Question

I’m trying to find all data items that have long/lats that are contained by a polygon made up of an array of long lats could be many points. I know you can do this kind of stuff with the new geospacial datatypes in SQL 2008 but i’m using SQL 2005 and C#. Would this be best done on at the DB end or in C#.

Thanks.

Answer 1

I have some code written for SQL2000 to do this. It uses the ‘angle’ method for determining if a point lies within a polygon.

First, the GetAngle user defined function:

ALTER Function [dbo].[GetAngle](
@Ax Decimal(8,5), 
@Ay Decimal(8,5), 
@Bx Decimal(8,5), 
@By Decimal(8,5), 
@Cx Decimal(8,5), 
@Cy Decimal(8,5))

Returns Float
As
Begin

Declare @dot_product Float
Declare @cross_product Float

Declare @BAx Decimal(8,5)
Declare @BAy Decimal(8,5)
Declare @BCx Decimal(8,5)
Declare @BCy Decimal(8,5)

--' Get the vectors' coordinates.
Set @BAx = Sign(@Ax - @Bx) * dbo.CalculateDistance(@Ax, @Ay, @Bx, @Ay)
Set @BAy = Sign(@Ay - @By) * dbo.CalculateDistance(@Ax, @Ay, @Ax, @By)
Set @BCx = Sign(@Cx - @Bx) * dbo.CalculateDistance(@Cx, @Cy, @Bx, @Cy)
Set @BCy = Sign(@Cy - @By) * dbo.CalculateDistance(@Cx, @Cy, @Cx, @By)

--' Calculate the dot product.
Set @dot_product = @BAx * @BCx + @BAy * @BCy

--' Calculate the Z coordinate of the cross product.
Set @cross_product = @BAx * @BCy - @BAy * @BCx

--' Calculate the angle.
return ATn2(@cross_product, @dot_product)

End

---

Next I assume there is a table variable of Latitude/Longitude pairs and a sequence number (indicating the order in which the LAT/LONG pairs define the polygon). It’s important that the first point in this table is the same as the last point in the table.

Also, I have several variables for Min and Max Latitude & Longitude. This effectively creates a bounding box so that I can quickly eliminate points NOT within a rectangular region bounding the polygon.

Select  Address.AddressId
From    @Temp As A
        Inner Join @Temp As B
            On A.SequenceNumber = B.SequenceNumber - 1
        Inner Join Address
            On Address.XCoord Between @MinLongitude And @MaxLongitude
            And Address.YCoord Between @MinLatitude And @MaxLatitude
Group By Address.AddressId
Having Abs(Sum(dbo.GetAngle(A.Longitude, A.Latitude, Address.XCoord, Address.YCoord, B.Longitude, B.Latitude))) > 3.14