QGIS Atlas Expression Query

Question

I have set up a page layout using the Altas Feature in QGIS v3.2.3. The layout has been configured to show the location of a polygon and the location of vertices that make up each of these polygons. Alongside this I have added an attribute table to list the X and Y values of each vertex for every polygon. The Atlas feature itself is linked to the polygon shape layer whilst the attribute table is linked to a vertex point layer. And whilst there have been no problems exporting this set-up there is one aspect which I cannot seem to resolve which is that in some instances the attribute table lists vertices from different polygons (rather than listing vertices only for the Altas Feature). I have tried toggling with some of the item properties for attribute table e.g. selecting 'show only features intersecting altas feature' however I believe that the solution lies with the expression filter. Unfortunately I have not been able to set up an expression filter that will work with the Atlas Feature and therefore I am looking for some advice on this.

I have attached an image for reference to the above.

Answer 1

If you need something like that it's possible to use a custom expression function to dump out the nodes of a given geometry into a HTML table.

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

@qgsfunction(args='auto', group='Custom')
def dump_table(geometry, feature, parent):
    nodes = geometry.asMultiPolygon()[0][0]
    template = """
    <style>table, th, td {{ border: 1px solid black; }}</style>
    <table>
    <tr>
        <th>PlotNo</th>
        <th>No.</th>
        <th>X</th>
        <th>Y</th>
        </tr>
        {}
    </table>
    """
    rows = []
    for count, node in enumerate(nodes):
        row = """<tr>
        <td>{}</td>
        <td>{}</td>
        <td>{}</td>
        <td>{}</td>
        </tr>""".format(feature['PlotNo'], count, node.x(), node.y())
        rows.append(row)
    return template.format("".join(rows))

You can add custom expressions using the Function Editor in the expression builder:

The output will look like this:

Called like this dump_table($atlas_geometry)

Answer 2

Make sure the point layer has a unique attribute in common with the atlas layer, where the points for a given atlas feature have the same value as that feature.

Set up a filter using this attribute, such as

"point_id" =  attribute(@atlas_feature, "atlas_id")

In this example, the common attribute is called "point_id" in the point layer, and "atlas_id" in the atlas layer.