Count points belonging to a country but outside of country polygon (coastal)

Question

I'm still getting my head around GIS and am using QGIS 3.6.2.

I'd like to do a simple count of points within countries. The problem is that many of my points are located on coastlines or islands, so my current counts come up short because they do not fall within the defined polygons of the country world map. I guess this may be a resolution issue of the countries shape file?

Is there a way to add a buffer zone of a couple of kilometers to coastal polygon borders - but not to land borders? Or is there a world countries shape file that reflects accuracy of google maps or similar?

I've attached a screenshot of an example of two points in the Floria keys as seen by QGIS and by Google Maps.

Answer 1

I come from SQL land, so not sure how applicable this to QGIS.

But in SQL I would think of two tables: one with original country polygons, another with country polygons buffered by whatever coastal buffer you want to use. Of course, the "buffered countries" would overlap on the land side, but that's simple to fix: you only count points within "buffered country" that don't belong to some other "original country".

In practice you don't need to create actual "buffered country" table, rather use ST_DWithin to include buffer. But it is a good mental model. For a single country the query would be something like

select * from points p
where 
    -- within `buffer_distance` of our country
    ST_DWithin(p.geom, 
               (select geom from countries where country_id = 123),
               buffer_distance)
    -- not within any other country
    and not exists (select 1 from countries o 
                    where o.country_id <> 123 and 
                    st_intersects(p.geom, o.geom))

Or you can actually create "buffered countries" table and preprocess buffered polygons by subtracting from each one all other original countries - effectively removing land-side buffer.

There is still an issue of points within multiple "buffered countries" - if it matters, it can be fixed by finding nearest original country, with more complicated SQL though.