Hello all

Maybe some of you have QGIS 2.16.

When using Vector | Research tools | Select by location

The interface is not very informative about what is the purpose of the 
'Precision' parameter.


Well, somebody at 
http://gis.stackexchange.com/questions/210948/how-does-the-precision-parameter-in-select-by-location-tool-work?noredirect=1#comment321716_210948

pointed out a link on github.... qgis development.


Precision:

https://github.com/qgis/QGIS/pull/2341

Improve robustness of geometry predicates with new precision parameter in by 
location algorithms.

Snap geometries to a virtual grid of step precision before applying geometry 
predicate.
This also affect bbox used for index intersection.

These features were funded by: MEDDE (French Ministry of Sustainable 
Development)

Just a little theoretical explanation of this would be very helpful... ...how 
the virtual grid and the bounding box work for the algorithm.

I tested it with very small numbers but I don't know if this is affected by the 
extent of the layer...


I'm wondering if this would be a kind of 'clustering tolerance' or fuzzy 
tolerance, like that on ArcGIS products.


This 'tolerance' issue is important but, can it be applied into QGIS, and at 
the same time, complying to Simple Features Standard?


I made some tests with dummy geodata in a spatialite db and Java Topology Suite 
(JTS).

In QGIS, select by location (intersection) does not select a point that lies on 
the boundary of a polygon or at the interior of a line segment. (using the SFA 
definitions of boundary and interior....


Turns out...

if I want that bordering coincident point to be selected, then a vertex must 
lie in the same place of that point (i tested it using ST_Snap in DB Manager's 
SQL Window...).


This is true for all programs using SF but not for JTS.  JTS seems more 
'robust' applying predicate rules.  I don't know why in JTS a point in a 
boundary or line interior is selected using intersection but not in other 
products... it is not the same algorithm? JTS java and GEOS


I guess is not that simple (infinite points in a line segment) but the rule for 
intersects predicate states that an intersection is true if geometries share at 
least a point, being on their interiors or boundaries....


...

By the way... having a 'buffer' parameter on select by location would be very 
useful...

is somebody willing to fund it? crowd-sourcing?



________________________________
Iván Santiago
GIS Specialist
Information Technologies
PR Office of Management and Budget
787.725.9420 x 2409
http://gis.pr.gov/<http://gis.pr.gov/>
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