Mysql – Optimal column type for latitude and longitude on Rails and MySQL

I'm wondering what's the best column type to store latitude/longitude on MySQL + Rails.

• Precision must be enough to store every bit of lat/lng degrees obtained from mobile devices and/or geocoders.
• Storage requirement should be minimal for the best query performance.

From Google's official document:

http://code.google.com/apis/maps/articles/phpsqlajax_v3.html

With the current zoom capabilities of
Google Maps, you should only need 6
digits of precision after the decimal.
To keep the storage space required for
your table at a minimum, you can
specify that the lat and lng
attributes are floats of size (10,6).
That will let the fields store 6
digits after the decimal, plus up to 4
digits before the decimal, e.g.
-123.456789 degrees.

So, actually FLOAT(10,6) is recommended by Google.

However, with Rails 3, there seems no easy way to define FLOAT column type with precision after the decimal point. For instance, you could write a migration with raw SQL as follows:

def self.up
  execute <<-SQL
  ALTER TABLE places
    ADD `lat` FLOAT(10,6),
    ADD `lng` FLOAT(10,6)
  SQL
  add_index :places, [ :lat, :lng ]
end

But the schema.rb as a consequence will look like this:

t.float "lat", :limit => 10
t.float "lng", :limit => 10

which is missing the precision for the fractional part.

Here I can see several options:

• Use FLOAT(10,6) for optimal production performance, and don't dump schema (e.g. rake db:test:load) on development.
• Use DECIMAL(10,6), which is supported by Rails, but it takes 6 bytes, 1.5 times larger than FLOAT (see: http://dev.mysql.com/doc/refman/5.1/en/storage-requirements.html). Maybe this would be a good compromise?
• Use DOUBLE, which is much roomier than Google's requirement, and takes 8 bytes, 2 times larger than FLOAT. It's simple as well.

What's your recommendation?