Why following ST_Intersects SQL returns false

Question

I run following code in PostGIS. Both SRID is default, which is 4326.

select ST_Intersects('POINT(104.042455 30.657381)'::GEOGRAPHY(POINT), ST_MakeEnvelope(80, 30, 140, 40, 4326)::GEOGRAPHY(POLYGON))

'POINT(104.042455 30.657381)'::GEOGRAPHY(POINT): This represents a point geography object with longitude 104.042455 and latitude 30.657381.

ST_MakeEnvelope(80, 30, 140, 40, 4326)::GEOGRAPHY(POLYGON): This creates a rectangular polygon (bounding box) with the specified corner coordinates (80,30), (140,40) in the WGS 84 coordinate system (SRID 4326). The rectangle spans from longitude 80 to 140 and latitude 30 to 40.

The point (104.042455, 30.657381) is within the longitude range (80 to 140) and the latitude range (30 to 40) of the bounding box.

Given this, the query should logically return true since the point is clearly within the bounds of the specified rectangle.

But, following return true:

select ST_Intersects('POINT(108.963413 34.284203)'::GEOGRAPHY(POINT), ST_MakeEnvelope(80, 30, 140, 40, 4326)::GEOGRAPHY(POLYGON))

Answer 1

Because when PostGIS handles geography objects it converts them into a local UTM zone and then uses Cartesian maths. In this case your point doesn't fall inside the polygon.

If you want it to then you need to densify your polygon so that the edges bend with the projection.

In PostGIS you need to use the ST_Segementize function.

Answer 2

Your statement "ST_MakeEnvelope(80, 30, 140, 40, 4326)::GEOGRAPHY(POLYGON): This creates a rectangular polygon (bounding box)" is false. It is not a square because it has been casted to geography.

When using geography, a line connecting two points is not a straight line but rather a great circle line.

The coordinate of the point is slightly above the southernmost latitude, but since it remains below the great circle line, st_intersects is false.