Google Maps provides several useful APIs for accessing data: a geocoding API to convert addresses to latitude and longitude, a search API to provide locations matching a term, and a details API for retrieving location metadata.<\/p>\n
For many mapping tasks it is valuable to get a large list of locations (restaurants, churches, etc) – since this is valuable, Google places a rate limiter on the information, and encourages caching query results.<\/p>\n
You can load a specific area of a map – the best way to find the starting point for the latitude and longitude is to enter an address in a geocoding API:<\/p>\n
\nmap = new google.maps.Map(document.getElementById('map-canvas'), {\n mapTypeId: google.maps.MapTypeId.ROADMAP,\n center: new google.maps.LatLng(curLat, curLong),\n zoom: 15,\n styles: [\n {\n stylers: [\n { visibility: 'simplified' }\n ]\n },\n {\n elementType: 'labels',\n stylers: [\n { visibility: 'off' }\n ]\n }\n ]\n});\n<\/pre>\nTo run a search, you can use the radarSearch API, which appears to return up to 200 results. However, this only returns latitudes and longitudes – not place names or anything you’d really want to a full application.<\/p>\n
\ngoogle.maps.event.addListenerOnce(map, 'bounds_changed', performSearch);\n\nfunction performSearch() {\n var request = {\n bounds: map.getBounds(),\n keyword: 'church'\n };\n service.radarSearch(request, callback);\n}\n<\/pre>\nOnce that finishes, it runs a callback – in this we save off the results so far, and set up a timer to get the full address of each entity. I determined experimentally that the Maps API won’t let you run a query more than once every two seconds – this adds a little extra lag because I’d rather the script continue than risk an error being slightly too soon.<\/p>\n
\nfunction callback(results, status) {\nfor (var i = 0, place; place = results[i]; i++) {\n createMarker(place);\n\n setTimeout(loadPlace, 2200 * i);\n}\n<\/pre>\nEach “place” is hydrated using the getDetails function on the maps API, then saved back to a server:<\/p>\n
\nfunction loadPlace() { \n place = places[placeIdx++];\n\n service.getDetails(place, \n function(result, status) {\n if (status !=\n google.maps.places.PlacesServiceStatus.OK) {\n return;\n }\n $.post(\n \"save.php\",\n {text: JSON.stringify(result)},\n function() {\n next();\n }); \n });\n}\n<\/pre>\nThis requires a simple PHP file- the results can be extracted later or used as a cache.<\/p>\n
\n$text = $_POST['text'];\n$json = json_decode($text, true);\n\n$id = md5($text);\nfile_put_contents('db\/' . $id, $text);\n<\/pre>\nUp to this point, we only have the ability to script a specific segment of a map – in reality we likely want to loop back and forth across an area. I found a bounding box that encompasses Philadelphia and the surrounding counties relatively well experimentally, by loading the map in several areas until I found good edges.<\/p>\n
Interestingly, Google Maps does not seem to have the same scale for latitude and longitude, as I found about one map unit area to be about 20x longitude as latitude (ideally this is slightly smaller than one box – this gives a little overlap and record a few entries twice)<\/p>\n
\nvar minLat = 39.873;\nvar minLong = -75.483;\nvar maxLat = 40.453;\nvar maxLong = -75.163;\n\nvar dLat = 0.01;\nvar dLong = 0.2;\n<\/pre>\nFinally, we need to define a function which moves the current map location over to the right or down, back and forth, until we read the entire area we want:<\/p>\n
\nfunction next() {\n if (placeIdx >= places.length) {\n curLat += dLat;\n if (curLat > maxLat) {\n curLong += dLong;\n curLat = minLat;\n }\n if (curLong <= maxLong) {\n setTimeout(initialize, \n Math.max(\n 2100, \n 2100 * (places.length - placeIdx)));\n }\n }\n}\n<\/pre>\nThis function must be called in a few places- anywhere there could be an error or a finished task which would otherwise stop the script. If we don't do this, it will stop partway through:<\/p>\n
\nif (status != google.maps.places.PlacesServiceStatus.OK) {\n placeIdx = 1000000; \n next();\n return;\n}\n\nplaces = results;\n\nif (!results) { \n next();\n return;\n}\nif (results.length == 0) {\n next();\n return;\n}\n<\/pre>\n","protected":false},"excerpt":{"rendered":"Google Maps provides several useful APIs for accessing data: a geocoding API to convert addresses to latitude and longitude, a search API to provide locations matching a term, and a details API for retrieving location metadata. For many mapping tasks it is valuable to get a large list of locations (restaurants, churches, etc) – since … <\/p>\n