CODEWITHSUNDEEP
google-app-engine
Peter
Peter

Reputation: 1541

google app engine : query geohash

I have a db.StringProperty() of geohash, by given a hashcode, how do I find the closer 10 result?

I tried below but doesn't seem to be right

pois = POI.all().filter('geohash <', h_latlng).order('-geohash').fetch(10)

Upvotes: 0

Views: 1010

Answers (1)

cat
cat

Reputation: 2895

A geohash cannot accomplish the task to find the n-nearest results. You can find the contents of any square region by prefix. But to find a reliable result containing the n-nearest you need to fetch at least 9 prefixes, making it a quite expensive query. Complicating the matter is that prefixes of the 9 squares need to be calculated.

IMO this problem is currently a hard problem to solve efficiently on app-engine. So far, I am on it since a year and have not found a sophisticated and fast solution. A Relational DB with geo index or 2 inequalities will perform such tasks better and faster. But I am interested in good solutions, too. :-)

Citation David Troy:

Geohash also has the property that as the number of digits decreases (from the right), accuracy degrades. This property can be used to do bounding box searches, as points near to one another will share similar Geohash prefixes.

However, because a given point may appear at the edge of a given Geohash bounding box, it is necessary to generate a list of Geohash values in order to perform a true proximity search around a point. Because the Geohash algorithm uses a base-32 numbering system, it is possible to derive the Geohash values surrounding any other given Geohash value using a simple lookup table.

See: https://github.com/davetroy/geohash-js

Upvotes: 1

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