Geolocation on Rails

identication of a real-world geographic
location

identication of a real-world geographic
location
=

context

Brief history: early days

many 1000s BC: smoke signals
as early as 3200 BC: celestial navigation
from 1000 BC: homing pigeons
between 1100-1200 AD: the magnetic compass

Brief history: modern era

early 1900s: radio triangulation
1960s: satellite GPS
1990s: automotive GPS navigation

Brief history: web era
late 1990s: IP lookup
mid 2000s: WiFi, GSM relevation
2004: A-GPS on smartphones

Location-aware apps

Later also Google, Facebook, and Twitter

Cool!
Let's join the party

introducing:
Stalk my Friends

1st step: display a map
We'll use Google Maps

# app/views/layouts/application.html.slim
= javascript_include_tag https://maps.googleapis.com/maps/api/js

// app/assets/javascripts/maps.js
$(document).ready(function() {
var mapOptions = {
zoom: 8,
center: new google.maps.LatLng(43.7710332, 11.2480006),
mapTypeId: google.maps.MapTypeId.ROADMAP
};
var map = new google.maps.Map($(’#map’)[0], mapOptions);
});

git: google maps

2nd step: geolocate
if (navigator.geolocation) {
navigator.geolocation.getCurrentPosition(
function(p) {
var latlng = new google.maps.LatLng(p.coords.latitude,
p.coords.longitude);
$(’span#position’).text(latlng);
if (myMarker) myMarker.setMap(null);
myMarker = new google.maps.Marker(
{position: latlng, map: map, title: This is me}
);
},
function(error) {
alert(error.message);
},
{maximumAge: 600000}
);
}
else { console.error(No HTML5? What are you using, IE6?); }
git: HTML5 geolocation

Geolocation API
supported by all current browers
location sources:
1. GPS
2. WiFi/Bluetooth MAC address
3. GSM/CDMA cell IDs
4. IP address
one-shot vs continuous

Geocode
New feature: set the position manually.

Geocode
We need to geocode the address.

Geocode
We need to geocode the address.
Several services:
Google: 2.500 requests/day [1]
Yahoo!: 50.000 requests/day [2]
Bing: 50.000 requests/day [3]
Nominatim: 1 request/second [4]
FreeGeoIP: 1000 requests/hour [5]
Geocoder.ca and Geocoder.us: ?
many others...

Gems for geocode

Geokit (deprecated, still lacks support for Rails 3) [7]
Graticule [6]
Geocoder [8]

Geocoder gem in action
# Gemfile
gem geocoder

# map_controller.rb
def geocode
position = Geocoder.coordinates(params[:query])
respond_to do |wants|
wants.json { render :json = position }
end
end

// applications.js
$(’[data-action=cheat]’).click(function() {
$.getJSON(’/geocode/’, {query: $(’#address’).val()}, map.cheat);
}); git: geocoding

Awesome! We have a user position.
Now to interact with other users we need:
store lat/long (and address, eventually) in a DB
query the DB for nearby points

1st approach: D.I.Y. I
# migration
class CreateUsers ActiveRecord::Migration
def change
create_table :users do |t|
t.string :name
t.float :latitude
t.float :longitude
t.string :address

t.timestamps
end
end
end

1st approach: D.I.Y. II

# app/controllers/users_controller.rb
class UsersController ApplicationController
respond_to :json

def index
@users = @user.nearbys(params[:radius])
respond_with @users
end
end

1st approach: D.I.Y. III

# app/models/user.rb
class User ActiveRecord::Base
attr_accessible :address, :latitude, :longitude, :name

def nearbys(radius)
# ... oh s@#!t
end
end

Let’s do some math

Earth is a sphere: coordinates are in degree, distance is in
meters.
Solve the second (inverse) geodetic problem
Given two points, determine the azimuth and length of the
line (straight line, arc or geodesic) that connects them. [9]

spherical model: Haversine formula (approximation)
oblate spheroid model: Vincenty's formulae

Haversine formula

d = 2r arcsin h(φ2 − φ1 ) + cos(φ1 ) cos(φ2 ) h(ψ2 − ψ1 ) (1)

=2 r arcsin sin2
φ2 − φ1
2
+ cos(φ1 ) cos(φ2 ) sin2
ψ2 − ψ1
2
(2)

where:
h() is the haversine function: h(θ) = sin(θ/2) 2
= 1−cos(θ)
2

d distance
r radius of the sphere (6371.0 km)
φ1 , φ2 latitude of point 1 and latitude of point 2
ψ1 , ψ2 longitude of point 1 and longitude of point 2

Geocoder to the rescue!
geocoded_by :address
reverse_geocoded_by :latitude, :longitude
after_validation :reverse_geocode

# app/controllers/users_controller.rb
def index
@users = @user.nearbys(params[:radius])
respond_with @users
end

Geocoder implements the Haversine formula
directly in the SQL query.
git: Geocoder backend

Geocoder goodies
# Nearest point
user = User.near(Firenze, 50, :order = distance).first

# distance from arbitrary point to user
user.distance_from([40.714, 11.234])

# Find the bearing (direction) between places
bearing = user.bearing_to(Paris, France)
Geocoder::Calculations.compass_point(bearing)
= NW

# find the geographic center (aka center of gravity)
polygon = [user1, user2, [40.22,-73.99], user4]
Geocoder::Calculations.geographic_center(polygon)
= [35.14968, -90.048929]

Geocoder considerations

Pros
simple and easy to use

Cons
only circle areas
SLOW (depends on the DBMS)

2nd approach: PostGIS

PostGIS [10] is an extention for PostgreSQL that adds
support for geographic objects.
Alternatives are:
MySQL: implements the datatype geometry
Sqlite: with SpatiaLite extension

PostGIS setup
# install on a ubuntu box
sudo apt-get install postgresql-9.1-postgis

# prepare the db
createdb template_postgis
createlang plpgsql template_postigs
psql -Upostgres -d template_postgis -f [...]/postgis.sql
psql -Upostgres -d template_postgis -f [...]/spatial_ref_sys.sql
psql -Upostgres -d template_postgis -f [...]/postgis_comments.sql
# inside postgres console
UPDATE pg_database SET datistemplate = TRUE
WHERE datname = ’template_postgis’;

PostGIS features

PostGIS follows the OpenGIS Simple Features
Specication for SQL [11]
data types for points, linestrings, polygons, multipoints,
multilinestrings, multipolygons and geometrycollections
functions for measurement like area, distance, length
and perimeter
spatial indexes for high speed spatial querying

PostGIS with Ruby

We'll use gem activerecord-postgis-adapter
to integrate postgis data types in ActiveRecord.

ActiveRecord PostGIS
# config/database.yml
development:
adapter: postgis
encoding: unicode
database: stalk_my_friends
pool: 5
username: nebirhos
template: template_postgis

# migration
add_column :users, :position, :point, :geographic = true
add_index :users, :position, :spatial = true

self.rgeo_factory_generator = RGeo::Geographic.simple_mercator_factory
# SRID 4326, used by Google Maps

PostGIS query

def nearbys(radius)
radius = radius.to_i*1000
User.where( ST_Distance(position, ?, false) = ? ,
position, radius ).where(id ?, id)
end

git: master

Thanks for watching!

Francesco Disperati | @nebirhos
http://nebirhos.com

Source code of the demo app available at
https://github.com/nebirhos/stalk-my-friends

Special thanks to Cantiere Creativo and Silvia Shell

Random stuff cited
Google Geocode API https://developers.google.com/maps/documentation/geocoding/

Yahoo! PlaceFinder http://developer.yahoo.com/geo/placefinder/guide/responses.html

Bing API http://msdn.microsoft.com/en-us/library/ff701715.aspx

Nominatim http://wiki.openstreetmap.org/wiki/Nominatim

FreeGeoIP http://github.com/fiorix/freegeoip/blob/master/README.rst

Graticule http://graticule.rubyforge.org/

Geokit http://geokit.rubyforge.org/

Geocoder http://www.rubygeocoder.com/

Geodesy on Wikipedia http://en.wikipedia.org/wiki/Geodesy

PostGIS http://postgis.refractions.net/

OpenGIS http://www.opengeospatial.org/standards/sfs

PostGIS ActiveRecord Adapter https://github.com/dazuma/activerecord-postgis-adapter

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