Day 6 - PostGIS
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This document introduces PostGIS, an extension to PostgreSQL that adds support for geographic objects allowing location queries to be run in SQL. It discusses geospatial data types and functions in PostGIS for working with spatial features like points, lines, polygons, and rasters. PostGIS allows importing and exporting geospatial data, integration with GIS software, and access to open mapping data sources. It also covers spatial queries and analysis in PostGIS using functions for distance, containment, intersections and more. Additional topics mentioned include pgRouting for routing/navigation, generating maps/images from PostGIS data, and real-world use cases.
Day 6 - PostGIS
- 2. What is Geospatial • Most common use – Narrow a search within a distance from a point of origin – Sort by that distance
- 3. Distance Formulas • Flat-surface formulae – Spherical Earth projected to a plane – Ellipsoidal Earth projected to a plane – Polar coordinate flat-Earth formula • Spherical-surface formulae – Haversine – Tunnel distance • Ellipsoidal-surface formulae – Lambert’s formula for long lines – Bowring’s method for short lines
- 5. What you usually care about • Given 2 latitude and longitude points – Calculate the distance so we can find what’s close • This is the common capability of databases with “geospatial” support
- 6. A few options… • Do the math in a query • Here’s what it looks like in SQL – (ACOS(least(1,COS(0.607198022186895)*COS(- 1.4410239410591847)*COS(RADIANS(latitude))*COS(RAD IANS(longitude)) + COS(0.607198022186895)*SIN(- 1.4410239410591847)*COS(RADIANS(latitude))*SIN(RAD IANS(longitude)) + SIN(0.607198022186895)*SIN(RADIANS(latitude))))*39 63.19)) <= 25) – Calculating within 25 miles of a point of origin based on an origin point – You can do that in any database (it’s just math) – Very intense query that slows as your dataset grows – This…is not using an index
- 7. Speed that up • Distance query a subset instead • Use the lat/lng to create query a box around the boundary • Queried with a numerical index • Then check the distance from center for the subset • NOTE: Doing this as part of another query is a lot easier with multi-index queries
- 8. And here’s what THAT looks like WHERE ( latitude > 34.42845936786603 AND latitude < 35.15130863213399 AND longitude > -83.00467808012291 AND longitude < -82.12450191987708 ) AND ( (ACOS(least(1,COS(0.607198022186895)*COS(- 1.4410239410591847)*COS(RADIANS(latitude))*COS(RADIANS(longitude))+ COS(0.607198022186895)*SIN(- 1.4410239410591847)*COS(RADIANS(latitude))*SIN(RADIANS(longitude))+ SIN(0.607198022186895)*SIN(RADIANS(latitude))))*3963.19) ) <= 25) Simple, am I right? Also, you need to drop that distance formula into the ORDER BY clause too.
- 9. There’s a Gem for that! Geokit https://github.com/geokit/geokit • Distance calculations between 2 points – Multiple formulas and units of measure • Multiple providers for Geocoding different data – Addresses • Yahoo • Geocoder.us/.ca • Geonames • Bing • Yandex • MapQuest • Geocode.io • Mapbox • Google • FCC • Open Street Map – IP Address • hostip.info • Geoplugin.net • RIPE • MaxMind (HIGHLY RECOMMEND) • freegeoip.net Geokit Rails (any database) https://github.com/geokit/geokit-rails • Premium Rails Integration • ActiveRecord distance finders • IP based location lookup • Cookie based user location tracking • Scopes: within, beyond, in_range, in_bounds, closest, farthest, by_distance • Generate the SQL • Auto-Geocoding • Mixin class Location < ActiveRecord::Base acts_as_mappable :default_units => :miles, :default_formula => :sphere, :distance_field_name => :distance, :lat_column_name => :lat, :lng_column_name => :lng end
- 10. Actually, there’s several… RGeo • Adapters for – MySQL – SQLite – PostGIS • Process GeoJSON • Read shapefiles • Uses C++ extensions for processing Geocoder • Object Geocoding – IP – Address • Reverse Geocoding – Address from lat/lng or IP • Center of Multiple Locations • Geographic Queries – Near / nearby – Distance – Direction • Easier to use • Seems to be some dispute about accuracy of data
- 11. What about PostgreSQL? • Provide database functions to handle this calculations – Distance in straight line or great circle – Convert lat/lng to point – Get lat/lng from a point – Calculate containment within a cube • Point datatype – Operator to get distance between Earth Distance extension http://www.postgresql.org/docs/9.2/s tatic/earthdistance.html Cube based Point based
- 12. SO WHAT DOES POSTGIS DO THEN? Funny you should ask…
- 13. EVERYTHING Seriously…it does pretty much everything
- 14. Import and Export Data • Import CSV • Import/export with GDAL (Geospatial Abstraction Library) – Shapefiles – OGR – Geospatial vector data files – OpenStreetMap data (openstreetmap.org) – Raster datasets • Import OGR files • ETL migration tools – GeoKettle
- 15. GIS Desktop Software Open Source • OpenJump • QuantumGIS • uDig • GvSig • OrbisGIS • PostGIS Viewer • pgAdmin plugin Commercial • CadCorp SIS • Manifold.net • MapInfo Professional • AutoCAD • ESRI ArcGIS
- 16. Mapping Servers Integration • Mapserver • GeoServer • Deegree • QGIS Server • MapGuide
- 18. Data: US Census
- 19. Data: WorldClim
- 21. Natural Earth
- 22. MaxMind GeoIP Web Services / API Constantly Updated Databases Free and Paid Versions Check your IP to try it out https://www.maxmind.com/en/locate _my_ip
- 23. Working with Data • Datatypes – Geography • Ellipsoidal spatial data – Geometry • Planar spatial data • Indexes • Topology • Spatial Joins • 3D – Mapping (building, etc) – Image generation • pgRoute – Generate driving route • Generate raster images from SQL queries • Spatial Relationship Functions – ST_Contains – ST_Covers – ST_Crosses – ST_DWithin – ST_Intersects – ST_Distance • Clustered Queries for Huge Datasets
- 24. Fun data tricks • Use a TRIGGER to populate a geographic data column for transparent indexing • Create a Geospatial View • Use table inheritance to centralize commonality among different data types – Geographic Data – Search Data
- 25. AR PostGIS Adapter Migrations create_table :locations do |t| t.column :shape1, :geometry t.geometry :shape2 t.line_string :path, :srid => 3785 t.point :lonlat, :geographic => true t.point :lonlatheight, :geographic => true, :has_z => true t.index :lonlat, :spatial => true end Datatypes :geometry -- Any geometric type :point -- Point data :line_string -- LineString data :polygon -- Polygon data :geometry_collection -- Any collection type :multi_point -- A collection of Points :multi_line_string -- A collection of LineStrings :multi_polygon -- A collection of Polygons ActiveRecord Location.where(:lonlat => 'POINT(-122 47)').first Location.where("ST_Distance(latlon, 'POINT(-122.330779 47.604828)') < 10000") scope :distance_from, ->(lat, lon, dist) do where(“ST_Distance(latlon, ‘POINT(? ?)’) < ?, lat, lon, dist) end Location.where("ST_Intersects(latlon, 'POLYGON(( -122.19 47.68, -122.2 47.675, -122.19 47.67, -122.19 47.68))')") https://github.com/rgeo/active record-postgis-adapter Features • Uses Rgeo gem • Spatial Migrations • Spatial Datatype • Spatial Queries • Create / Update PostGIS DB • Support central schema
- 26. Assignment 2 • Refine and improve your application • Add authentication • Add authors (or some type of user) to created data • Display author info on your data • Implement simple_form somewhere • Add a new validation rule to a model • Add a page to show related data for an author • Use slim (or haml) to create one of your views • Add a BASIC geographic capability (optional) – Use your own discretion for how deep you go – Suggestions / Ideas • Geocode user IP addresses to get city/state/zip info • Add address fields to your data type to make it geographically relevant • Use a distance filter in search