[OSGeo Africa] QGIS Field Calculator - attributes for 2 layers
dave at kirchhoffsurveyors.co.za
Fri Jul 8 07:07:43 PDT 2016
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
SET B.long = A.long
B.Lat = A.lat
ON ST_Intersects(A.geom, B.geom)
Would that not be an easier method?
On 08/07/2016 12:37, Tim Sutton wrote:
>> 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
> 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
> 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.
>> Any thoughts anyone?
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