Local Kriging: grid split into 500m blocks, each block uses only
sample points within block+buffer zone. Covariance matrix shrinks
from 8242x8242 to local nxn. 10M+ grid cells now finish in minutes.
Complexity:
Global: O(G * P * logP * K^3) = hours for large grids
Local: O(sum(block*grid * block*points * 15^3)) = minutes