[mmalone]

1. A geohash is a z-curve.

2. It won't be better than a k-d tree. Dimensionality reduction is usually done when you have really truly huge numbers of dimensions that are sparsely populated and you don't care much about some information loss (e.g., for machine learning) or, in this case, when you have an easy way to create a single dimensional index and you want to force multi-dimensional data into it. In the general case a k-d tree would be objectively better in terms of performance.

[AstralStorm]

And in very many or highly sparse dimensions with few or lazy updates, R-tree or derivatives.