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Amazon CloudSearch
Developer Guide (API Version 2013-01-01)

Searching and Ranking Results by Geographic Location in Amazon CloudSearch

If you store locations in your document data using a latlon field, you can use the haversin function in an Amazon CloudSearch expression to compute the distance between two locations. Storing locations with your document data also enables you to easily search within particular areas.

Searching Within an Area in Amazon CloudSearch

To associate a location with a search document, you can store the location's latitude and longitude in a latlon field using decimal degree notation. The values are specified as a comma-separated list, lat,lon—for example 35.628611,-120.694152. Associating a location with a document enables you to easily constrain search hits to a particular area with the fq parameter.

To use a bounding box to constrain results to a particular area

  1. Determine the latitude and longitude of the upper-left and lower-right corners of the area you are interested in.

  2. Use the fq parameter to filter the matching documents using those bounding box coordinates. For example, if you include a location field in each document, you could specify your bounding box filter as fq=location:['nn.n,nn.n','nn.n,nn.n'] . In the following example, the matches for restaurant are filtered so that only matches within the downtown area of Paso Robles, CA are included in the results.

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    q='restaurant'&fq=location:['35.628611,-120.694152','35.621966,-120.686706']&q.parser=structured

Sorting Results by Distance in Amazon CloudSearch

You can define an expression as part of your search request to sort results by distance. Amazon CloudSearch expressions support the haversin function, which computes the great-circle distance between two points on a sphere using the latitude and longitude of each point. (For more information, see Haversine formula.) The resulting distance is returned in kilometers.

To calculate the distance between each matching document and the user, you pass the user's location into the haversin function and reference the document locations stored in a latlon field. You specify the user latitude and longitude in decimal degree notation and access the latitude and longitude stored in a latlon as FIELD.latitude and FIELD.longitude. For example, expr.distance=haversin(userlat,userlon, location.latitude,location.longitude).

To use the expression to sort the search results, you specify the sort parameter.

For example, the following query searches for restaurants and sorts the results by distance from the user.

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q=restaurant&expr.distance=haversin(35.621966,-120.686706,location.latitude,location.longitude)&sort=distance asc

Note that you must explicitly specify the sort direction, asc or desc.

You can include the distance calculated for each document in the search results by specifying the name of the expression with the return parameter. For example, return=distance.

You can also use the distance value in more complex expressions to take other characteristics into account, such as a document's relevance _score. In the following example, a second rank expression uses both the document's calculated distance and its relevance _score.

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expr.distance=haversin(38.958687,-77.343149,latitude,longitude)&expr.myrank=_score/log(distance)&sort=-myrank

For more information about using expressions to sort search results, see Controlling Search Results.