LocationLocationLocation

Gavin Heavyside - ACCU 2013 - 12 April 2012gavin@heavyside.co.uk @gavinheavyside

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Friday, 12 April 13

GPS, Location, GIS, and Spatial Relationships

• GPS Technology

• GIS and Mapping Concepts

• Spatial DBs and SQL

• Practical Example

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• www.mydrivesolutions.com

• Insurance telematics

• Linux,Ruby, Rails, Postgres, PostGIS, NoSQL, Chef, AWS, C++

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• I have no idea what I’m doing

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GPS

• Currently 32 satellites

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GPS

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Basic GPS Concepts

• 24+ Satellites orbit the earth

• Atomic clocks

• Each constantly transmits its time and location

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Basic GPS Concepts

• Normally 4 or more sats used by receiver

• Distance to each satellite calculated using speed of light

• Triangulate position

• Derived attributes - speed, bearing

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http://www.ausairpower.net/TE-GPS-Guided-Weps.html

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Almanac and Ephemeris

• Almanac: GPS constellation info, orbits

• Ephemeris: orbital and clock correction

• With both in memory, lock is faster

• A-GPS gets ephemeris from internet

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NMEA 0183

• Serial ASCII data format

• CSV ‘sentences’ for different types of data

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NMEA Sentences

• GGA - Fix information

• GSA - DOP and active satellites

• GSV - Satellites in view

• RMC - Time, date and position

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Problems with GPS

• Noise

• Urban Canyons

• Multipath signals

• Solar flares and atmospherics

• Mounting of receiver

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Others

• GLONASS

• BEIDOU/COMPASS

• GALILEO

• IRNSS

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Smartphone Support

• All major smartphones have GPS sensors

• Balance accuracy, frequency, battery life

• Usually subscribe to service and get called back with location updates

• Alerts: Geofencing, Significant Change

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HTML5 GeoLocation

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Digital Maps

• Commercial

• NAVTEQ, Tele Atlas

• Free

• OSM

• Online

• Google, Bing, etc

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Display

• Usually map tiles at different zoom levels

• Server will return tiled images for a bounding box

• Plenty of libraries commercial or free to embed in apps or web pages

• Add your own overlay markers and layers

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Leaflet

• JS plugin

• Uses OSM or other tile servers

• HW accelerated, touch-compatible

• Markers, Polygons, Layers, GeoJSON

• http://leafletjs.com

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Digital maps for GIS

• Vector representation of your map

• Commercial or OSM

• Use the road or feature geometry, in conjunction with attributes of the features

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OpenStreetMap

• http://www.openstreetmap.org/

• Community generated and curated

• XML geometry and metadata composed of nodes, ways, and relations

• Can be downloaded in regions, updated incrementally

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• Some people when faced with a problem think “I know, I’ll use XML”. Now they have

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Data Formats

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Geometry Types

• Points

• Linestrings

• Polygons

• Compound: MultiPoint etc

• 3D: all above with elevation, Polyhedrals

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Well-Known Text (WKT)

• markup language for vector geometry

• ISO standard, originally by OGC

• Always uses X Y, so watch out when using lat and lon: needs to be (lon lat)

• Also an equivalent Well Known Binary used by databases to store spatial data

• Can embed SRID (see later)

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WKT Examples

• Point(30 10)

• LINESTRING (30 10, 10 30, 40 40)

• POLYGON ((30 10, 10 20, 20 40, 40 40, 30 10))

• POLYGON ((35 10, 10 20, 15 40,45 45, 35 10),(20 30, 35 35,30 20, 20 30))

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KML

• XML data format used by Google Earth

• Supports points, lines, markers, polygons, overlays, and much much more

• Zip up with assets (e.g. Collada 3D models) into a .kmz file

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KML

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GeoJSON

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ShapeFiles

• Created by ESRI

• Widely used and understood

• Points, Lines, Polygons

• Collection of files: .shp, .shx, .dbf, others

• Can be imported straight into PostGIS with shp2pgsql

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Spatial Reference Systems

• The Earth is not flat

• Neither is it a spheroid

• nor even an ellipsoid

• Spatial Reference Systems approximate the geoid of the earth to an ellipsoid

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Geoid

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WGS84

• The most common (and most useful) spatial reference system

• best fitting ellipsoid model to the reference points on the 1996 Earth Gravitational Model geoid

• Used by GPS

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Projections

• Projections

• Cylindrical

• Conic

• Azimuthal

• Orientation

• Oblique

• Equatorial

• Transverse

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Transverse Mercator

• Some projections look good over most of the globe

• Some are only useful locally (e.g. UTM)

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Geospatial Databases

• Ordinary RDBs with support for spatial data types

• NoSQL DBs

• MongoDB

• https://github.com/neo4j/spatial

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PostGIS

• http://postgis.org/

• Geospatial Extensions for PostgreSQL

• Excellent support for spatial data, operations, indexes

• Actively developed

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Creating a PostGIS DB

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Spatial Relationships

• Many ways to compare geometries

• Some can compare different types

• Some only valid for same type

• CONTAINS, CROSSES, DISJOINT, DISTANCE, DWITHIN, EQUALS, INTERSECTS, OVERLAPS, TOUCHES, WITHIN

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ST_Equals

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ST_Intersects

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ST_DWithin

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ST_Transform

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Geography Data Type

• Usually measurements are in the units of the spatial reference system

• Radians for SRID 4326 (WGS84)

• The geography data type can help

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Geospatial Indexes

• Indexes on geo columns are necessary

• Covers bounding boxes

• Reduces search space

• Geos used for calculations

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EXPLAIN

• Use EXPLAIN to check the planner is using spatial indexes, just like other SQL queries

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Ordnance Survey OpenData

• http://www.ordnancesurvey.co.uk/oswebsite/products/os-opendata.html

• Vector & Raster data

• Maps, Roads, Boundaries, Postcodes

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Code-Point Open

• CSV "Database" of UK postcode

• (Not Northern Ireland)

• 1.6m+ postcodes

• Eastings and Northings for each entry

• Also county/district & NHS info

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Data Format

• Postcode in field 1

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Data Format

• Eastings, Northings in fields 3,4

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OS National Grid

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Geocoding

• Find the geographic location of a feature, e.g. address, business, POI

• Reverse: Find a feature from a geographic location (e.g. lat/lng)

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Example App

• Put it all together:

• Open Data

• PostGIS

• CRS transformations

• Geospatial DB queries

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GeoPostCoder

• https://github.com/gavinheavyside/geopostcoder

• Bash and SQL to load postcode geo data

• Simple Ruby web app to query DB

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Loading Data

• Filter Code-Point Open just to the columns we want

• Load into a temporary table

• Transform National Grid coordinates to WGS Geography data type

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Recap

• Lots of sources of location data

• You probably have a GPS receiver with you

• Lots of data available for free

• Plenty of free and open-source tools querying and manipulating spatial data

• Go do something interesting with it :)

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@gavinheavysidegavin@heavyside.co.uk

Friday, 12 April 13