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Blake CrosbyOctober 25, 2011

Introduction to GIS

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What is GIS?

Geographic Information System is the merging

of cartography, statistical analysis,

and database technology.

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Earliest Use

• John Snow, MD

was the man.

• Discovered the

cause of a cholera

outbreak in the UK

in 1854.

• Revolutionized

epidemiology.

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Basic Principals

• Three key items:– Reliable data source (database)

– A way to query your data (statistical analysis)

– A way to display your result (cartography)

• We’ll Cover:– Spatial object types (of the vector kind)

– Geography 101 (the Earth is not round!)

– The power of a spatially aware database

– Displaying the end results to users.

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Basic Vector Spatial Object Types

• A Point– (-75.6692 45.3225)

• A Line String– (-75.99 36.89,-74.16 38.78,-73.36 39.97,-72.86 40.10)

• A Polygon– ((-75.99 36.89,-74.16 38.78,-73.36 39.97,-72.86 40.10,-75.99 36.89))

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Cartesian what?

• What is the shortest distance between two points?

A Straight Line

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Spherical Trig? Ugh!

• What is the shortest distance between two points on a

sphere?

A Geodesic Path

(aka: Great Circle)

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What do you mean it’s not round?

• The Earth:– Is not flat (thank you Pythagoras!)

– Is not round (eh?)

– Is actually an ellipsoid

SphereSpheroid

(Ellipsoid)

It is important to know this distinction!

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So who cares? Sphere Vs. Ellipsoid

• What’s the distance between Ottawa and Amsterdam?

• If the Earth was a perfect sphere:– 5,631.441 KM

• Using the WGS84 Geoid– 5,648.054 KM

• A difference of 17 KM!

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Geoids: Everything there is to know.

• Over 4000 “definitions” for the earth.

• Most common is WGS84 (Latitude / Longitude)

• Second most common is UTM (Meters)

• Some items to think about:– If your source data is in UTM, how do you plan on converting to WGS84?

– Can your libraries support calculating “real world distances” (meters/kilometers)

from hours, minutes, and seconds (latitude/longitude)?

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The $1,000,000 question

• How do you plan on answering these questions?– Give me all the news stories within 10km of my current location.

– Will the forest fire in this story burn down my house?

– I want to know all the news events that affects the “Oakwood” Neighbourhood

• Javascript?

• PHP? Java?

• Spatially aware DB + SQL?

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Javascript Stylez: News stories within 10 KMs?

• Doesn’t scale well.– Calculating distances for 10 points is fast, but for 100? 1000? 10000?

• Not particularly fast (especially on mobile devices)

• How do you handle other objects (like polygons?)

var R = 6371; // km

var dLat = (lat2-lat1).toRad();

var dLon = (lon2-lon1).toRad();

var lat1 = lat1.toRad();

var lat2 = lat2.toRad();

var a = Math.sin(dLat/2) * Math.sin(dLat/2) +

Math.sin(dLon/2) * Math.sin(dLon/2) * Math.cos(lat1) * Math.cos(lat2);

var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));

var d = R * c;

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SQL Stylez: News stories within 10 KMs?

• Scales well– Using bounding boxes and spatial indexes to eliminate points

• Fast!– Runs on the server side

• Need a spatially aware database backend (such as

PostGIS)

SELECT storyid FROM storytable WHERE

st_within(userlocation,st_buffer(location,10000));

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That’s great! But how do I show data to my users?

• Google Maps API– Most popular.

– Easy to use functions

– Moderately Customizable

• OpenLayers– Completely customizable

– No license restrictions

– 100% Pure JS library

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I don’t like my base map.

• Do you want your base map to look like this:

• No control over look & feel

• At the “mercy” of Google

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I don’t like my base map.

• Or like one of these:

• Complete control

• Data may be out of date

(crowd sourced)

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OL vs. GM

Feature Google Maps Open Layers

Customizable Base Map? 4 predefined ones

(Unlimited in V3 API)

Unlimited

Map Controls Fully Customizable Fully Customizable

Mobile Compatible? Yes No

Data Sources KML only KML, Mapserver, GML,

WKT.

Spatial Functions? Yes (a few) Yes (a lot)

Learning Curve Medium Steep

Reprojection? No (everything must

be in lat/long)

Yes

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“Ultimate” Solution/Technologies

• Custom Base Map– Data: Open Street Map

– Renderer: Mapnik+Tile Cache

• Datastore– PostgreSQL+PostGIS

• Google Map API– PHP backend serving it data via JSON

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“Probable” Solution/Technologies

• Datastore– PostgreSQL

– Lat/Lon stored as int in database

• Google Map API– PHP backend serving it data via JSON

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Questions We Need to Answer

• How are we going to geo-tagging news stories?

• How are we going to store UGC?

• What spatial questions is the application going to answer?

• Is the primary interface a map?

• Do we need to overlay only points? Or polygons as well?