如何根据纬度和经度搜索半径？

Question

I want to show data based on latitude and longitude with specified radius.

Example:

I have a record with latitude 55.0628 and longitude -162.3056 without specified state.

How can I show only records that within one state, using entity to linq?

If I have state Florida to show just records that within Florida.

Table Data

id            item             latitude                 longitude
1             townhome          55.0628                 -162.3056


Table postal codes

id               state                 city            latitude            longitude
1                alaska                Akutan          54.143              -165.7854
2                Alabama               Huntsville      34.7448             -86.6704

Answer 1

I would execute the query as close to the actual data as possible (which probably means circumventing LINQ and calling a stored procedure).

Here is a SQL user defined function that I use to calculate the distance between two locations. It leverages the new geography functionality introduced in SQL Server 2008.

CREATE FUNCTION [dbo].[GetDistanceBetween]
(
    @Lat1 float,
    @Long1 float,
    @Lat2 float,
    @Long2 float
)
RETURNS float
AS
BEGIN

    DECLARE @RetVal float;
    SET @RetVal = ( SELECT geography::Point(@Lat1, @Long1, 4326).STDistance(geography::Point(@Lat2, @Long2, 4326)) / 1609.344 );

RETURN @RetVal;

END

Function returns distance in miles and is very fast in my experience (this will obviously depend on how many comparisons you need to make).

You could call it using something like:

DECLARE @StartingLatitude FLOAT, @StartingLongitude FLOAT;
DECLARE @MaxDistance FLOAT = 50;

SELECT * FROM PostalCodes 
WHERE dbo.GetDistanceBetween(@StartingLatitude, @StartingLongitude, latitude, longitude) <= @MaxDistance;

Answer 2

To know a destination is within a given larger region, you need to not just have the coordinates of a point in the larger region, but coordinates that map out it's entire boundary.

If you have this data, it becomes a matter of the Point in Polygon problem. I've found that ray-tracing was quite easy to implement in both SQL and C# so a nice one to have as a function that you make Linq-aware, though I only did it Linq2SQL not Linq2Entities.

If you only have the co-ordinates of central points, you can find the closest such point, using STDistance . This could though, easily mis-identify a location near the border of a larger state as being in the smaller state, because it is is close to the centre of the smaller state than the larger.

If you do go with that, but you aren't using SQL 2008 (so no STDistance) you can get a crude approximation by ordering on ((lat1 - lat2) * (lat1 - lat2)) + ((lng1 - lng2) * (lng1 - lng2)) . This gives the relative distance as if the world really were a rectangle like the Mercator projection suggests. The inaccuracy gets worse the further the points are from the equator, so it might be just about tolerable for US states, but fail with Canadian provinces.

Answer 3

The proper and fastest way is to start with your lat-lon pair of a zip code and find the East, West, North and South points for a given distance (radius). Follow this link for the relative formulas. Then use these points to get zip codes from the database; something like:

select *
from zipcodes
where
    lat >= South
    and lat <= North
    and lon >= West
    and lon <= East
    -- and state = 'FL' -- use it optionally

Now having all zip records within the square defined by East, North, West and South points (as lines) proceed with distance calculations (use Tim's code) and keep only the records within the radius. (If all points in the square where records, then something like 22% would be out of the radius).
Obviously if you use CTE you will perform the whole sql thing with only one query.