Creating Point-IDs in QGIS based on where was created and how many were already created

Question

CONTEXT

I´m creating a Qfield project in QGIS and I´m creating a Point-shapefile to work with in offline mode in the field.

In my project there are different plots where the field work will be developed. On each plot, trees will be sampled and registered as points (offline shapefile). When a point is created within one of these plots, the attribute value of the name of this plot will be transfered to a column of this point. Hence, it will be known in which plot the point was created. The name of the Plot is, for example, "P1".

When a point (tree) is created within a plot, the ID of this point is constructed as: Plot_ID + Tree_ID. Hence, it will be known where the tree was created and the ID of this tree within this plot. For example, the first tree recordered in the Plot 1, will be written as "P1B1". Then, the second will be written as "P1B2", and so on.

QUESTION

So far I´ve managed to set up the column of Tree_ID so that its ID number increases. This is, when several trees are created in the same plot, the values are created as expected: "P1B1"; "P1B2"; "P1B3". However, when I try to create another tree in a different plot, the plot is not differentiated, and the ID number continues increasing, rather than starting from 1 again. For example, if I create another tree in P2, the name will be wrongly created as "P2B4". This tree should be created as "P2B1", since this would be the first tree created within this plot.

EDITED

SOLUTION

After trying different approaches, the approach that gave me the result I was looking for is the following, where Point_trees is the points-shp that are being created, and plots_layer is the polygon-shp containing the plots where the field work will be made:

count('Point_trees', group_by:="Plot_ID", filter:= "Plot_ID"= aggregate(layer:='plots_layer', aggregate:='max', expression:="Name_of_plot", filter:=intersects($geometry, geometry(@parent))))+1

This code counts the number of features which Plot_ID is equal to the actual Plot_ID of the point which is being created (through the intersection part after filter:=)

Answer 1

Does each tree feature include separate attributes for plotid and treeid. If so to return next number for plot

Coalesce(Maximimum(treeid, filter:=plotid=plotid)+1,0)+1

If treeid contains plotid, then you need to use string functions to extract the number part and perform maximum on this.

I suggest it makes sense to have plotid as an attribute of trees, it can then serve as a foreign key for a relation. I'd also use a separate attributes for the number part.

EDIT

So I've created a small test project with layers 'tree' and 'plot'.

'plot' has attribute 'plotid' 'tree' has attributes 'plotid', 'treeid'

I tried the following code and it worked in preview but failed when using the form in QGIS. I then changed the data type of 'treeid' from integer' to 'text' and it works. I have not tested in QField.

In 'tree'.'plotid' I have set default value to

array_first(overlay_intersects('plot',"plotid"))

In 'tree'.'treeid' I have set default value to

maximum("treeid", "PlotID")+1

In the above expression "PlotID" is the Group By argument of maximum(). If the expression for 'tree'.'plotid' fails, try replacing the layer name with the full unique reference generated in QGIS expressions dialog.

Regards

Answer 2

Assuming you have the plot geometry as a polygon layer, you can set an expression for the default value of the treeid field. The treeid is generated by getting the plot_id of the plot that the tree point falls within, and combining it with how many tree points already exist inside that plot +1.

with_variable(
    'plot',
    overlay_within('plots', "plot_id")[0],      -- get plot id
    
    with_variable(
        'num_trees_in_plot',
        num_geometries(                         -- count how many points exist in the current plot
            intersection(
                aggregate(
                    @layer,
                    'collect',
                    $geometry
                ),
                geometry(get_feature('plots', 'plot_id', @plot))
            )
        ),  
        
        -- construct the tree id by concatenating the elements
        -- set counter to 1 if no points are already in the plot
        @plot || 'B' || case when @num_trees_in_plot is null then 1 else @num_trees_in_plot + 1 end
    
    )
)

The default value is set via the layer properties > Attributes Form: