Query ShapeFile to get informations from coordinates

Question

I have a shapefile that represents all the Hydrological Basins in the world. You can download the file from here:

http://www.fao.org/geonetwork/srv/en/resources.get?id=38047&fname=Major_hydrological_basins.zip&access=private

In that file, there's the whole world divided into areas (all the Hydrological Basins in the world). I need to create a function that query the shapefile with latitude and longitude, and gets back the values of that point, that are: MAJ_BAS, MAJ_NAME and MAJ_AREA

I need to do this server side, without a GUI. To me seems simple, when I do in my QGIS I just click on a point and get back the informations I need, so I thought "there'll be a function that you'll give latitude and longitude, and the function will answer with the datas of that point". But I've been looking for 2 days and still get no solution about this problem.

I'm quite new to QGIS.

Answer 1

If you are using PyQGIS then I think you can use the QGSVectorLayer.select() method to select objects in a given search rectangle.

See @nathan-w example code here which seems a good starting point: How do I allow users to select features with the mouse?

layer.select( layer.pendingAllAttributesList(), rect, True, True)    
for feature in layer:

Answer 2

And thanks for your precious answers! I see your methods are much more rigorous than what I found out yesterday night, working on my project, and I appreciate all your answers.

Since I need just a few queries, what I did was to create a table in MySQL with polygon objects and then use the algorythim of point in polygon. I created a function in MySQL that's the follwing

DETERMINISTIC
BEGIN 
DECLARE n INT DEFAULT 0; 
DECLARE pX DECIMAL(9,6); 
DECLARE pY DECIMAL(9,6); 
DECLARE ls LINESTRING; 
DECLARE poly1 POINT; 
DECLARE poly1X DECIMAL(9,6); 
DECLARE poly1Y DECIMAL(9,6); 
DECLARE poly2 POINT; 
DECLARE poly2X DECIMAL(9,6); 
DECLARE poly2Y DECIMAL(9,6); 
DECLARE i INT DEFAULT 0; 
DECLARE result INT(1) DEFAULT 0; 
SET pX = X(p); 
SET pY = Y(p); 
SET ls = ExteriorRing(poly); 
SET poly2 = EndPoint(ls); 
SET poly2X = X(poly2); 
SET poly2Y = Y(poly2); 
SET n = NumPoints(ls); 
WHILE i<n DO 
SET poly1 = PointN(ls, (i+1)); 
SET poly1X = X(poly1); 
SET poly1Y = Y(poly1); 
IF ( ( ( ( poly1X <= pX ) && ( pX < poly2X ) ) || ( ( poly2X <= pX ) && ( pX < poly1X ) ) ) && ( pY > ( poly2Y - poly1Y ) * ( pX - poly1X ) / ( poly2X - poly1X ) + poly1Y ) ) THEN 
SET result = !result; 
END IF; 
SET poly2X = poly1X; 
SET poly2Y = poly1Y; 
SET i = i + 1; 
END WHILE; 
RETURN result; 
End

If the function returns 1, the point is in the polygon. If it returns 0, it's not.

Then, when I need to find if a point is inside a polygon, I'll just query like this

select * from *Polygon_table* where myWithin(PointFromText( concat( 'POINT(', *{$long}*, ' ', *{$lat}*, ')' ) ), *Polygon_table*.polygon )

where myWithin is the name of the function and the polygon in the Polygon table has been saved as a Polygon object. This function returns the polygon(s) that contain(s) that point.

I know this is not the fastest solution, but for my case (one single query not often) was a great solution.