Create dynamic text in legend for visible Layer on map layout using SQL expression

Question

QGIS 3.16

Looking to insert an expression to my layer, Survey Coverage (text), in the legend. I want dynamic text to reflect the attribute value from the field, VIS_RADIUS (Integer64) so it looks something like:

Survey Coverage (20 m)

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Survey Coverage map layer id: Spatial_SurveyCoverage_Mission

Field: VIS_RADIUS

Map Item ID: Working Map

At present I have testing different expressions in the Legend Item Properties as follows:

Survey Coverage ([%to_string(display_expression(@Spatial_SurveyCoverage_Mission, "VIS_RADIUS"))%] m)

Survey Coverage ([%to_string(display_expression(@Spatial_SurveyCoverage_Mission, "VIS_RADIUS", map_get(item_variables('Working Map'), 'map_extent')))%] m)

Survey Coverage ([%layer_property('Survey Coverage', 'VIS_RADIUS')%] m)

All result in...

Survey Coverage ( m)

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Ultimate goal is to reduce the amount of revisions to layouts for multi-map projects which I can share with colleagues to be more user-friendly.

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Edit: Additional dynamic text using SQL to update as Map Extent is updated/moved

Map Canvas

Map Layout Text Box Properties

[% map_get(item_variables('Working Map'),'map_crs_description') %]
Base Map Source: ESRI World Topo
Map data sources from BC Gazetteer [%layer_property(@NTS, 'title')%] [%map_get(item_variables('Working Map'), 'map_layers')%]
[%attribute(get_feature('nts_snrc_50k', 'NTS_SNRC', 'IS NOT NULL'), 'NTS_SNRC')%]
[%@map_layers%]

As seen in the below image, the 'map_crs_description' reveals text in a way I want. However, the text after BC Gazetteer does not appear in the text box display.

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RE: aggregate expression

Solution provided yields no result...

Survey Coverage Layer Properties > Variables

note: the "display as" in Source is greyed out, unsure why or if it even effects this at all.

Answer 1

You can use the aggregate function for that, something like the expression

survey radius: [% aggregate( 'Survey Coverage', 'sum', "VIS_RADIUS")%] m

will get the aggregated sum of all values within the field VIS_RADIUS. If you want to limit it on the visible area in your Map, you can use the filter criteria of the aggregate function and intersect the geometries of the layer with the mapextent.

Answer 2

I understand your want to get only one specific information and not an aggregate like a sum or count. So here is two solution, only working if you use atlas.

Solution 1

Condition : your atlas coverage layer is your layer containing the field VIS_RADIUS.

You can access attribute of your current atlas feature really easily. Here is the expression for your example :

attribute(@atlas_feature, 'VIS_RADIUS') 

Solution 2

Condition : your atlas coverage layer share a common attribute with your layer Spatial_SurveyCoverage_Mission.

I assume your atlas coverage layer has field named survey_coverage_id to connect with the field id of your layer Spatial_SurveyCoverage_Mission. For your layer Spatial_SurveyCoverage_Mission the id must be unique.

Here is the expression to get the value you want :

attribute(
 get_feature(
  'Spatial_SurveyCoverage_Mission',
  'id',
  attribute(@atlas_feature,'survey_coverage_id')
 )
,'VIS_RADIUS')

You can adapt your expression to your layer name and field name.