How do I use spatials to search a radius of zip codes?

Question

Background

I'm writing an application which finds events within a certain radius of a zip code. You can think of this like ticketmaster, where you type in your zip code and all of the concerts in the radius of x show up.

I have a database table which has the zip code, and each zip codes' Latitude and Longitude. I also have an 'EventListings' table where each 'Event' has a ZipCode field.

Problem

Currently, I'm using the Haversine formula in a Linq-to-Entities query in my service layer to find which events are within the radius. Right now, I'm using it as a filter in the where clause. I'd also like to place it in the select clause so on the website I can show "this is 4.6 miles away", etc.

I can't move this code into a separate C# method because Linq-to-Entities will complain that it can't convert it to sql, so that leaves me with duplicating the entire formula in the select statement too. This is very ugly. I tried to fix it.

What I've Tried

I edited the Entity, and added a special scalar property of "DistanceFromOrigin". I then created a stored procedure which brought back all the entity data, plus a hard coded value (for testing purposes) for the new field "DistanceFromOrigin".

I then came to realize that I can't tell entity framework to use my sproc for its select statement on the EventListings entity... Phil suggested spatials, so thats what I went with.

Question

How can I use Spatials to search for events within a radius of zip codes?

Answer 1

So, Using Phil's suggestion, I re-wrote a lot of this using spatials. This worked out great, you just need .NET4.5, EF5+ and sql server (2008 and above I believe). I'm using EF6 + Sql Server 2012.

Set-Up

The first step was to add a Geography column to my database EventListings table (Right click on it -> Design). I named mine Location:

Next, since I am using the EDM with database-first, I had to update my model to use the new field that I created. Then I received an error about not being able to convert Geography to double, so what I did to fix it was select the Location property in the entity, go to its properties and change its type from double to Geography:

Querying within a Radius & Adding Events With Locations

Then all you have to do is query your entity collection like this:

 var events = EventRepository.EventListings
                             .Where(x => x.Location.Distance(originCoordinates) * 0.00062 <= radiusParam); 

The Distance extension method gets the distance from the current object, to the "other" object that you pass in. This other object is of type DbGeography. You just call a static method and it creates one of these puppies, then just throw your Longitude & Latitude in it as a point:

DBGeography originCoordinates = DBGeography.fromText("Point(" + originLongitude + " " + originLatitude + ")");

This isn't how I created my originCoordinates. I downloaded a separate database that had a list of all zip codes and their Latitudes & Longitudes. I added a column of type Geography to that as well. I'll show how at the end of this answer. I then queried the zipcode context to get a DbGeography object from the Location field in the ZipCode table.

If the user wants a more specific origin than just a zipcode, I make a call to the Google Maps API (GeoCode to be more specific) and get the latitude and longitude for the users specific address via a webservice, and create a DBGeography object from the Latitude & Longitude values in the response.

I use google APIs when I create an event also. I just set the location variable like this before adding my entity to EF:

someEventEntity.Location = DBGeography.fromText("Point(" + longitudeFromGoogle+ " " + latitudeFromGoogle + ")");

Other Tips & Tricks & Troubleshooting

How did I get the Distance Extension Method?

To get the extension method Distance you must add a reference to your project: System.Data.Entity After you do that you must add the using: using System.Data.Entity.Spatial; to your class.

The Distance extension method returns the distance with a unit of measure of meters (You can change it I think, but this is default). Here, my radius parameter was in miles so I did some math to convert.

Note: There is a DBGeography class in System.Data.Spatial. This is the wrong one and it would not work for me. A lot of examples I found on the internet used this one.

How to convert Lat/Long to Geography Column

So, if you were like me and downloaded a zipcode database with all the Latitude & Longitude columns, and then realized it didn't have a Geography column... this might help:

1) Add a Location column to your table. Set the type to Geography.

2) Execute the following sql

UPDATE [ZipCodes]
SET Location = geography::STPointFromText('POINT(' + CAST([Longitude] AS VARCHAR(20)) + ' ' + CAST([Latitude] AS VARCHAR(20)) + ')', 4326)

How to view the details of a Geography item

So, when you query your EventListings table in Sql Server Management Studio after you've inserted some DbGeography items, you'll see the Location column holds a hex value like: 0x1234513462346. This isn't very helpful when you want to make sure the correct values got inserted.

To actually view the latitude & longitude off of this field you must query it like so:

SELECT Location.Lat Latitude, Location.Long Longitude
FROM [EventListings]