implementing a proximity search in sql server

or "find stuff near me" and how i did it. (long read sorry)

after my last post about using the proximity search article for sql server 2000, i received some requests for a simple walkthrough on how i implemented my app.  well, here's an attempt at a very simple sample.

first, so we are clear, my scenario was as follows: given a known single point, search a database of other known points and return a result set of those points that are known to be within a certain mile radius of the single known point.  better put "find locations near me" :-)

let's use a sample database for the known points.  i'm using sql server 2005 express which is free.  here's my table structure for known points (for this sample):

   1:  CREATE TABLE [dbo].[Locations](

   2:      [LocationId] [int] IDENTITY(1,1) NOT NULL,

   3:      [LocationName] [nvarchar](50) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL,

   4:      [Address] [nvarchar](50) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL,

   5:      [City] [nvarchar](50) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL,

   6:      [State] [nvarchar](2) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL,

   7:      [Zip] [nvarchar](9) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL,

   8:      [Latitude] [float] NOT NULL,

   9:      [Longitude] [float] NOT NULL,

  10:      [XAxis] [float] NOT NULL,

  11:      [YAxis] [float] NOT NULL,

  12:      [ZAxis] [float] NOT NULL,

  13:   CONSTRAINT [PK_Locations] PRIMARY KEY CLUSTERED 

  14:  (

  15:      [LocationId] ASC

  16:  )WITH (PAD_INDEX  = OFF, IGNORE_DUP_KEY = OFF) ON [PRIMARY]

  17:  ) ON [PRIMARY]

once you have the locations, there are two key steps here: 1) geocoding those locations and 2) establishing the proximity calculations for the axis points.  the first is rather simple.  you essentially have to use a geocoding service to perform the action.  in my application i use my geocode provider and the yahoo geocode apis.  i've found them to be accurate and current.  google also has some as well.  both are 'free' (certain number of transactions) but you have to register your application.  i think yahoo is also implementing referrer-based checking as well, which might be a consideration.  either way, these will geocode your address.

suggestion: you can abstract this from your users very easily.  when they enter an address to save into your application UI, take an intermediary step to verify and geocode the address...once you have the lat/long then you are ready to save it to the database.

the next step is calculating the axis.  i'm honestly not going to sit here and pretend to tell you that i'm a geospacial expert on the laws of longitude, because i'm not.  basically the math that is performed makes calculations based on the radius and axis point of the earth.  no, i'm serious.  you'll see later one of the parameters is earth radius.  once you have the known lat/long of your addresses, then you have to calc the axis points.  to simplify this for me, i created three functions in sql server like this one:

   1:  CREATE FUNCTION [dbo].[XAxis] 

   2:      (

   3:      @lat float,

   4:      @lon float

   5:      )

   6:  RETURNS float

   7:  AS

   8:      BEGIN

   9:      RETURN COS(4 * (4 * atn2(1, 5) - atn2(1, 239)) / 180 * @lat) * COS(4 * (4 * atn2(1, 5) - atn2(1, 239)) / 180 * @lon)

  10:      END

the others are similar and you can get the scripts at the bottom of this post.  once you have those in your database for reusability, you have two options: stored procedure or trigger.  stored procedure (or rather, implementing within your insert/update routine) is probably most ideal.  you want to ensure the axis calculations (and also the geocoding) maintains integrity.  after all, if the user updates the address, you want to update all that information!  so within your stored proc you can simply update the XAxis, YAxis, and ZAxis fields using these functions using something like:

   1:  UPDATE Locations SET XAxis = dbo.XAxis(@lat, @long)

you could also do this in a trigger (as i was using), but it was pointed out to me (thanks bert) that using an AFTER UPDATE trigger might send this in an infinite loop as my trigger was performing an update statement itself.  stored proc is the way to go if you can.  i've included the INSERT trigger in the files just for your perusal though.

okay, with that done, you should have a sequence of when an address is entered (again possibly adding the intermediary step of geocoding for the user) you now have all the information you need.  now when your user needs to perform a "find near me" query the process is simple.

first, you'll need to geocode their asking point.  again, this is simple and can be accomplished by various geocoding apis.  once you have the lat/long of the asking point, you can feed them to the stored proc that will use your previous proximity information and find the ones nearest you.  here's the stored proc:

   1:  CREATE PROCEDURE [dbo].[FindNearby]

   2:     @CenterLat float, 

   3:     @CenterLon float, 

   4:     @SearchDistance float, 

   5:     @EarthRadius float

   6:  AS

   7:  declare @CntXAxis float

   8:  declare @CntYAxis float

   9:  declare @CntZAxis float

  10:   

  11:  set @CntXAxis = cos(radians(@CenterLat)) * cos(radians(@CenterLon))

  12:  set @CntYAxis = cos(radians(@CenterLat)) * sin(radians(@CenterLon))

  13:  set @CntZAxis = sin(radians(@CenterLat))

  14:   

  15:  select *,  ProxDistance = @EarthRadius * acos( XAxis*@CntXAxis + 

  16:    YAxis*@CntYAxis + ZAxis*@CntZAxis)

  17:  from  Locations

  18:  where  (@EarthRadius * acos( XAxis*@CntXAxis + YAxis*@CntYAxis + 

  19:    ZAxis*@CntZAxis) <= @SearchDistance) AND latitude is not null

  20:  order by ProxDistance ASC

you'll notice the parameters of radius and earth radius.  since i'm using miles, i use the earth radius of 3961 miles.  you can search on live.com and other places for another number, but this seemed to be a general consensus of the radius of the earth in miles.  i put this in my configuration file in case i needed to change it.  no, not in case the earth changed, but in case it needed to be kilometers.  the SearchDistance param needs to be in the same unit of measurement as the earth radius.  feed those and your lat/long in and you get those near the point -- as well as the approximate distance in the same unit of measure.  boom, you are done.  do with the data as you wish.

using virtual earth, you can easily plot those points as i did in my sample application.  heck using virtual earth you could also geocode for you in your user interface.

suggestion: you could present a map in your UI.  have the user enter an address...if found you can have them click on the point to verify and capture that lat/long from virtual earth, then sending back to your procedure and skipping that functional step on the backend.

that's it.  below are the files for the scripts to play around if you'd like.

Files: ProximitySampleSqlScripts