[Qgis-user] Filtering an ArcGIS Feature Service layer

[Seggie, Graeme via Qgis-user]

PUBLIC

Hello,

Is the functionality to filter a layer via query builder an option for ArcGIS 
based service layers, within the layer's properties, Query Builder is disabled? 
Looks like not according to this (link below), but just wondered if any updates 
on that being resolved. Or are there workarounds to filter a layer once in QGIS 
I can use instead to filter a service layer, other than symbology.

https://issues.qgis.org/issues/19811

Regards,
Graeme Seggie



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[Qgis-user] Graphical Modeler to add same datasource multiple times with different styles

[Seggie, Graeme via Qgis-user]

PUBLIC

Hello,

I am trying to use the graphical modeller to do something which seems simple 
but will help users set up a map quickly based on refreshed data and existing 
styles. Basically, add the same geopackage source to map and style using the 
algorithm 'Load layer into project' then style with algorithm 'Set style of 
vector layer' several times using different qml files. Even with multiple pairs 
of this setup, every successive added layer overwrites in the table of contents 
the same, single layer as the ID of the layer seems to be retained from first 
use of  'Load layer into project' and is therefore use for each new style 
rather than a new, separate layer.

I tried to get round this by keeping simple and running the pair of steps as a 
batch call of process with different style each time, same result. Then tried 
calling the running a parent model, calling other models doing the work, but I 
still end up with a single layer on the map, the last one styled. Am I missing 
something to refresh the layer added ID so that each one is unique and styled 
separately?

Regards,

Graeme Seggie



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Re: [Qgis-user] Iterate through all elements of a layer

[Seggie, Graeme via Qgis-user]

PUBLIC

Hello,
Have you tried the plugin ‘Go2NextFeature3’. It is very useful, allows you to 
cycle in turn through features of a specified layer, in order of some specified 
field value and, on all features, or only on a selection of them. You can pan 
to or zoom to each feature identified by the tool

Regards,

Graeme Seggie
From: Qgis-user  On Behalf Of Roland 
Spielhofer via Qgis-user
Sent: 23 August 2022 13:56
To: qgis-user 
Subject: [Qgis-user] Iterate through all elements of a layer

CAUTION: This email originated from outside the organisation. Do not click 
links or open attachments unless you recognise the sender and know the content 
is safe.
Hi,
for quality checking I woul like to have a simple way of iterating 
"click-by-click" through all elements of a layer.
Like "zoom map to the selected rows" in the attribute table - but maybe without 
manually selecting one row after the other (with the fear of omitting one by 
not clicking correctly).
All I would need are two buttons (or even better two keyboard shortcuts) like 
"zoom to next feature" and "zoom to previous feature".
Any hints?

Thanks and
Regards,
Roland


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[Qgis-user] relation_aggregate

[Seggie, Graeme via Qgis-user]

PUBLIC


Hello,

I am having issues getting relate_aggregate to do anything within expression 
builder. Ideally, I would like a features (1) to (m) attributes table setup 
correctly with a relationship to allow a user to select attributes based on 
attribute criteria have the corresponding related features selected / shown on 
map as related. Given how little I am uncovering online about ways to do this, 
it might not be possible, but even getting the expression builder to update a 
field in my features with a count of corresponding values in attributes is not 
updating feature's values, I am wondering if I am doing something wrong. Should 
this work?

Within select by attributes for the feature table:
relation_aggregate( 'relation _ID','count',1) > 3

The preview of the expression returns a value for a feature (without the >3 at 
end), but when I try to calculate a field of this count - every value returns 
NULL. With the >3 it does not work at all to select anything.

It does look like 'relation_aggregate( 'relation _ID','count',1)' is fine for 
the basis of evaluating a field of values based on preview but it returns null 
on running the field calculator.

I am using QGIS 3.16

Regards,
Graeme


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[Qgis-user] Raster Calc in QGIS of PostGIS based Rasters.

[Seggie, Graeme]

PUBLIC

I am trying to add rasters from a PostGIS database to QGIS - I can see them in 
QGIS, but I cannot see them in Raster Calculator tool to undertake any Raster 
Algebra. Other non-PostGIS based rasters are available for this tool. Do I need 
to do something else in PostGIS to make them visible to this? I notice that the 
extent (within DB Manager) is Unknown and cannot be found out by clicking on 
the URL to Find Out. Is this related to the issue? They have primary key and an 
index on convex hull of their extents.

Regards,
Graeme Seggie



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[Qgis-user] QGIS - Can a plugin be installed with Python code

[Seggie, Graeme]

Hello, 

Is it possible to install QGIS Plugins with python code?

I would like to be able to provide users who have a basic QGIS image served up 
to them at login, to be able to quickly install all their required plugins with 
a tailored Python script to save them manually installing each via the usual 
menu route, one at a time.

I cannot see any code that runs in logs after a successful plugin install and 
no clues online in my searches.

Graeme
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Re: [Qgis-user] colorramp starting from white

[Seggie, Graeme]

Would adapting and using this code help at all? 

https://gis.stackexchange.com/questions/134435/what-is-replacement-for-setcolorshadingalgorithm-in-qgis-2-6/134589#134589

Graeme



-Original Message-
From: Qgis-user  On Behalf Of Raymond Nijssen
Sent: 15 June 2020 15:34
To: DelazJ 
Cc: qgis-user 
Subject: Re: [Qgis-user] colorramp starting from white

Attached is an example image.

Currently it is very hard to distinguish lakes from areas in the lowest class, 
for example in the southwest area.

In the map at the right, I manually adjusted the color ramp to start at a light 
green color. Should that be the default?

Raymond



On 15-06-2020 16:04, DelazJ wrote:
> Hi Raymond,
> 
> Instead of changing manually, why not "Invert the Color ramp" when 
> applying it?
> https://docs.qgis.org/testing/en/docs/user_manual/introduction/general
> _tools.html#the-color-ramp-drop-down-shortcut
> 
> Regards,
> Harrissou
> 
> Le lun. 15 juin 2020 à 15:53, Raymond Nijssen  > a écrit :
> 
> Am I the only one who would like the color ramps in QGIS to NOT start
> from (super nearly) white, but from a light version of the color?
> 
> For example, "Reds" should go from (light) pink to red.
> 
> Currently, the first class in my legend always looks white, as many
> other parts of my map often do. Therefore it is hard to see where my
> data is and where not.
> 
> Another thing, changing it manually is hard because of the 7 stops in
> the ramps.
> 
> I'm happy to figure this out and make a PR, but would like to know
> other's opinions first.
> 
> Regards,
> Raymond
> 
> 
> -- 
> Terglobo
> Fahrenheitstraat 1
> 5223 BJ 's-Hertogenbosch
> The Netherlands
> +31 (0) 6 25 31 49 83
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Re: [Qgis-user] Adding folder location to Browser - Favorites with Python

[Seggie, Graeme]

I actually got a response on Stack Exchange regarding this query, so in the 
interests of sharing the response...

Code to run in QGIS Python console is:

browserModel = iface.browserModel()
dirs = ['U:\dir1', 'U:\dir2', 'U:\dir3', 'U:\dir4']
for dir in dirs:
browserModel.addFavoriteDirectory(dir)

browserModel.reload()

Ref:
https://gis.stackexchange.com/questions/362638/qgis-python-to-add-folder-location-to-browser-favorites



Hello,

I am struggling to get something to work, which would seem to be an easy thing 
to do, but cannot quite make my understanding of the python docs work in 
practice.

What I would like to do, is run some python to add some folders to the 
favorites area in the browser within QGIS. This is to get round an 
environemental constraint of not being able to save a profile i.e. quickly add 
a python script to setup the favorites each time log on.

This is what I have tried thus far:

url = "U:\Tyres"
qgis.gui.QgsBrowserDockWidget(addFavoriteDirectory, uri)

-->

Traceback (most recent call last):
  File "C:\OSGEO4~1\apps\Python37\lib\code.py", line 90, in runcode
exec(code, self.locals)
  File "", line 1, in 
  File "", line 3, in 
NameError: name 'addFavoriteDirectory' is not defined

AND

url = "U:\Tyres"
qgis.gui.QgsBrowserDockWidget(addFavorite, uri)

-->

Traceback (most recent call last):
  File "C:\OSGEO4~1\apps\Python37\lib\code.py", line 90, in runcode
exec(code, self.locals)
  File "", line 1, in 
  File "", line 3, in 
NameError: name 'addFavorite' is not defined

Should I be calling the addFavorite code differently? This is within the Python 
console and script area of same.

Regards,
Graeme

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[Qgis-user] Adding folder location to Browser - Favorites with Python

[Seggie, Graeme]

Hello,

I am struggling to get something to work, which would seem to be an easy thing 
to do, but cannot quite make my understanding of the python docs work in 
practice.

What I would like to do, is run some python to add some folders to the 
favorites area in the browser within QGIS. This is to get round an 
environemental constraint of not being able to save a profile i.e. quickly add 
a python script to setup the favorites each time log on.

This is what I have tried thus far:

url = "U:\Tyres"
qgis.gui.QgsBrowserDockWidget(addFavoriteDirectory, uri)

-->

Traceback (most recent call last):
  File "C:\OSGEO4~1\apps\Python37\lib\code.py", line 90, in runcode
exec(code, self.locals)
  File "", line 1, in 
  File "", line 3, in 
NameError: name 'addFavoriteDirectory' is not defined

AND

url = "U:\Tyres"
qgis.gui.QgsBrowserDockWidget(addFavorite, uri)

-->

Traceback (most recent call last):
  File "C:\OSGEO4~1\apps\Python37\lib\code.py", line 90, in runcode
exec(code, self.locals)
  File "", line 1, in 
  File "", line 3, in 
NameError: name 'addFavorite' is not defined

Should I be calling the addFavorite code differently? This is within the Python 
console and script area of same.

Regards,
Graeme

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Re: [Qgis-user] Make selected features dominant in view

[SEGGIE Graeme]

Brilliant, thanks Richard - that works perfectly. As you say would be good if 
was default, but not too onerous and perhaps speeds up rendering to not have it 
doing so for every layer etc.

Graeme.


Hi Graeme,

I do not know an answer, but I checked and it's not that selected items always 
would end up on top...

This sounds very reasonable though... and helpfull :-) If it is not already 
there, it would make a nice Feature Request!

There is in the Layer Properties/Symbology/Layer Rendering an option to 
'Control feature rendering order' ...
Ok, so you can make an  an expression like:

 is_selected( $currentfeature )

which then makes your selected features draw last (on top).
You could at least try that.
I'm wondering if that would/could not be default painting behaviour?

Regards,

Richard Duivenvoorde

On 12/02/2020 17.34, SEGGIE Graeme wrote:
> Is there a way to have selected features dominate in the map view? I
> mean when selected not all selected features of a layer show as yellow
> (default) if there are other features in the same layer above them in
> the drawing order. In Arc – anything selected is always shown as such,
> which is a lot handier for tracking things down spatially, quickly. Am
> wondering if there is a preference that sets this, which I have not
> done.  I know I can create a sub-layer with filtering as an
> alternative to do this, but that seems like more work than it should be.
>
>
>
> Graeme
>
>
>
>
>
>
>
> This message has been scanned for malware. This message and any
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[Qgis-user] Make selected features dominant in view

[SEGGIE Graeme]

Is there a way to have selected features dominate in the map view? I mean when 
selected not all selected features of a layer show as yellow (default) if there 
are other features in the same layer above them in the drawing order. In Arc - 
anything selected is always shown as such, which is a lot handier for tracking 
things down spatially, quickly. Am wondering if there is a preference that sets 
this, which I have not done.  I know I can create a sub-layer with filtering as 
an alternative to do this, but that seems like more work than it should be.

Graeme



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[Qgis-user] QGIS - PostGIS interaction - click on feature and update other layer based on related database items

[SEGGIE Graeme]

Hello,

Is there a set of steps I can follow in QGIS and PostGIS in order to have QGIS 
act as an interactive frontend to data held in a PostGIS database? I can't 
quite find anything online to point me in the right direction, but by simple 
example I have some stops (bus) and associated timetable data stored. I can 
show the stops easily, but what I would like is to be able to click on a given 
stop and for an existing layer change (or new one be added) that shows the 
stops which are related to the selected one by a connecting route.

My initial thought is that I should have a second layer which is based on a 
dynamic query based on the selected stop. Maybe a table for results which is 
empty but on clicking on a stop, this is populated with routes associated with 
the stop and the QGIS view is refreshed to pick up the new content?

Can anyone get me started or point me at some docs that would get me going 
please?

Graeme Seggie


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Re: [Qgis-user] QGIS and ESRI's ArcMAp

[SEGGIE Graeme]

That will vary on what level of licensing you have for ArcGIS. If you are on a 
Basic license then you will probably find QGIS will fill in quite a lot of gaps 
in your functionality. Off the top of my head these would include Erase / 
Difference geoprocessing and some proximity analysis.
It will also vary accoring to any extensions you have licensed in Arc - so any 
route networking will be available (in QGIS if you have plugins and suitable 
data)) whereas in Arc you would need the Network Anlayst extentsion. Any raster 
analysis is available in QGIS, but requires Spatial Analyst extension in Arc...

Graeme



From: Qgis-user [mailto:qgis-user-boun...@lists.osgeo.org] On Behalf Of Crosby, 
Scott
Sent: 29 June 2018 13:00
To: qgis-user@lists.osgeo.org
Subject: [Qgis-user] QGIS and ESRI's ArcMAp

Good morning,

Are there tools and/or capabilities unique to QGIS that are not available in 
ArcMap? I'm currently using ArcMap 10.5.1, and recently installed QGIS.

Scott Crosby, C.F.
Forestry Supervisor II, Etoniah Creek State Forest
Florida Forest Service
Florida Department of Agriculture and Consumer Services
Certified Forester #12604, Society of American Foresters

Office: (386) 329-2555
Fax: (386) 329-2554
scott.cro...@freshfromflorida.com

www.FreshFromFlorida.com

Please note that Florida has a broad public records law (Chapter 119, Florida 
Statutes). Most written communications to or from state employees are public 
records obtainable  by the public upon request. Emails sent to me at this 
e-mail address may be considered public and will only be withheld from 
disclosure if deemed confidential pursuant to the laws of the State of Florida.




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[Qgis-user] Posted email to group not appearing

[SEGGIE Graeme]

Dear Sir / Madam,

Is there something that stops certain email addresses posting to the QGIS-User 
email list? I have noted in the past delays when I have done so, but this last 
one (below), which I posted on the 18th (05/18) has still not appeared on the 
email list.

Regards
Graeme Seggie
Senior Consultant
Prospect House, 5 Thistle Street, Edinburgh, United Kingdom, EH2 1DF

Direct Dial:  +441314609559
Main Office:   +441314601847
Website:  www.systra.co.uk

[cid:image001.png@01D3F0E6.A7867EB0]
https://twitter.com/SYSTRA_LTD
www.linkedin.com/company/36421

SYSTRA Limited is a company registered in England and Wales, (number 03383212). 
 Registered office:  Dukes Court, Duke Street, Woking, Surrey, GU21 5BH. 
Registered VAT number: GB1823826/95



From: SEGGIE Graeme
Sent: 18 May 2018 17:11
To: Qgis-user@lists.osgeo.org
Subject: Selected features (by attribute) not showing on map as selected

Is there a setting that controls whether features when selected by expression 
against attribute values, show as highlighted (yellow) or not? I am sure this 
worked in the past, but no longer does so on my install (2.18). The features 
when selected by mouse show as yellow on map and selected in attribute view, 
but not the other way round.

Regards
Graeme Seggie



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[Qgis-user] Selected features (by attribute) not showing on map as selected

[SEGGIE Graeme]

Is there a setting that controls whether features when selected by expression 
against attribute values, show as highlighted (yellow) or not? I am sure this 
worked in the past, but no longer does so on my install (2.18). The features 
when selected by mouse show as yellow on map and selected in attribute view, 
but not the other way round.

Regards
Graeme Seggie





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

[SEGGIE Graeme]

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 <gseg...@systra.com<mailto:gseg...@systra.com>>
To: "Qgis-user@lists.osgeo.org<mailto:Qgis-user@lists.osgeo.org>" 
<Qgis-user@lists.osgeo.org<mailto:Qgis-user@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 

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

[SEGGIE Graeme]

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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et susceptibles de contenir des informations couvertes par le secret 
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Re: [Qgis-user] Question about results of Union in QGIS

[SEGGIE Graeme]

Thanks Luigi,


That doesn't seem to help unfortunately.


I tried with the same two shapefiles in 2.14 and in both cases get 463 zones 
unioned to 463 resulting in over 4000 features with nulls in attributes and 
lots of features which seem to not highlight any polygon when selected.


Simplifying things, I created a new polygon shapefile with just three polygons  
- one overlapping each of the other two. Result of unioning this with itself is 
17 features. Doing same on two copies of same shapefile also results in 17 
features. However, two different shapefiles, same 3 features in one and a 
single feature in another, which overlaps two of the 3, results in 6 features 
when unioning which makes some sense.


Graeme


From: Luigi Pirelli <lui...@gmail.com>
Sent: 25 October 2016 20:46:42
To: SEGGIE Graeme
Cc: Qgis-user@lists.osgeo.org
Subject: Re: [Qgis-user] Question about results of Union in QGIS

may you test using 2.14? if I well remember that version suffered a
severe regression on ftools geoprocessing modules due the introduction
of the new 3d geometry. During the hackmeeting in Gran Canaria we
seriously though to announce the deprecation of that version, but a
fix was found => and a new qgis version + processing tests where
introduced.

regards
Luigi Pirelli

**
* Boundless QGIS Support/Development: lpirelli AT boundlessgeo DOT com
* LinkedIn: https://www.linkedin.com/in/luigipirelli
* Stackexchange: http://gis.stackexchange.com/users/19667/luigi-pirelli
* GitHub: https://github.com/luipir
* Mastering QGIS 2nd Edition:
* 
https://www.packtpub.com/big-data-and-business-intelligence/mastering-qgis-second-edition
**


On 25 October 2016 at 10:44, SEGGIE Graeme <gseg...@systra.com> wrote:
> Hello,
>
> I was doing some analysis on UNION against the same feature (on itself) to 
> get an understanding of what happens with slivers and overlaps. I started in 
> ArcGIS and was able to understand what happened with the combinations 
> available there as follows:
>
> Zone system used as input features composed of 463 zones including some 
> accidental overlap.
>
> File with accidental overlaps:
> Case 1 - self union, 1 ref to source, no gaps  - Generates 525 - due to 
> lakes (3) and slivers (16) and overlaps (43)
> -- overlaps times number of contributors i.e. two zones overlap, two overlap 
> features with same area but different source zone
> -- slivers -1 as source (since there is none) and single feature.
> Case 2 - self union, 1 gaps  - 506
> - due to overlaps
> Case 3 - self union, 2 refs to source, no gaps  - 577
> Case 4 - self union, 2 gaps  - 558
>
> Having done all this in Arc, I now wanted to check that QGIS produced the 
> same thing. I could not check for gaps / no gaps as there is no setting to 
> select regarding this. Also, the two tools I used via  (1) menu Vector --> 
> Geoprocessing --> Union and (2) Toolbox Geoalgorithms --> Vector --> Polygons 
> --> Union do not allow for a single reference to the source feature set.
>
> What puzzles me though is that instead of feature counts of the order listed 
> above, I get much more features and many, but not enough with nulls and 0 for 
> source ID fields etc. I cannot see an obvious way to get the resultant 4,139 
> features (with tool 2) down to approx 500 or so.  Tool 1, returns 3,950 
> features on the same input feature set.
>
> I also when selecting features in the area with overlaps return normal counts 
> of features i.e. 2, not multiple orders higher as I'd expect given the count 
> of features in the attribute table. Do some (lots) of them have no geometry? 
> - Actually having just added a field of area, I see this is not the case 
> either as too many have geometry on that basis.
>
> Am I missing a key difference employed in the processing of QGIS union and a 
> way to get to the relevant features which approximate to the 500+ I expect?
>
> Am using 2.12 on Windows if that has a bearing on this.
>
> Graeme.
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[Qgis-user] Question about results of Union in QGIS

[SEGGIE Graeme]

Hello,

I was doing some analysis on UNION against the same feature (on itself) to get 
an understanding of what happens with slivers and overlaps. I started in ArcGIS 
and was able to understand what happened with the combinations available there 
as follows:

Zone system used as input features composed of 463 zones including some 
accidental overlap.

File with accidental overlaps:
Case 1 - self union, 1 ref to source, no gaps  - Generates 525 - due to 
lakes (3) and slivers (16) and overlaps (43)
-- overlaps times number of contributors i.e. two zones overlap, two overlap 
features with same area but different source zone
-- slivers -1 as source (since there is none) and single feature.
Case 2 - self union, 1 gaps  - 506- 
due to overlaps
Case 3 - self union, 2 refs to source, no gaps  - 577
Case 4 - self union, 2 gaps  - 558

Having done all this in Arc, I now wanted to check that QGIS produced the same 
thing. I could not check for gaps / no gaps as there is no setting to select 
regarding this. Also, the two tools I used via  (1) menu Vector --> 
Geoprocessing --> Union and (2) Toolbox Geoalgorithms --> Vector --> Polygons 
--> Union do not allow for a single reference to the source feature set.

What puzzles me though is that instead of feature counts of the order listed 
above, I get much more features and many, but not enough with nulls and 0 for 
source ID fields etc. I cannot see an obvious way to get the resultant 4,139 
features (with tool 2) down to approx 500 or so.  Tool 1, returns 3,950 
features on the same input feature set.

I also when selecting features in the area with overlaps return normal counts 
of features i.e. 2, not multiple orders higher as I'd expect given the count of 
features in the attribute table. Do some (lots) of them have no geometry? - 
Actually having just added a field of area, I see this is not the case either 
as too many have geometry on that basis.

Am I missing a key difference employed in the processing of QGIS union and a 
way to get to the relevant features which approximate to the 500+ I expect? 

Am using 2.12 on Windows if that has a bearing on this.

Graeme.
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Re: [Qgis-user] Minimum bounding circle from cluster of points

[SEGGIE Graeme]

I don’t have enough experience of coding steps in QGIS yet, but could you do 
the following?

For each set of points sharing a plot ID attribute value:
Identify the distances between all points.
Take the furthest apart pair (or one set of if more pairs are 
equal), which would become a diameter.
From this chosen pair, get the mid-point, which becomes the midpoint of the 
circle.
Buffer the identified midpoints as circles with size of radius (half max 
distance identified) to get circle containing all others in each set.

I think that would get all of them, or at least I can’t think of a situation 
where any longest pair identified could be further away from another point to 
be left out.

Graeme.


From: Qgis-user [mailto:qgis-user-boun...@lists.osgeo.org] On Behalf Of Tina 
Cormier
Sent: 08 July 2016 14:50
To: qgis-user@lists.osgeo.org
Subject: [Qgis-user] Minimum bounding circle from cluster of points

Hi all,

Looking for help aggregating some field data subplots to the plot level. In the 
attached screenshot, you can see clusters of 4 points (4 subplots = 1 plot). 
I'd like to create a circle around each cluster that is the smallest circle 
that would encompass all 4 points. Sort of like a convex hull, but a circle 
(convex hull, in this case, would give me a triangle). Is there a tool or a 
series of steps to accomplish this without doing it manually? In the attribute 
table, I have a unique ID for each plot/cluster. So for each plot ID (in this 
case, consisting of 4 subplots), I'd like to build a circle around all of the 
subplots.

I should also mention that it's not always 4 points (subplots), and they aren't 
always covering the same size area on the ground.

I have QGIS 2.14.2 as well as one of the nightly builds from a few weeks ago 
(can download the newest one if necessary).

Thanks in advance for any ideas/insights! Google has not helped me so much yet 
with this one!
Tina


[Inline image 2]


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Re: [Qgis-user] Correct the attribute table automatically

[SEGGIE Graeme]

Which version of QGIS are you using? The later ones (certainly 2.12) allows use 
of a Virtual Attribute. In field calculator – check the box for ‘create virtual 
field’ and set the correct time to hold geometric area. This should then update 
itself accordingly.

Regards,

Graeme

From: Qgis-user [mailto:qgis-user-boun...@lists.osgeo.org] On Behalf Of 
Josefine Petrenz
Sent: 10 February 2016 09:18
To: qgis-user@lists.osgeo.org
Subject: [Qgis-user] Correct the attribute table automatically

hej hej,
at first, I would like to apologize for my english.

I have a shape-file with several areas. In my attribut table, each area has an 
information about the area size.
Step by step the areas become bigger or smaller. Every time, when this 
happened, i have to recalculate the area size.  is there a way, that the area 
size correct automatically in the attribute table, when i change the area?
thanks in advance
josi


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