Question

SELECT `NAME` FROM world_boundaries 
WHERE Within(GeomFromText('POINT(8.5929098 50.0286713)'), ogc_geom);

Why does this query return 3 countries when it should return one.
It returns:
1. France
2. Germany (correct one)
3. America

Is my country boundaries data flawed or is there some trick with these geometry functions?

Answer 1

MySQL can’t check a point in poly yet. It will check if your point is within the ([minX, maxX], [minY, maxY]). Also, I don’t see within in MySQL documentation.

Note

Currently, MySQL does not implement these functions according to the specification. Those that are implemented return the same result as the corresponding MBR-based functions. This includes functions in the following list other than Distance() and Related().

These functions may be implemented in future releases with full support for spatial analysis, not just MBR-based support.

From here

I used a point-in-poly ray-tracing algorithm, which can be quite accurate outside of polar regions if the lines aren’t long. Otherwise you’ll need to solve spherical triangles.

The simple implementation is like this:

create table point (x int, y int...) /* int values (int(lat * 10^6)) for better performance */
create table poly (x1 int, y1 int, x2 int, y2 int...) /* temporary table for 1 poly only */

query that selects points inside the poly:

select point_id from point, poly
where ((y<y2 and y>=y1) or (y<y1 and y>=y2)) and ((x<x2 and x>=x1) or (x<x1 and x>=x2))
group by point_id
having sum(x1*(y2-y1)*sign(y2-y1) < x*(y2-y1)*sign(y2-y1) - (y-y1)*(x2-x1)) % 2

Counts 20,000 points vs a poly of 40 lines in 0.06 seconds on Pentium with 2.5Ghz. 🙂