It sound like this performed well enough, but it seems like a more efficient solution would be to crawl the grid from left to right and at each point only searc

It sound like this performed well enough, but it seems like a more efficient solution would be to crawl the grid from left to right and at each point only search the neighbors of the previous closest grid from the larger grid, assuming that a local grid cell will never be have a height or width greater than a global cell. This would be linear in the number of local grid cells.