To do that you need to change a stored procedure. In the procedure prcGetRouteForMap, you need to add TOP 1 and ORDER BY gpsTime DESC. The new stored procedure will look like this:

  SELECT TOP 1 latitude '@latitude', longitude '@longitude',
  speed '@speed', direction '@direction', distance '@distance',
  locationMethod '@locationMethod', CONVERT(VARCHAR(25), gpsTime, 100) '@gpsTime',
    phoneNumber '@phoneNumber',  sessionID '@sessionID', accuracy '@accuracy',
    isLocationValid '@isLocationValid', extraInfo '@extraInfo'
  FROM gpslocations
    WHERE sessionID = @sessionID
    AND phoneNumber = @phoneNumber
    ORDER BY gpsTime DESC
  FOR XML PATH('locations'), ROOT('gps')

Nick