fix: Make capture area geometry valid TDE-1306 (#1163)

Caveat: Shapely's `BaseGeometry.buffer` produces different results on
different machines, even with exactly the same Python package versions
installed. To reproduce, change `test_should_make_valid_capture_area` to
run with a buffer distance of `0.3`, and revert the change in
`merge_polygons`. At this point the test passes on my machine, but fails
on @schmidtnz's machine! We tracked this down to the line
`union_buffered.buffer(-buffer_distance, cap_style=BufferCapStyle.flat,
join_style=BufferJoinStyle.mitre)`. With the changes mentioned above,
this returns a `MultiPolygon` on my machine (basically doing the same as
`make_valid`, and therefore making the test pass), and a `Polygon` on
@schmidtnz's machine (making the `make_valid` necessary to produce a
valid polygon).

### Motivation

Ensure that published `capture-area.geojson` files are valid.

### Modifications

Use [Shapely's
`make_valid`](https://shapely.readthedocs.io/en/2.0.4/reference/shapely.make_valid.html)
before saving the geometry.

### Verification

- [x] Unit test
- [x] Verify in QGIS:
   1. `nix-shell --packages qgis --run qgis`
   3. Press <kbd>Ctrl-l</kbd>
4. Open one of the `capture-area.geojson` files created by the unit test
(for example by pausing the test at the assert)
   5. Go to Vector → Geometry Tools → Check Validity
   6. Press _Run_
   7. Verify that it prints `INVALID_COUNT: 0` and `ERROR_COUNT: 0`

scripts/stac/imagery/capture_area.py

@@ -4,6 +4,7 @@
 
 from linz_logger import get_log
 from shapely import BufferCapStyle, BufferJoinStyle, to_geojson, union_all, wkt
+from shapely.constructive import make_valid
 from shapely.geometry.base import BaseGeometry
 from shapely.ops import orient
 
@@ -67,7 +68,7 @@ def merge_polygons(polygons: Sequence[BaseGeometry], buffer_distance: float) ->
     # Ref: https://datatracker.ietf.org/doc/html/rfc7946#section-3.1.6
     oriented_union_simplified = orient(union_rounded, sign=1.0)
 
-    return oriented_union_simplified
+    return make_valid(oriented_union_simplified)
 
 
 def generate_capture_area(polygons: Sequence[BaseGeometry], gsd: Decimal) -> dict[str, Any]:

scripts/stac/imagery/tests/collection_test.py

@@ -9,11 +9,13 @@
 from moto import mock_aws
 from moto.s3.responses import DEFAULT_REGION_NAME
 from pytest_subtests import SubTests
+from shapely.predicates import is_valid
 
 from scripts.datetimes import format_rfc_3339_datetime_string
 from scripts.files.files_helper import ContentType
 from scripts.files.fs import read
 from scripts.files.fs_s3 import write
+from scripts.stac.imagery.capture_area import merge_polygons
 from scripts.stac.imagery.collection import ImageryCollection
 from scripts.stac.imagery.item import ImageryItem, STACAsset
 from scripts.stac.imagery.metadata_constants import CollectionMetadata
@@ -342,6 +344,27 @@ def test_capture_area_added(fake_collection_metadata: CollectionMetadata, fake_l
         assert collection.stac["assets"]["capture_area"]["file:size"] in (269,)  # geos 3.11 - geos 3.12 as yet untested
 
 
+def test_should_make_valid_capture_area() -> None:
+    # Given two touching triangles
+    polygons = [
+        shapely.geometry.shape(
+            {
+                "type": "MultiPolygon",
+                "coordinates": [[[[0, 0], [0, 1], [1, 1], [0, 0]]]],
+            }
+        ),
+        shapely.geometry.shape(
+            {
+                "type": "MultiPolygon",
+                "coordinates": [[[[1, 0], [2, 2], [1, 2], [1, 0]]]],
+            }
+        ),
+    ]
+
+    capture_area = merge_polygons(polygons, 0.1)
+    assert is_valid(capture_area)
+
+
 def test_event_name_is_present(fake_collection_metadata: CollectionMetadata, fake_linz_slug: str) -> None:
     fake_collection_metadata["event_name"] = "Forest Assessment"
     collection = ImageryCollection(fake_collection_metadata, any_epoch_datetime, fake_linz_slug)