Mysql Distance from test point to a polygon - mysql

Question

I have stored some map zones to a table using Geometry type field.
So the inserts are like this:
INSERT INTO zones (zoneName, coords) VALUES ('name',
PolygonFromText('POLYGON((
41.11396418691335 1.2562662363052368,
41.11370552595821 1.2560248374938965,
41.11851079510035 1.2459397315979004,
41.11880984984478 1.2461864948272705,
41.11396418691335 1.2562662363052368))'));
Then I have the user position, and I need to know if he is inside some zone. This works well with this:
SELECT id
FROM zones
WHERE MBRContains(coords,GeomFromText('POINT(41.117783 1.260590)'))
But sometimes, user position is not perfect, so I think its better to know wich zone is closest to user position.
That is the part that I don't have any idea about... I found some queries to get distance between two points, but not a point and polygons.

Answer 1

The MBR series of functions (like MBRContains) are unsuitable for what you're trying to do; they only test bounding rectangle inclusion.
You may wish to jump forward to MySQL 5.6.1, and use the ST_ functions, like ST_Contains. These functions actually test the geometry.
The problem you're working on can be defined as an uncertainty in the position of your POINT when you go to compare it to your collection of boundary POLYGON items.
Try this: create a POLYGON from your point that is a square with the size of your uncertainty. You can think of this square as a "fuzzy" point. (You could also use an octagon or another closer approximation of a circle in place of a rectangle, but your querying speed will slow.)
Then use ST_Within to see if you have a unique polygon that entirely contains your fuzzy point. If you get just one polygon, you're done.
If you get multiple polygons that entirely contain your fuzzy point, that means some of your boundary polygons overlap other ones. You need to figure out what this means in your problem space. If your data is intended to be properly structured cartographic boundary data, it means you have a data mistake. (NOTE: This is not unheard of :-)
If you get no polygons that entirely contain your fuzzy point, then your fuzzy point may or may not overlap the boundary of at least one polygon. Use ST_Overlaps to find those polygons.
If you get just one, you're done -- your fuzzy point is near the boundary of just one polygon.
If you get none, you're done -- your fuzzy point is away from the boundaries of all your polygons.
If you get more than one hit, you have an ambiguity -- your fuzzy point is near the boundary of more than one polygon.
This is the hard case to sort out. You could reduce the size of the fuzzy point and try again. This MIGHT yield just one polygon result. But, you could deceive yourself into thinking that your points are more accurate than they are by doing this.
MySQL doesn't have the geometric operator Area(Intersection(Polygon, FuzzyPoint)). If it did you could choose the polygon with the biggest area of intersection with your fuzzy point, and that would be a good disambiguator. But it would still be as inaccurate as the position of your point.
Maybe your application should handle the category of result "too near the boundary of A, B, and C to be sure."