[Qgis-user] Atlas: Filter Empty Pages

[Albin Blaschka]

Am 23.03.2016 um 07:27 schrieb Karl-Magnus Jönsson:
> Hi!
> QGIS likes to have something unique. Try to add something like this in your select statement. This is a snippet from a view I made that works:
>
> 	SELECT row_number() OVER (ORDER BY o.id, s.id) AS uid,
>     	 s.id,
> 	...
>
> Row_number creates a counter and then you have to decide the order in "OVER()". o.id and s.id is just what I use to sort on in my tables.

Hi!
Thank you for your suggestion, but I *do have* a primary key, which I 
selected in the dialogue for loading it as a postgis layer... It is not 
only QGIS, which likes something unique, every table in a database likes 
it, too ;-)
With the workaround in the Database Manager, everything is ok...

best regards,
Albin


> -----Ursprungligt meddelande-----
> Från: Qgis-user [mailto:qgis-user-bounces at lists.osgeo.org] För Albin Blaschka
> Skickat: den 22 mars 2016 15:59
> Till: qgis-user at lists.osgeo.org
> Ämne: Re: [Qgis-user] Atlas: Filter Empty Pages
>
> Hello!
>
> So, finally, I was successful: The idea with the virtual layer made it simple, as the point data is a postgis layer anyway, after storing the grid in the database, I created a view, where I have my grid and the count of points per cell, which I use for the creation of the atlas.
> Brings other advantages as well... (...wondering why I had this idea not
> earlier...)
> The only caveat was that QGIS did not want to load the view as layer directly - It said, the layer is not valid. But after doing a "select * from myview" in the DB-Manager and adding the result as layer, it worked. Will look into it later...
> (Postgres 9.4.6, PostGIS 2.1, pretty standard Ubuntu server - QGIS 2.14 on Win7 - maybe somebody has an idea on that, too?)
>
> Thank you all for your help and specially Andreas!
> best regards,
> Albin
>
> Am 21.03.2016 um 17:00 schrieb Andreas Neumann:
>> hm - actually, I was wrong.
>>
>> It is not so easy to filter dynamically - as $geometry equals
>> @atlas_geometry and you don't have a relation with the points.
>>
>> My next try would be to use a "Virtual Layer" and add a spatial
>> relationship between your atlas coverage layer (the grid) and a column
>> containing the count of points within the atlas feature.
>>
>> See
>> https://www.qgis.org/en/site/forusers/visualchangelog214/index.html#fe
>> ature-virtual-layers
>>
>> You need QGIS 2.14 to use the virtual layers.
>>
>> You can add a virtual column to the atlas coverage layer that uses a
>> spatial relation, group by and count. If the count is zero you can
>> filter that coverage feature away.
>>
>> This should hopefully work. Let us know if you are stuck.
>>
>> Andreas
>>
>> On 21.03.2016 16:41, Andreas Neumann wrote:
>>> Hi Albin,
>>>
>>> You can do this dynamically. The Atlas filter also accepts geometry
>>> relationship functions. Have a look at the QGIS expression editor.
>>>
>>> You can try with the intersects / within / overlaps tests and test
>>> $geometry against @atlas_geometry.
>>>
>>> Your filter could look like:
>>>
>>> within( $geometry,  @atlas_geometry )
>>>
>>> Can you please report back if that works?
>>>
>>> Greetings,
>>> Andreas
>>>
>>>
>>>
>>> On 21.03.2016 14:47, Albin Blaschka wrote:
>>>> Am 21.03.2016 um 14:20 schrieb Bernd Vogelgesang:
>>>>> sounds maybe to easy, but why don't you just make a spatial query
>>>>> on your coverage grid to select and save those elements which do
>>>>> have points inside (Grid Layer Contains Points Layer e.g.) and then
>>>>> use that new coverage layer to produce your Atlas?
>>>>>
>>>>> Or do you search for a more clever approach?
>>>>>
>>>>> Cheers
>>>>> Bernd
>>>> Hello Bernd, Joris & List!
>>>>
>>>> Ah, yes - that would be a possibility, which I did not think of -
>>>> Thank you both!
>>>> ...but in general I was on the quest for a "more clever" meaning
>>>> here "dynamic" approach, I have to admit: So, the aim would be in
>>>> future, just opening the project, printing the atlas without further
>>>> intervention and getting an "up-to-date" atlas...
>>>> But in the meanwhile, it should be ok with the "manual" approach.
>>>>
>>>> Thanks and greetings!
>>>> Albin
>>>>
>>>>> Am 21.03.2016, 13:53 Uhr, schrieb Albin Blaschka
>>>>> <albin.blaschka at standortsanalyse.net>:
>>>>> Hello List!
>>>>>
>>>>> I am starting to use the Atlas feature for the first time and had
>>>>> the following problem:
>>>>> I have a coverage-layer (a vector grid) and a point layer
>>>>> (collection sites). At the moment, there are grid cells in which no
>>>>> site (point) is located.
>>>>> My problem is: I would like to filter those "empty"
>>>>> atlas-elements/pages out. In other words, if the atlas-page
>>>>> contains no element of the point layer, it should not get
>>>>> printed/produced.
>>>>> I tried myself and failed, I googled and failed...
>>>>>
>>>>> Is this possible and if yes, how?
>>>>>
>>>>> Thanks in advance,
>>>>> Albin
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