[Qgis-user] Print one map for every category in layer with fixed extent

Sun Feb 5 15:51:54 PST 2017

So,

Imagining that you have layer as a regular grid. Each cell has, for
different species, the number of individuals found stored in columns. You
want to keep the map always with the same extent and iterate over the
columns (species).

With atlas, all you need is a layer with a list of the names of all the
columns you want to iterate. It does not have to be spatial, you can just
create it as a simple CSV and import that in QGIS. You can also add it
other columns with information that you want to use in the maps. Here an
example:

"column_name", "common_name", "scientific_name" <-- first row can be used
for column names
"specie_1", "specie 1 common name", "specie 1 scientific name"
"specie_2", "specie 2 common name", "specie 2 scientific name"
"specie_3", "specie 3 common name", "specie 3 scientific name"

After loading the layer and give it a name ("list_of_species" for this
example) Use that layer atlas layer.

Let's say we want to draw the cell with some color if there are more that 0
elements of a particular specie and white for the rest. In a rule-base
simbology, you would need something like:

"specie_1" > 0      --> color
ELSE          --> white

Since we what to iterate the column name, we can get it from the atlas
feature, and we can recreate a string that match the expression above. But
we need to pass it trough the eval() function to make it work

eval('"' + attribute( @atlas_feature, 'column_name') + '" > 0')

Something similar can also be used directly in a graduated style, without
the ">0" part:

eval('"' + attribute( @atlas_feature, 'column_name') + '"')

I have planned to do a blog post about this, but it's in my long to do list.

Hope it helps.

Alexandre Neto

Blumentrath, Stefan <Stefan.Blumentrath at nina.no> escreveu no dia domingo,
5/02/2017 às 19:41:

> Hi Alexandre,
>
>
>
> Currently I have a layer per species, but species could also be in one
> layer with names of the different species in the attribute table…
>
>
>
> I already compiled a little Python script for my use case, however, if
> there is an existing function for that is better of course…
>
>
>
> Thanks for helping.
>
>
>
> Kind regards,
>
> Stefan
>
>
>
> *From:* Alexandre Neto [mailto:senhor.neto at gmail.com]
> *Sent:* søndag 5. februar 2017 19.59
> *To:* Blumentrath, Stefan <Stefan.Blumentrath at nina.no>; Neumann, Andreas <
> a.neumann at carto.net>
>
>
> *Cc:* Qgis-user at lists.osgeo.org; Terje Blindheim <terje at biofokus.no>
> *Subject:* Re: [Qgis-user] Print one map for every category in layer with
> fixed extent
>
>
>
> In the layer, where is the species recorded? In Columns?
>
> If so, I have a workflow for that.
>
>
>
> A sáb, 4/02/2017, 08:38, Blumentrath, Stefan <Stefan.Blumentrath at nina.no>
> escreveu:
>
> Thanks Andreas for the excellent procedure description.
>
>
>
> Good to know that Python is not required for that, and that there is a
> nice, flexible, and efficient solution!
>
>
>
> For someone who uses QGIS on a regularly basis, this is actually much less
> work than it might seem at a first look.
>
> However, I am afraid, if I imagine that I try to explain that to my
> colleagues, many would not be able or not willing to follow, I am afraid…
>
>
>
> I found some more code snippets online, which I compiled into a small
> script. That could be easily wrapped into a plugin if none exists for that
> yet…??
>
>
>
> Cheers
>
> Stefan
>
>
>
> *From:* Neumann, Andreas [mailto:a.neumann at carto.net]
> *Sent:* torsdag 2. februar 2017 15.05
> *To:* Blumentrath, Stefan <Stefan.Blumentrath at nina.no>
> *Cc:* Qgis-user at lists.osgeo.org; Terje Blindheim <terje at biofokus.no>
> *Subject:* Re: [Qgis-user] Print one map for every category in layer with
> fixed extent
>
>
>
> Hi Stefan,
>
> It is possible without the help of Python.
>
> You would have to split up the species into layers.
>
> Then create an atlas layer that is the geometry you want joined with every
> possible layer. You can use QGIS virtual layers for that which you can
> define in DB manager. Or use a Postgis view for it.
>
> So for each geographic unit you want to print you need a combination with
> each layer you want.
>
> E.g.
>
> Province 1 - Species 1
>
> Province 1 - Species 2
>
> Province 1 - Species 3
>
> Province 2 - Species 1
>
> Province 2 - Species 2
>
> Province 2 - Species 3
>
> In this layer, the geometry is redundant, but since you can use Postgis
> views or virtual layers, it is only virtually redundant.
>
> -------------
>
> Finally, in print composer you would use data-defined layers or visibility
> presets based on attributes in the atlas coverage layers.
>
> I did exactly that - print an Atlas for several districts in my city and a
> loop of historic maps. For each district I printed around 15 different
> historic time snapshots (old maps).
>
> Hope this helps to get started.
>
> Andreas
>
> On 2017-02-02 11:21, Blumentrath, Stefan wrote:
>
> Dear all,
>
>
>
> In a project I like to produce a sort of species distribution atlas, where
> I like to print out (export to image) a map from the print composer for
> every species recorded within one and the same municipality.
>
>
>
> So, the map extent should not change and layout is fixed too. With these
> things fixed, I want to get a map for each single species mapped and thus
> also legend entries change. The species are in one single layer, but I
> could also split them if that makes it easier.
>
>
>
> Is there a function in Atlas for that which I overlooked or a plugin or
> the like.
>
> If not, is there any Python code I could repurpose for that? Found only
> this [1] and [2], where [1] does not work…
>
>
>
> But before I start looking into a pygqis solution I like to make sure that
> this feature does not exist yet…
>
> This is a not uncommon use case in ecology btw., so a built in function
> would be neat.
>
>
>
> I am grateful for any hint.
>
>
>
> Kind regards,
>
> Stefan
>
>
>
> [1]
> http://gis.stackexchange.com/questions/183589/using-pyqgis-to-create-images-from-the-qgis-print-composer-each-showing-a-differ
>
> [2]
> http://kartoza.com/en/blog/how-to-create-a-qgis-pdf-report-with-a-few-lines-of-python/
>
>
>
>
>
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> --
>
> Alexandre Neto
>
> ---------------------
>
> @AlexNetoGeo
>
> http://sigsemgrilhetas.wordpress.com
>
> http://gisunchained.wordpress.com
>
-- 
Alexandre Neto
---------------------
@AlexNetoGeo
http://sigsemgrilhetas.wordpress.com
http://gisunchained.wordpress.com
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