[Qgis-user] Dynamic Line

L.W. eaglelw at gmx.de
Sat Nov 14 04:53:25 PST 2020 

I tried a lot ...

I have a point-layer, I put this code in symbol -> single symbol ->
geometrygenerator

What is wrong?


geom_to_wkt
(
     make_line
     (
         make_point
         (
             5826542.703128459,
             797907.3304607988
         ),
         make_point
         (
             5827473.536854242,
             796987.0074038045
         ),
         make_point
         (
             5827167.893279332,
             795128.2317764781
         ),
         make_point
         (
             5826835.7783904355,
             793227.8583700341
         )
     )
)

Am 13.11.2020 um 15:24 schrieb Andreas Neumann:
>
> Hi Nicolas,
>
> Yes - but that doesn't magically turn a "non geometry" table into a
> "geometry table" - as far as I know. So you probably need to create a
> "Virtual Layer" and create the geometry similar to what you propose.
>
> Andreas
>
> Am 13.11.20 um 15:22 schrieb Nicolas Cadieux:
>> Hi,
>> You can create a geometry from WKT if you have x and y fields using
>> and expression in the field calculator.
>> |geom_from_wkt('POINT('||"x"||' '||"y"||')')|
>>
>> https://en.wikipedia.org/wiki/Well-known_text_representation_of_geometry
>>
>> Nicolas Cadieux
>>
>>> Le 13 nov. 2020 à 09:07, Andreas Neumann <a.neumann at carto.net> a écrit :
>>>
>>> 
>>>
>>> Hi
>>>
>>> Neither GeometryGenerators nor Vector Field Markers work on tables
>>> without geometry, unfortunately. You didn't inform us, that you
>>> don't have geometry in your table.
>>>
>>> You can, however, create a "Virtual Layer" and write some SQL to
>>> create a geometry from your attributes: menu "Layer" --> "Create
>>> Layer" --> "New Virtual Layer" - where you can use your existing
>>> tables and create a new geometry column with SQL. See
>>> https://docs.qgis.org/testing/en/docs/user_manual/managing_data_source/create_layers.html#creating-virtual-layers
>>>
>>> Or, if your table is a CSV file, then you can assign attribute
>>> columns to a point geometry and then just hide the point rendering
>>> if you don't need it.
>>>
>>> This is an unfortunate limitation of QGIS, that there is no "easy"
>>> way to add a geometry column to table that doesn't have geometry (or
>>> if it is possible, then I don't know it).
>>>
>>> Hope this helps you,
>>>
>>> Andreas
>>>
>>> Am 13.11.20 um 14:52 schrieb L.W.:
>>>>
>>>> Does it works on Layers with no geometry?
>>>>
>>>> My layers do not have a geometry.
>>>>
>>>>
>>>> Am 13.11.2020 um 11:56 schrieb Andreas Neumann:
>>>>>
>>>>> Yes, you need point data to start from, but then vectors can
>>>>> bei/are generated from the attributes you mentioned.
>>>>>
>>>>> Andreas
>>>>>
>>>>> Am 13.11.20 um 11:47 schrieb L.W.:
>>>>>>
>>>>>> from the docs ...
>>>>>>
>>>>>> ****It can only be used to render point data****
>>>>>>
>>>>>>
>>>>>>         12.2.2.2.6.The Vector Field Marker
>>>>>>         <https://docs.qgis.org/3.10/en/docs/user_manual/style_library/symbol_selector.html?#id15>
>>>>>>
>>>>>> The vector field marker is used to display vector field data such
>>>>>> as earth deformation, tidal flows, and the like. It displays the
>>>>>> vectors as lines (preferably arrows) that are scaled and oriented
>>>>>> according to selected attributes of data points. ****It can only
>>>>>> be used to render point data****; line and polygon layers are not
>>>>>> drawn by this symbology.
>>>>>>
>>>>>> The vector field is defined by attributes in the data, which can
>>>>>> represent the field either by:
>>>>>>
>>>>>>  *
>>>>>>
>>>>>>     *cartesian*components (|x|and|y|components of the field)
>>>>>>
>>>>>>  *
>>>>>>
>>>>>>     or*polar*coordinates: in this case, attributes
>>>>>>     define|Length|and|Angle|. The angle may be measured either
>>>>>>     clockwise from north, or Counterclockwise from east, and may
>>>>>>     be either in degrees or radians.
>>>>>>
>>>>>>  *
>>>>>>
>>>>>>     or as*height only*data, which displays a vertical arrow
>>>>>>     scaled using an attribute of the data. This is appropriate
>>>>>>     for displaying the vertical component of deformation, for
>>>>>>     example.
>>>>>>
>>>>>> The magnitude of field
>>>>>>
>>>>>>
>>>>>> Am 13.11.2020 um 11:34 schrieb Andreas Neumann:
>>>>>>> Or even easier: use the "Vector Field Marker":
>>>>>>>
>>>>>>> https://docs.qgis.org/3.10/en/docs/user_manual/style_library/symbol_selector.html?#the-vector-field-marker
>>>>>>>
>>>>>>>
>>>>>>> Am 13.11.20 um 11:33 schrieb Andreas Neumann:
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> Yes - you can use Geometry Generators for that. Use the
>>>>>>>> make_point() and make_line() expressions for that.
>>>>>>>>
>>>>>>>> See
>>>>>>>> https://docs.qgis.org/3.10/en/docs/user_manual/style_library/symbol_selector.html?highlight=geometry%20generator#the-geometry-generator
>>>>>>>>
>>>>>>>> Greetings,
>>>>>>>> Andreas
>>>>>>>>
>>>>>>>> Am 13.11.20 um 11:26 schrieb L.W.:
>>>>>>>>> Hi,
>>>>>>>>>
>>>>>>>>> is it possible to draw automatically a line between two points
>>>>>>>>> defined
>>>>>>>>> by 4 fields (x1,y1 - x2,y2) in an attributetable?
>>>>>>>>>
>>>>>>>>> but I do not want to add a line-layer based on this table nor
>>>>>>>>> want to
>>>>>>>>> draw the line by hand.
>>>>>>>>>
>>>>>>>>> thanks
>>>>>>>>>
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