[Qgis-user] Dynamic Line

[Andreas Neumann]

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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