Hey there! Let's break down exactly how to generate your desired closed polygon in QGIS, using your two isobaths as side boundaries and the 33°S/43°S latitudes as north/south edges. Here's a step-by-step workflow that’s easy to follow:

1. Prep Your Input Data #

First, make sure all your line layers are ready to work with:

• Your two isobath lines (they need to be vector line layers, not raster data)
• The 33°S and 43°S latitude lines. If you don’t have these already:
  • You can create a new line shapefile layer, then use the digitizing toolbar to draw the latitudes. Or use the Virtual Layer tool with a simple SQL query to generate them automatically:

    SELECT ST_SetSRID(ST_MakeLine(ST_Point(-180, -33), ST_Point(180, -33)), 4326) AS geometry
    UNION ALL
    SELECT ST_SetSRID(ST_MakeLine(ST_Point(-180, -43), ST_Point(180, -43)), 4326) AS geometry
    

  • Pro tip: Ensure all layers use the same coordinate reference system (CRS). A projected CRS (like UTM zones) works better here than geographic WGS84 to avoid distortion issues.

2. Align Line Endpoints to Form a Closed Loop #

This is the most critical step—your four lines need to connect end-to-end to form a single, unbroken ring:

• Enable the Snapping Toolbar (right-click the toolbar area and check "Snapping Toolbar")
• Set snapping to "Vertex" mode for all four line layers, with a small tolerance (adjust based on your CRS; 10 meters is a good starting point)
• Enter Edit Mode for each line layer one by one:
  • For each isobath, move its northern endpoint to snap exactly to the corresponding point on the 33°S latitude line
  • Move its southern endpoint to snap exactly to the corresponding point on the 43°S latitude line
  • Trim the 33°S and 43°S lines so they only span between the two isobaths (use the Trim/Extend Features tool in the digitizing toolbar to cut off excess segments outside your isobaths)

3. Merge All Boundary Lines into One Layer #

Once all endpoints are connected and lines are trimmed to the right length:

• Use the Merge Vector Layers tool (found under Processing > Toolbox > Vector general > Merge Vector Layers)
• Select your two isobath layers and the two trimmed latitude layers as inputs
• Save the merged output as a new line layer

4. Convert the Closed Line Ring to a Polygon #

Now turn that closed line loop into a solid polygon:

• Use the Lines to Polygons tool (under Processing > Toolbox > Vector geometry > Lines to Polygons)
• Select your merged line layer as the input
• Save the output as a new polygon layer—this is your desired closed polygon!

Quick Troubleshooting Tips #

• If the polygon doesn’t form, double-check that all line endpoints are fully snapped (no gaps or overlaps between segments)
• If you end up with multiple polygons, make sure your merged lines form exactly one continuous closed ring (no extra, unconnected line segments)

内容的提问来源于stack exchange，提问作者Alfonsina López