•A mapping of keys (cell
values) and values (cell identifiers)
is a general way of storing grid data.
–Efficient in terms of memory use where a default value can be set, and if there are only a small number of non-default mappings in the chunk (compared to the number of cells in the chunk)
–Offer the means to generating some statistics about a chunk very efficiently
•the diversity
(number of different values)
•mode