[Qgis-user] Dynamic Line

Nicolas Cadieux njacadieux.gitlab at gmail.com
Sat Nov 14 12:50:56 PST 2020 

Start by joining both files using a common field (right click 
property/joins).

This could give you a file that looks like this:

fid,x1,y1,x2,y2
1,0,0,1,1
2,10,10,100,100
...

Save this file as geopackage (line).  This way, you can modify it.

x1,y1 are from the first file and x2,y2 are from the second file. These 
coordinates can be generated by creating a new field and updating it 
with $x or $y functions in the field calculator.

Then, using the field calculator, select "modify and existing field" and 
select <geometry>.  Update this field using the following line.

geom_from_wkt('LINESTRING(' ||  "x1"  || ' ' ||  "y1"  || ',' || "x2"  
|| ' ' ||  "y2"  || ')')

This will create the lines.

Nicolas

On 2020-11-14 7:53 a.m., L.W. wrote:
>
> 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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