Introduction ¶

The article describes a project simulating South African 1 in 50k topographic maps. The implementation uses QGIS Server for printing, with all logic configured in the print composer to generate dynamic maps based on user-selected areas.

Problem Statement ¶

The core challenge involved determining which map sheets intersect with a printed area. Since layer attributes weren’t directly exposed through built-in expressions, a custom solution was necessary.

Solution: Getting Map Extent ¶

The author demonstrates retrieving a map’s extent using:

map_get(item_variables('main_map'), 'map_extent')

Where main_map represents the map’s Item ID, returning a QGIS Geometry object suitable for intersection operations.

Custom QGIS Function ¶

A Python function was created to perform map intersections:

from qgis.core import *
from qgis.gui import *

@qgsfunction(args='auto', group='Custom')
def map_index(source_layer, map_extent, source_attribute, feature, parent):
    map_layer = QgsProject.instance().mapLayersByName(source_layer)[0]
    map_extent_bounds = map_extent.boundingBox()
    records = []
    for f in map_layer.getFeatures():
        f_bounds = f.geometry().boundingBox()
        if map_extent_bounds.intersects(f_bounds):
            field_name_idx = f.fieldNameIndex(source_attribute)
            field_value = f.attributes()[field_name_idx]
            records.append(field_value)
    result = ','.join(records)
    return result

Implementation ¶

In the map composer, a text box implements the custom function:

replace(map_index('index1in50k', map_get(item_variables('main_map'),
'map_extent'), "sh_no"), ',', ' \n')

This generates the list of intersecting map sheets from the index1in50k layer using the sh_no attribute.