Geohashes - Why is interleaving index values necessary?

Question

I have had a look at this post about geohashes. According to the author, the final step in calculating the hash is interleaving the x and y index values. But is this really necessary? Is there a proper reason not to just concatenate these values, as long as the hash table is built according to that altered indexing rule?

Answer 1

From the wiki page

Geohashes offer properties like arbitrary precision and the possibility of gradually removing characters from the end of the code to reduce its size (and gradually lose precision).

If you simply concatenated x and y coordinates, then users would have to take a lot more care when trying to reduce precision by being careful to remove exactly the right number of characters from both the x and y coordinate.

Answer 2

There is a related (and more important) reason than arbitrary precision: Geohashes with a common prefix are close to one another. The longer the common prefix, the closer they are.

54.321 -2.345 has geohash gcwm48u6
54.322 -2.346 has geohash gcwm4958

(See http://geohash.org to try this)

This feature enables fast lookup of nearby points (though there are some complications), and only works because we interleave the two dimensions to get a sort of approximate 2D proximity metric.

As the wikipedia entry goes on to explain:

When used in a database, the structure of geohashed data has two advantages. First, data indexed by geohash will have all points for a given rectangular area in contiguous slices (the number of slices depends on the precision required and the presence of geohash "fault lines"). This is especially useful in database systems where queries on a single index are much easier or faster than multiple-index queries. Second, this index structure can be used for a quick-and-dirty proximity search - the closest points are often among the closest geohashes.

Note that the converse is not always true - if two points happen to lie on either side of a subdivision (e.g. either side of the equator) then they may be extremely close but have no common prefix. Hence the complications I mentioned earlier.