable? As a human I can determine it. I am
> aware about the edge cases where a point is on the line, but in my case
> this can't happen as the data comes from a correctly defined polygon mosaic.
>
>  From the lines of a polygon mosaic (directed lines node to node, no
> double lines) I want to label what is left and what is right of a
> polygon border. My theme is usage zones and I should label what is left
> and right of the polygon border.
>
> Andreas
>
> Am 23.07.2013 16:48, schrieb Denis Rouzaud:
>> Oops, here is the complete code:
>>
>> pointGeom << ST_Intersection( ST_MakeLine(
>>               ST_MakePoint(ST_XMin(lineGeom), ST_Y(pointGeom)),
>>               ST_MakePoint(ST_XMax(lineGeom), ST_Y(pointGeom))
>>                            ), lineGeom
>>                                                                  )
>>
>>
>>
>> On 07/23/2013 04:31 PM, Denis Rouzaud wrote:
>>> Hi Andreas,
>>>
>>> Do you mean you want to test to a segment?
>>>
>>> Because, I can't see a true answer for a line.
>>>
>>> If you want to do test to a segment, it should do the job to test if
>>> the intersection with the horizontal line (dy=0) over your point and
>>> your input line is on the left/right to your point.
>>>
>>> So, maybe test:
>>>
>>> pointGeom << ST_Intersection( ST_MakeLine(
>>>              ST_MakePoint(ST_XMin(lineGeom), ST_Y(pointGeom)),
>>>              ST_MakePoint(ST_XMax(lineGeom), ST_Y(pointGeom))
>>>                                                                 )
>>>
>>> You have to handle the case where your segment is horizontal, but then
>>> a simple test with ST_Xmin/max should be enough.
>>>
>>> Cheers,
>>>
>>> Denis
>>>
>>> On 07/23/2013 03:54 PM, Andreas Neumann wrote:
>>>> Hi,
>>>>
>>>> I want to test a point if it is left-of or right-of a given LINESTRING.
>>>> I found the << >> and &< &> but they do not behave like I expected.
>>>>
>>>> Perhaps it is because they only test on the bounding box level and not
>>>> on the real geometry?
>>>>
>>>> Is there a left/right of test available on the geometry instead of the
>>>> bounding box?
>>>>
>>>> Thank you,
>>>> Andreas
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