Saturday, February 19, 2011

Searching Near By Addresses on Map using Haversine formula in SQL Server

In one of my projects, we have a requirement to search near by locations. Our application front is Silverlight and backend is SQL Server 2008. In this article I am sharing my solution from SQL Server point of view. I have used Haversine formula to solve the problem. Click here to see more details on Haversine formula.

I have Locations table. This table has below fields:
  • LocationId
  • LocationName
  • StreetAddress
  • City
  • Zip
  • State
  • Country
  • Description
  • Logitude
  • Latidue
In this application we have used Bing Map Provider. Click here to see more details on Bing Map Provider. While saving data in Locations, the application checks for valid address. And stores Latitude and Longitude of the address.

In the Nearby search screen, allow user to enter full address of location where he/she wants to search for nearby locations. If it is valid address then we extract latitude and longitude of the address user entered.

Now front end passes latitude, and longitude to business layer. Let us see on how we can extract nearby locations of given address (latitude and longitude). SQL Server Stored proedure accepts maximum distance of search radius. You can make this configurable item (web.config entry) in web service.

Step #1: Create a function in SQL Server. The details of the function are:

Function Name: IsLocationFallInRadius
Purpose: This function takes latitude and longitude of two locations. Also it takes allowed maximun distance between these two locations. It retuns 1(true) if two locations fall within maximum distance. Returns 0 (false) if if two locations does not fall within maximum distance.

create function dbo.IsLocationFallInRadius
@Latitude1 float,
@Longitude1 float,
@Latitude2 float,
@Longitude2 float,
@MaxDistance float
returns bit
Function: IsLocationFallInRadius
Computes the Great Circle distance in miles
between two points on the Earth using the
Haversine formula distance calculation.

Input Parameters:
@Longitude1 - Longitude in degrees of point 1
@Latitude1 - Latitude in degrees of point 1
@Longitude2 - Longitude in degrees of point 2
@Latitude2 - Latitude in degrees of point 2
declare @radius float
declare @lon1 float
declare @lon2 float
declare @lat1 float
declare @lat2 float
declare @a float
declare @distance float
declare @IsInRadius bit

-- Sets average radius of Earth in miles
set @radius = 3959.0E
-- Convert degrees to radians
set @lon1 = radians( @Longitude1 )
set @lon2 = radians( @Longitude2 )
set @lat1 = radians( @Latitude1 )
set @lat2 = radians( @Latitude2 )

set @a = sqrt(square(sin((@lat2-@lat1)/2.0E)) +  (cos(@lat1) * cos(@lat2) * square(sin((@lon2-@lon1)/2.0E))) )

set @distance = @radius * ( 2.0E *asin(case when 1.0E < @a then 1.0E else @a end ))
if(@distance > @MaxDistance)
 SET @IsInRadius = 0
 SET @IsInRadius = 1
return @IsInRadius

Step #2: Create Stored Procedure to retrive records from Locations table.
Stored Procedure Name: SearchLocationsNearBy
Purpose: It takes location latitude and longitude of search input. It also takes maximum distance of search. It returns locations which fall within maximum distance (within radius).

CREATE PROCEDURE [dbo].[SearchLocationsNearBy]

@Latitude float,
@Longitude float,
@MaxDistance float
 select * from Locations
   Where  dbo.IsLocationFallInRadius(@Latitude,@Longitude, Latitude, Longitude,@MaxDistance) = 1


In the where clause of query we I have called IsLocationFallInRadius function to vaidate if the records falls within the given rage. Business layer can invoke SearchLocationsNearBy stored procedure for searching.

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