[Qgis-user] Dynamic Line

L.W. eaglelw at gmx.de
Sat Nov 14 05:00:41 PST 2020 

Selected Geometrietype: LineString / MultLineString


Am 14.11.2020 um 13:53 schrieb L.W.:
>
> 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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