Combining polygon datasets

Question

I need to combine 2 large polygon datasets. One of these datasets contains more recent data, so the polygons from this dataset need to be copied into the existing dataset.

There are three different instances where the existing dataset would need to be modified. See below for screenshots and explanations.

(In all screenshots the dashed red outline represents the existing data. The green polygons are new data.)

---

Situation 1 - Entirely new polygon:

Action - New polygon needs to be added to existing dataset. Polygons that remain the same can be ignored.

---

Situation 2 - An existing polygon has changed shape:

Action - Existing polygon(s) needs to be deleted. New polygon then needs to be added to existing dataset.

---

Situation 3 - Multiple new polygons within an existing polygon

Action - Existing containing/large polygon needs to be deleted. New polygon then needs to be added to existing dataset.

---

So, my question is; would it be possible to combine these datasets according to the rules above, either in QGIS or PostGIS?

Answer 1

I believe you can do all of this in 2 steps. First, delete all existing polygons where the new polygons table intersects them. In case 1, the existing polygons will not be touched. In cases 2 and 3, all existing polygons that have either grown (and possibly merged into one) or shrunk (and possibly split into smaller ones) will be deleted. Then you simply insert the new polygons into the existing table which will cover cases 2 and 3.

DELETE FROM existing_polygons a USING new_polygons b
WHERE ST_Intersects(a.geom, b.geom);

INSERT INTO existing_polygons 
SELECT * FROM new_polygons;