BoundingBox Shape

Question

In my Android mapping activity, I have a parallelogram shaped area that I want to tell if points (ie:LatLng) are inside. I've tried using the:

 bounds = new LatLngBounds.Builder()
                   .include(latlngNW)
                   .include(latlngNE)
                   .include(latlngSW)
                   .include(latlngSE)
                   .build();

and later

 if (bounds.contains(currentLatLng) {
    .....
 }

but it is not that accurate. Do I need to create equations for lines connecting the four corners?

Thanks in advance.

Answer 1

The LatLngBounds appears to create a box from the points included. Given the shape that I'm trying to monitor is a parallelogram, you do need to create equations for each of the edges of the shape and use if statements to determine which side of the line a point lies.

Not an easy solution!

Answer 2

If you wish to build a parallelogram-shaped bounding "box" from a collection of points, and you know the desired angles of the parallelogram's sides, your best bet is to probably define a 2d linear shear transform which will one of those angles to horizontal, and the other to vertical. One may then feed the transformed points into normal "bounding box" routines, and feed the corners of the resulting box through the inverse of the above transform to get a bounding parallelogram.

Note that this approach is generally only suitable for parallelograms, not trapezoids. There are a few special cases where it could be used to find bounding trapezoids [e.g. if the top and bottom were horizontal, and the sides were supposed to converge at a known point (x0-y0), one could map x' = (x-x0)/(y-y0)] but for many kinds of trapezoids, the trapezoid formed by inverse mapping the corners of a horizontal/vertical bounding rectangle may not properly bound the points that are supposed to be within it.