CODEWITHSUNDEEP
c#mathgeospatial
markpirvine
markpirvine

Reputation: 1592

Latitude/Longitude Combinations within Bounding Box

Given a bounding box, I would like to be able to calculate all possible latitude/longitude combinations within that box, to a certain accuracy e.g. 3 decimal places. At the moment the calculations are done is real time, which is very slow. To help speed things up I plan to use something like AppFabric Caching to cache a dataset of all points/distance combinations. If done correctly this should reduce the distance calculation to a lookup.

The only piece of the puzzle that I'm missing is an efficient way to calculate the combinations of lat/lon in the bounding box - can anyone help?

Mark

Upvotes: 0

Views: 2129

Answers (3)

user1473461
user1473461

Reputation: 369

I would concur with the other answers that normally you wouldn't want to create a list of all latitude / longitude combinations.

If you're using SQL Server 2008 or 2012, using the STContains() or STIntersects() methods are pretty easy and with a spatial index, very fast.

Upvotes: 0

Keith
Keith

Reputation: 6834

Still a bit mystified as to why you want all such points rather than just checking a given point is in the box on the fly (this does not take much computation).

However, given a small box, we can treat is as locally flat.

That means we transform any point to new coordinates:

(llat, llong) -> (x = llong * cos(llat), y = llong)

[Note the cos() is not an expensive function - unless you are still on a 486!]

The problem then reduces to that for a cartesian plane.

Assuming that by "box" you mean a rectangle, not necessarily oriented.

The obvious thing to do is rasterize it.

  • Find the extremal latitude points from your box.
  • For each latitude within the range find the pair of longitudes which intersect the edges. [It's for this edge interesection that you use the local flatness and just consider straight line intersections.]
  • All longitudes within the interval defined by that pair of longitudes are in the set you seek.
  • Maybe all your actually store are the interval end points rather than all points.

Upvotes: 1

Reinderien
Reinderien

Reputation: 15233

My guess is that calculating "all of the latitude and longitude combinations" within a certain box shouldn't be sped up; it shouldn't be done at all. Instead, you should derive functions that run along the four edges of the bounding box. For instance, for varying x and constant y_top, derive functions lat(x) and long(x). Given two functions for four edges = eight functions total, you should be able to develop fast logic that determines whether or not any given coordinate is in the bounding box.

Upvotes: 1

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