geocoding

Olly Betts

Motivation

Geo-search features for Xapian text search library.

Common Retrieval Operations

• Restrict by distance
• Restrict by shape
• Weight by distance
• Sort by distance
• Sort by coordinate (N-S or E-W)

Why another format?

Goals

• Compact
• Efficient to use
• Nice rounding behaviour
• Truncates "nicely"
• Precision: human scale

First Observation

180 × 360 = 64,800 < 65,536

So nearest degree takes two bytes.

Second Observation

60 * 60 = 16 * 15 * 15

So nearest second of arc takes five bytes.

Precision

One second ~ 30m

so 1/16th of second < 2m

So six bytes gives precision better than 2m.

Demo

The End

Any questions or comments?