Sunday, September 4, 2011

Example #19 - Rats! - a crude app

We can combine the functionality of previous examples to construct a simple app.

Let's build rats.py. Instead of potholes, we'll search the Chicago Data Portal dataset, 311 Service Requests - Rodent Baiting (2011) for locations of rodent complaints and display a map with markers showing nearby rat sightings.

Example #18's getgeo.py has the basics for taking a street address as an argument and determining the latitude and longitude.  This is the center of our circle and the user can take a default radius or specify the radius of the circle.

Example #17's simple geo query code has the basics to search the Rodent Baiting dataset to find rat complaints inside that circle.  The search will return the latitude and longitude of those nearby rat complaints.

Example #18's code for creating a Google map has the basics for constructing a URL with all the rat complaint locations and sending it to Google to display a map on the user's browser.

The code is lengthy.  It's posted at the end.  Soon, it'll be on github.

Here are two examples of running it and the output.  It runs from the command prompt.  Displays a short output message in that window and opens up a browser tab with a static Google map.

Example with default radius
Check for rats at 2400 W. Fullerton Ave in the default circle radius of 100 meters.

 

There are eight rat complaints in this circle.  Here's the map of them.


Example with bigger radius
Ask again with a bigger circle.  Try 300 meters.



There are 85 complaints in this bigger circle.  Only the first 20 are displayed on the map.  Trying to display all of these as markers makes the URL too long for Google to process and generates an error.


Comments
rats.py is a prototype.  It gives you the basics and the idea of one way to use the data.  There are many, many improvements that could be made:
  • turn it into a web app where the user enters the address from a web browser and sees the results in the same window.
  • make it a mobile app where the user's current location is used for the center of the circle.
  • use dynamic maps to allow the user to pan and zoom the map
  • page the results from the query back in chunks of 20 rows so the user can see all the rats in the circle
  • add flyovers to the markers so the user can click on the marker to get details in a balloon of text
  • remove duplicate rat complaints
  • use different colored markers for open and closed complaints. 
  • use a custom map marker that looks like little rat
  • and so on.

 Code
#
#  rats   - Takes a Chicago street address as input and opens a map in your
#           default browser with nearby locations of rat complaints.  The
#           address is the center of a circle that is <radius> meters.  Rodent
#           baiting requests in that circle will appear on the map.  The total
#           number of rats found is printed in the command prompt window.
#
#           The maximum number of markers on the map is maxNumRatsInMap. If the
#           URL has too many markers, it becomes too long for Google and
#           generates and error.  Playing with this max keeps the URL short
#           enough to process.  If more rats than the max are found, the first
#           maxNumRatsInMap are displayed on map.  A message is printed to the
#           command prompt window informing user the number of rats in the map.
#
#           Uses Google Maps API for geocoding and map display.
#           Uses Chicago Data Portal 311 Service Requests for Rodent Baitig in
#           2011.
#           Does not check for duplicate complaints or open/closed complaints.
#
#  usage:   python rats.py <street> [radius]
#
#  example: python rats.py "2400 West Fullerton"
#           python rats.py "2400 w fullerton" 200
#
#  args:    <street> - street address in quotes.  Assumes city is Chicago and
#               state is IL
#           <radius> - find rat complaints in circle centered on <street> that is
#               <radius> meters in radius.  Default is 100.  Must be greater than 0.
#
import httplib
import sys
import pprint
import json
import urllib
from urllib2 import Request, urlopen, URLError, HTTPError
import os


#
#   geocode(street, city, state) returns the latitude and longitude of a
#       street address using Google's map API.  Note, there is a limit on
#       the number of geocode requests you can make each day to Google.
#
#   Args:
#       street - string - street address
#       city - string - city name
#       state - string - state name
#
#   Returns:
#       err - string - error information.  If empty, geocode was successful
#       lat - string - latitude of the street address
#       lng - string - longitude of the street address
#
def geocode(street, city, state):
    err = ""
    lat = ""
    lng = ""
    url =  "http://maps.googleapis.com/maps/api/geocode/json?address=" + \
        urllib.quote_plus(street) + ',' + \
        urllib.quote_plus(city)   + ',' + \
        urllib.quote_plus(state)  + \
        "&sensor=false"
    req = Request(url)
    try:
    u = urlopen(req)
    except URLError, e:
        if hasattr(e, 'reason'):
            err = e.reason
    elif hasattr(e, 'code'):
            err = e.code
    else:
        response = json.load(u)
        # Check that Google sent back valid data.  If so, get lat and long.
        if response['status'] == 'OK':
            lat = response['results'][0]['geometry']['location']['lat']
            lng = response['results'][0]['geometry']['location']['lng']
        # otherwise, Google returned an error
        else:
            err = 'Google error code: %s\n' % response['status']
    return err, str(lat), str(lng)

#
#   getRatLocs(lat, lng, rad) returns a list of the locations of rats
#       from the Chicago 311 service requests for rodent baiting.  The
#       locations will be within the circle that has a center point at
#       lat/lon and a radius of rad meters.
#
#   Args:
#       lat - latitude of the center point of the circle
#       lng - longitude of the center point of the circle
#       rad - radius, in meters, of the circle
#
#   Returns:
#       err - string with error information.  If empty, geocode was successful
#       locs - a list with each item being a sublist containing two
#           elements: latitude and longitude that represents the location of
#           a rat baiting request.
#
def getRatLocs(lat, lng, rad):
    # parameters used in the SODA POST request to do the search
    hostName   = "data.cityofchicago.org"
    service    = "/views/INLINE/rows"
    formatType = "json"
    parameters = "method=index"
    headers    = { "Content-type:" : "application/json" }
    # SODA inline query.  Lat, Lng, Radius are set to 0 here.
    query = {
        "originalViewId": "97t6-zrhs",
        "name": "Nearby rats",
        "query": {
            "filterCondition": {
                "type": "operator",
                "value": "within_circle",
                "children":
                    [
                        {
                            "type": "column",
                            "columnId": 2849547
                        },
                        {
                            "type": "literal",
                            "value": 0
                        },
                        {
                            "type": "literal",
                            "value":  0
                        },
                        {
                            "type": "literal",
                            "value": 0
                        }
                    ]
                }
            }
         }
    # constants used to index into inline filter children[]
    queryLat = 1
    queryLng = 2
    queryRad = 3
    # constants for table column numbers in query return data
    colNumLat = 22
    colNumLng = 23
    # constants used to index into locs
    locLat = 0
    locLng = 1

    # initialize return variables to be empty
    err = ''
    locs = list()

    # put lat, lng, radius into the inline query
    query["query"]["filterCondition"]["children"][queryLat]["value"] = lat
    query["query"]["filterCondition"]["children"][queryLng]["value"] = lng
    query["query"]["filterCondition"]["children"][queryRad]["value"] = rad
    # setup and send the inline query
    jsonQuery = json.dumps(query)
    request = service + '.' + formatType + '?' + parameters
    conn = httplib.HTTPConnection(hostName)
    conn.request("POST", request, jsonQuery, headers)
    response = conn.getresponse()
    # check for good response, pull data out of response and setup locs
    if response.reason != 'OK':
        err = "%s %s" % (response.status, response.reason)
    else:
        rawResponse = response.read()
        jsonResponse = json.loads(rawResponse)
        for rowData in jsonResponse['data']:
            locs.append([rowData[colNumLat], rowData[colNumLng]])
    return err, locs


#
#   createMapUrl(centerLat, centerLng, locs) returns the URL for a Google
#       static map that has a marker for the center of the map and markers
#       for the locations in locs.
#
#   Args:
#       centerLat - string - latitude of the center point of the map
#       centerLng - string - longitude of the center point of the map
#       locs - list - where each item is a location to be marked on the map.
#           Each item is a sublist with two strings: latitude and longitude
#       n - integer - the first n locations in locs will be marked on the
#           map.  There is an upper limit on the size of the URL sent to
#           the Google Map API.  A long list of locations in the URL will
#           not be accepted.  n keeps the URL within the limit.
#
#   Returns:
#       URL - string - URL to send to the Google Map API to create a static
#           map in the default browser.
#
def createMapUrl(centerLat, centerLng, locs, n):
    # define constants for indexing into the locs sublist
    locLat = 0
    locLng = 1
    # define parameters for the map
    urlMapAPI = "http://maps.googleapis.com/maps/api/staticmap"
    mapZoomLevel = "14"
    mapHeight = "512"
    mapWidth = "512"
    mapType = "roadmap"
    centerMarkerColor = "blue"
    centerMarkerLabel = "C"
    locMarkerColor = "red"
    locMarkerLabel = "R"

    url = \
        urlMapAPI + '?' + \
        'center=' + centerLat + ',' + centerLng + \
        '&' + \
        'size=' + mapHeight + 'x' + mapWidth + \
        '&' + \
        'maptype=' + mapType + \
        '&' + \
        'sensor=false' + \
        '&' + \
        'markers=color:' + centerMarkerColor + '%7C' + \
        'label:' + centerMarkerLabel + '%7C' + centerLat + ',' + centerLng

    i = 0
    for loc in locs:
        if i < n:
            url += '&' + \
                'markers=color:' + locMarkerColor + '%7C' + \
                'label:' + locMarkerLabel + '%7C' + \
                loc[locLat] + ',' + loc[locLng]
        i +=1
    return url



# constants used to index into addr[]
indexStreet = 0
indexCity = 1
indexState = 2
# defaults
circleRadius = 100   
maxNumRatsInMap = 20

if len(sys.argv) < 2 or len(sys.argv) > 3:
    sys.stderr.write("Usage: python %s \"street\" [radius]\n" % sys.argv[0])
    raise SystemExit(1)

addr = sys.argv[1]

if len(sys.argv) == 3:
    try:
        circleRadius = int(sys.argv[2])
    except:
        sys.stderr.write("Usage: python %s \"street\" [radius]\n" % sys.argv[0])
        raise SystemExit(1)

if circleRadius <= 0:
    sys.stderr.write("Usage: python %s \"street\" [radius]\n" % sys.argv[0])
    raise SystemExit(1)

err, circleCenterLat, circleCenterLng = geocode(addr, 'Chicago', 'IL')
if (err != ""):
    print "Geocoding error = %s" % err
    sys.exit(0)

err, ratLocations = getRatLocs(circleCenterLat, circleCenterLng, circleRadius)
if (err != ""):
    print "Query error = %s" % err
    sys.exit(0)

url = createMapUrl(circleCenterLat, circleCenterLng, ratLocations, maxNumRatsInMap)

numRats = len(ratLocations)
print "%d rats!" % numRats
if numRats > maxNumRatsInMap :
    print "The map only displays the first %d rats.\n" % maxNumRatsInMap

os.startfile(url)

sys.exit(0)



Note
If you get err = 400 Bad Request from getRatLocs(), check the columnID of the Location column.  The city redefined the view or something like that in the portal, the columnID changed on me, and this example started failing.  Run getcolumns.py on the view and compare the columnID it returns with the columnID in this code.  If they differ, update the code with the ID returned by getcolumns.py.  (You can also use your browser and look at the table in the portal and find the ID for the columns under Export tab -> API.)

No comments:

Post a Comment

Post a Comment