[Qgis-user] Update field based on a value from another layer

[Ben Hur Pintor]

Hi,

aggregate ( layer:='yourpolygonlayername', aggregate:='concatenate',
> expression:="yourfieldtoextract", filter:=contains($geometry,
> geometry(@parent)))
>

+1 for Harissou's answer. I'm gonna copy this one in my future workflows.
Thanks!

You can also try the Point Sampling Tool
<https://plugins.qgis.org/plugins/pointsamplingtool/> plugin. It allows for
multiple vectors and raster to be sampled. It creates a 3rd layer + it
results in 1 column/attribute per sampled attribute of the layers. If your
purpose is to have a single column/attribute hold the values sampled from a
series of vector layers (layer 1, layer 2, layer 3, etc.), you might need
to merge your layers to sample first.

You can also try the v.what.vect or v.what.rast GRASS modules within QGIS
for sampling vectors and rasters respectively. This option does not create
a third layer but simply updates the point layer with values from the
sampled layer although it only allows for one layer to be sampled at one
time resulting again in multiple columns if you want to sample multiple
layers.

In both cases, if you have multiple layers that you want to sample (with
similar attributes + no overlap), you can merge these layers first before
sampling them. If they have similar attributes/fields, you can then sample
the resulting layer. Of course this adds another step to your workflow.

All the best,
Ben Hur

On Wed, Jun 24, 2020 at 2:01 PM Harrissou <delazj at gmail.com> wrote:

> Hi Tony,
>
> You can use functions from the aggregate group in the field calculator of
> the point layer. No need to create a third layer. (Somehow) Picked from the
> QGIS function help panel
>
> aggregate ( layer:='yourpolygonlayername', aggregate:='concatenate',
> expression:="yourfieldtoextract", filter:=contains($geometry,
> geometry(@parent)))
>
> This will populate the point layer field with values of whatever polygon
> covers the point.
> You can also use the direct functions concatenate, concatenate_unique,
> minimum, maximum... depending on how your data are shaped and what you
> actually need.
>
> Hope that helps,
> Harrissou
>
> Le 24 juin 2020 05:29:58 GMT+02:00, "Tony Shepherd (FarmMaps NZ)" <
> tony at farmmaps.nz> a écrit :
>>
>> Hi All
>>
>>
>>
>> Am I missing something?  Is there a simple point and click tool in QGIS
>> to update an existing field in a QGIS layer based on a value in another
>> using a geographic join?  For example a series of points and a series
>> polygons.  I want to update a field in points from a value in a field in
>> polygons based on which polygon the point falls in.  Simple stuff.
>>
>>
>>
>> In a previous life using MapInfo we simply used query editor and
>>
>> update point_layer set col1 = colx from polygon_layer where obj within any
>>
>>
>>
>> To me, this seems like a very simple everyday thing to do to an existing
>> table.  I can, of course, use vector -> Join Attributes by Column but why
>> in this case would I needlessly complicate things by having to create a
>> third layer?  I don’t need an extra layer, I just need to update the one I
>> have already.
>>
>>
>>
>> I can see that the internet is littered with similar questions dating
>> back as long as QGIS has been around.  Perhaps I have missed something
>> obvious, and if so apologies for wasting the time of others, but can you
>> point me to the tool?
>>
>>
>>
>> Cheers
>>
>> Tony
>>
>>
>>
>> *Tony Shepherd*  *|*  GeoSpatial / Mapping Manager
>>
>> Ph 027 435 6193  *|*  E tony at farmmaps.nz
>>
>> FarmMaps NZ, 3 Kerwood Place, Gore 9740
>>
>>
>>
>> [image: FM_logo1_small (Custom)]
>>
>>
>>
>>
>>
>> <http://tonyshepherd8.wixsite.com/home>
>> <https://www.wunderground.com/personal-weather-station/dashboard?ID=ISTLGORE2>
>>
>>
>>
>
> --
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-- 
---
*Ben Hur S. Pintor*
bnhr.xyz
hi at bnhr.xyz


open spatial & data solutions
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