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

Tim Sutton tim at kartoza.com
Thu Jul 7 14:38:58 PDT 2016

Hi Zoltan

> On 07 Jul 2016, at 5:55 PM, Zoltan Szecsei <zoltans at geograph.co.za> wrote:
> Hi Everyone,
> Maybe someone can give me a prod in the right direction?
> I have two Shape files (both polygons).
> I'm trying to copy an attribute field value from B.shp into A.shp, using Lon and Lat from two of A.shp attribute fields to select the relevant polygon's attribute in B.shp.
> In other words:
> A.shp attribute fields are:  "Name", "Longitude" and "Latitude"
> B.shp attribute field is "Name"
> Using the LatLong coords in the attribute fields of A.shp, I want to select a polygon in B.shp, and write that polygons "Name" contents into this A.shp's "Name" field.
> Is the field calculator up for this?

I don't think so - its not really set up for doing inter-layer references. I wrote a few lines of python that will do it though (assumes you have two layers loaded with names a and b in the layers list:

reg = QgsMapLayerRegistry.instance()
a = reg.mapLayersByName('a')[0]
b = reg.mapLayersByName('b')[0]
collection = {}
index = QgsSpatialIndex()
for f in b.getFeatures():
   collection[f.id()] = f

with edit(b):
    for af in a.getFeatures():
        p = QgsPoint(af['lon'], af['lat'])
        box = QgsRectangle(p, p)
        for key in index.intersects(box):
            feature = collection[key]
            print feature['name'], af['name'], af['lon'], af['lat']
            feature['name'] = af['name']

Just paste that into the QGIS python console and it will update the name field of b with the value in a for the places where the attributes lon, lat in a intersect b.

Please test in a sandbox before trying on your production data....



> I've been trying to find ways of reference fields in separate maps (layers), but neither Google or I are winning at this.
> Regards, and TIA,
> Zoltan
> --
> ===========================================
> Zoltan Szecsei PrGISc [PGP0031]
> Geograph (Pty) Ltd.
> GIS and Photogrammetric Services
> P.O. Box 7, Muizenberg 7950, South Africa.
> Mobile: +27-83-6004028
> Fax:    +27-86-6115323     www.geograph.co.za <http://www.geograph.co.za/>
> ===========================================
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