finding locations within a particular distance using db2 - geolocation

Question

I am using html5 geolocation api to get my position in latitude and longitude. I want to store them in a table of locations and want to retrieve those locations within a particular distance.
my current latitudes and longitudes are stored in variables "latval", "longval", "distance"
My table is "location"
columns are "location", "lat", "long"
I am using DB2 Express C as database and latitude and longitude columns are set as double type now. What type should I use to store these values and what would be the query to get location names within a distance
Thank you.

Answer 1

It looks like there's an extension for Express C that includes Spatial processing. I've never used it (and can't seem to get access at the moment), so I can't speak to it. I'm assuming that you'd want to use that (find all locations within a radius is a pretty standard query).
If for some reason you can't use the extension, here's what I would do:
Keep your table as-is, or maybe use a float data-type, although please use full attribute names (there's no reason to truncate them). For simple needs, the name of the 'location' can be stored in the table, but you may want to give it a numeric id if more than one thing is at the same location (so actual points are only in there once).
You're also going to want indicies covering latitude and longitude (probably one each way, or one covering each column).
Then, given a starting position and distance, use this query:
SELECT name, latitude, longitude
FROM location
WHERE (latitude >= :currentLatitude - :distance
AND latitude <= :currentLatitude + :distance)
AND (longitude >= :currentLongitude - :distance
AND longitude <= :currentLongitude + :distance)
-- The previous four lines reduce the points selected to a box.
-- This is, or course, not completely correct, but should allow
-- the query to use the indicies to greatly reduce the initial
-- set of points evaluated.
-- You may wish to flip the condition and use ABS(), but
-- I don't think that would use the index...
AND POWER(latitude - :currentLatitude, 2) + POWER(longitude - :currentLongitude, 2)
<= POWER (:distance, 2)
-- This uses the pythagorean theorem to find all points within the specified
-- distance. This works best if the points have been pre-limited in some
-- way, because an index would almost certainly not be used otherwise.
-- Note that, on a spherical surface, this isn't completely accurate
-- - namely, distances between longitude points get shorter the farther
-- from the equator the latitude is -
-- but for your purposes is likely to be fine.
EDIT:
Found this after searching for 2 seconds on google, which also reminded me that :distance would be in the wrong units. The revised query is:
WITH Nearby (name, latitude, longitude, dist) as (
SELECT name, latitdude, longitude,
ACOS(SIN(RADIANS(latitude)) * SIN(RADIANS(:currentLatitude)) +
COS(RADIANS(latitude)) * COS(RADIANS(:currentLatitude)) *
COS(RADIANS(:currentLongitude - longitude))) * :RADIUS_OF_EARTH as dist
FROM location
WHERE (latitude >= :currentLatitude - DEGREES(:distance / :RADIUS_OF_EARTH)
AND latitude <= :currentLatitude + DEGREES(:distance / :RADIUS_OF_EARTH))
AND (longitude >= :currentLongitude -
DEGREES(:distance / :RADIUS_OF_EARTH / COS(RADIANS(:currentLatitude)))
AND longitude <= :currentLongitude +
DEGREES(:distance / :RADIUS_OF_EARTH / COS(RADIANS(:currentLatitude))))
)
SELECT *
FROM Nearby
WHERE dist <= :distance
Please note that wrapping the distance function in a UDF marked DETERMINISTIC would allow it to be placed in both the SELECT and HAVING portions, but only actually be called once, eliminating the need for the CTE.