[Qgis-user] FW: Simple methodology for user / data / map interactivity?

Fri Feb 24 06:34:34 PST 2017

Brent / Bernhard,

Thanks both for the detailed responses. It does look like I am re-inventing a process that can be done as an informed user, I suppose the example I suggested might be too simple to be worth going to the lengths I described, but I was thinking of some of the manual processes I see on editing data and wanted to refine a process to be such that repetitive steps could be minimised somehow.

I see some stuff from both your responses worth me looking into anyway even if not pursuing my original approach, which is good.

I have played some more with this, but am struggling to see how I would do the following, which seems like a logical thing to do with QGIS acting as a front end to a database.  If I have a zone layer and a road network layer, how do I link the two such that the selection in one layer impacts the view of the other? For example, if I select a zone, I would like that selected zone_id, to be passed to PostGIS, to identify the roads which intersect the selected zone and only display them (or display them as dominant) as selected in the network layer. Is there a way of filtering one layer on the selection of another i.e. filter on where zone_id in (selected zone_id in layer X)

Graeme.

Hi Graeme,

QGIS can do what you describe in your example out of the box.

Workflow:

0) copy layer

1) select polygon in layer

2) use the spatial query plugin

  - select source from layer_copy

  - Where the feature intersects

  - reference features of layer (tick 1 selected polygon)

- apply

3) zoom to selection of layer_copy

If you work with PostGIS you can use the layer's filter to enter any SQL thus restricting the features to those that fit into the filter rule.

Then zoom to the layer and you are done.

You can load the result of a query against a PostGIS DB as layer into QGIS, too, by using DB manager.

For automation Processing's graphical modeler might be worth looking at.

Of course you can do most things with a plugin that you e.g. connect to a layer action. I am always reluctant to write a plugin for workflows that can be performed using QGIS' standard tools, though.

I know users like to klick on a dedicated button and then something is run just the way they (think they) need it, however IMHO the smarter approach is to train the users to do these tasks themselves with the standard tools.

Bernhard

…

Sort of in the area you are asking about:

See the Action tool

Under Windows this can invoke a batch file to display the result of an sql for data about the (Postgis) feature clicked on. Even easier under Linux with a bash script. If one of the fields is an image file, you can create an action to display the image associates with the feature. A useful bit of functionality...

But to do exactly what you are asking, I think the Attribute table does this, so it is a matter of explaining the steps to your users, rather than writing code...

If you select a set of features in a Postgis (or most type of layer) layer rendered on the map, QGIS highlights the selected ones as defined in the project properties. Open the Attribute table for the layer, you will see the selected features, not only highlighted on the map, but also in the attribute list. Click the "move selected features to top" button, and the selected/highlighted features will always be at the top of the list. The attribute table has buttons at the top to zoom & pan to selected features...

Cheers,

Brent

________________________________
From: SEGGIE Graeme <gseggie at systra.com<mailto:gseggie at systra.com>>
To: "Qgis-user at lists.osgeo.org<mailto:Qgis-user at lists.osgeo.org>" <Qgis-user at lists.osgeo.org<mailto:Qgis-user at lists.osgeo.org>>
Sent: Friday, February 24, 2017 6:48 AM
Subject: [Qgis-user] Simple methodology for user / data / map interactivity?

I would like to use QGIS as a visual / interactive front to some PostGIS data and am not sure what the best route to doing so is. Happy to explore further myself, but wanted to get some input on what needs to be joined up to get started on building a little process.

Basically, if I wanted to present a map of features and have a button (like the attribute information one) which a user would click then use to select features, I want this to run a SQL query which to get related features for example and highlight them on the map view, possibly pan to the extent of the newly selected feature set.

Does this sound like a plugin – or would that be taking things too far?

Would the best way for the dynamic layer to be setup, to be a view – empty except when populated by the query and styled to be dominant over the full set of features?

As a simple example, say I had a district zone system of polygons and I wanted a user to be able to click on any polygon, and have all the neighbouring polygons highlighted, centred and fill the map view as fully as possible. What would be the best / simplest approach to this?

Graeme

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