[OSGeo Africa] QGIS Field Calculator - attributes for 2 layers

[Tim Sutton]

Hi


> On 08 Jul 2016, at 4:07 PM, David Barkhuisen <dave at kirchhoffsurveyors.co.za> wrote:
> 
> Hi Tim, Zoltan
> 
> Just a quick one with the query below.
> 
> Will it not be easier to move both tables into postgis db.
> Add empty lat and long columns to B then using SQL update the table to pull the data from A to B
> something like:
> 
> UPDATE B
> SET B.long = A.long
>         B.Lat = A.lat
> 
> From A
> JOIN B
> ON ST_Intersects(A.geom, B.geom)
> 
> Would that not be an easier method?
> 

Yes absolutely - my answer was more intended to be academic ("how could one do this in QGIS"). Given a choice doing it in PostGIS is going to be far more efficient...

Regards

Tim


> Regards
> David
> 
> 
> On 08/07/2016 12:37, Tim Sutton wrote:
>> Hi
>> 
>>> On 08 Jul 2016, at 11:30 AM, Zoltan Szecsei <zoltans at geograph.co.za <mailto:zoltans at geograph.co.za>> wrote:
>>> 
>>> Hi again,
>>> OK - it's not an index thing. More than one 'a' polygon has received the 'b' attribute from the wrong 'b' polygon even though the coordinate is inside the correct 'b' polygon.
>>> 
>>> So the next step is to suspect the index.intersects result.
>> 
>> Could you share a small testable dataset that replicates it? I made a noddy test dataset here when playing with it last night which worked fine, but it only had 2 polygons in it...
>> 
>>> Note the coords are in LatLong  "small numeric values. I can see we are using a null-length box (ie: a point) to select from 'b'.
>>> What is the internal precision of the QgsPoint?
>> 
>> the x and y properties of QgsPoint are both doubles...it depends on your platform what that means but typically
>> 
>> http://qgis.org/api/2.6/classQgsPoint.html <http://qgis.org/api/2.6/classQgsPoint.html>
>> 
>> According to wikipedia on a 64 bit machine it should be:
>> 
>> Sign bit <https://en.wikipedia.org/wiki/Sign_bit>: 1 bit
>> Exponent <https://en.wikipedia.org/wiki/Exponent> width: 11 bits
>> Significand <https://en.wikipedia.org/wiki/Significand> precision <https://en.wikipedia.org/wiki/Precision_%28arithmetic%29>: 53 bits (52 explicitly stored)
>> 
>> 
>> 
>>> My coord attributes are 'double Real 19,10'
>>> 
>>> In a script, when you select with a point, what is the snapping tolerance?
>> 
>> To use snapping you would need to use the QGIS snapping api - otherwise absolute geometry operations are being done.
>> 
>>> My project CRS is epsg 4326 - so it should be OK.
>> 
>> Yup. If you wanted to be fancy, you could extract a layer of the tie points (FptnLat. FptnLong) so that you can actually visualise the points in relation to the polygons. Maybe you have some that don't actually intersect a poly...
>> 
>>> 
>>> FWIW, b.name is string 60 which is copied into a.name string 32  -  but the contents are shorter that 32 chars.
>> 
>> I think QGIS will just truncate it if the name is longer than 32 chars.
>> 
>> T
>> 
>>> 
>>> Any thoughts anyone?
>>> 
>>> Regards,
>>> Zoltan
>>> 
>> 
>> 
>> 
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