Two closest points on boundary of Postgis geometry - postgresql

Question

I have a table geofences which stores geometry of polygon.
I also have a point A which is inside the geometry. What I have to do is find the two closest points from point A that lie on the surface of the polygon geometry.
Function in PostGIS:
CREATE OR REPLACE FUNCTION accuracyCheck(Polygon geometry
,decimal lat
,decimal lon)
RETURNS VARCHAR AS
$BODY$
DECLARE height DECIMAL;
DECLARE accuracy VARCHAR(250);
BEGIN
CREATE TEMPORARY TABLE closePointStorage AS
SELECT ST_AsText(ST_ClosestPoint(geometry
,ST_GeomFromText('POINT(lat lon)',0)
)
) AS closestPoint
FROM (
SELECT ST_GeomFromText(geometry) as geometry
FROM gfe_geofences
WHERE is_active=true
) As tempName;
CREATE TEMPORARY TABLE areaStorage ON COMMIT DROP AS
SELECT ST_Area(ST_GeomFromText('Polygon((23.0808622876029 96.1304006624291
,28.0808622876029 99.1304006624291
,100 200
,23.0808622876029 96.1304006624291
))'
,0)
) AS area;
CREATE TEMPORARY TABLE distanceStorage ON COMMIT DROP AS
SELECT ST_Distance(
ST_GeomFromText('POINT(23.0808622876029 96.1304006624291)',-1)
,ST_GeomFromText('POINT(28.0808622876029 99.1304006624291)',-1)
) AS distance;
height = (SELECT area FROM areaStorage)
/(0.5*(SELECT distance FROM distanceStorage));
IF height < (SELECT radius_meters
FROM gfe_geofences Where is_active=true) THEN
accuracy = "FullConfirm";
RETURN accuracy;
ELSE
accuracy = "PartiallyConfirm";
RETURN accuracy;
END IF;
END;
$BODY$ LANGUAGE plpgsql;
I just want to find two points on boundary of polygon geometry. Just like I have found one from the query:
CREATE TEMPORARY TABLE closePointStorage AS
SELECT ST_AsText(ST_ClosestPoint(geometry
,ST_GeomFromText('POINT(lat lon)',0)
)
) AS closestPoint
FROM (
SELECT ST_GeomFromText(geometry) as geometry
FROM gfe_geofences
WHERE is_active=true
)
AS tempName;
Other then this point I have to find one more with distance greater then the point find above but smaller then the rest of points.

Answer 1

Use ST_DumpPoints() to dump the points of the polygon, then select from that order by ST_Distance to A limit 2. ?
So it is something like
SELECT * from ST_DumpPoints(poly) order by ST_Distance(A,geom) asc limit 2;
(assumes that this is an inner select where poly is the polygon, A is the point to compare to and geom is the geom column of one of the points in the poly being compared)

Answer 2

There generally is no second closest point on the boundary polygon, if you include the lines. Just like there is no real number second closest to zero.
Either you only wish to consider the points at the corners, like Markus suggests.
Or you have only one closest point.

Answer 3

1) Kind of a left-field idea, but to find the second-closest point to your destination, why not find the closest point to the point you already found?
2) Or, more germaine to your specific question,
find the set of points within some reasonable range of the point,
find the intersection of that set with the set of points lying on the polygon border (which I am guessing may be another PostGIS function; haven't used postG in a while so I'm not sure)
3) Farther into left field, dump some of your dataset into Mongo and use the $near function... http://docs.mongodb.org/manual/reference/operator/near/

Answer 4

I am assuming you want to find the edge of the polygon that passes the closest to the point in question
To obtain the distance 'd' of point 'C' from line [A,B]
First translate all points so A is at 0,0
B -= A //vector subtraction
C -= A
Then normalize B so it is of length 1.0
len = sqrt( B . B) //dotproduct of two vectors is the length squared
B /= len //scalar divide by length
Find length from A that is at right angles to C
dotp = B . C //dot product again
closestPointOnLine = B * dotp //scalar multiply
Now get the distance
diff = (C - ClosestPointOnLine)
d = sqrt(diff . diff)
Not sure how to do that in SQL. You will need to do the above for each edge on your polygon, and then find the smallest value 'd'
By the way the sign of the cross-product of B and C will now tell you whether the point is on the inside of the polygon or not