Intro to PostGIS in Ruby

What is PostGIS

A set of geo-spatial extensions for the PostgreSQL database

And for those who don’t know, PostgreSQL is an SQL database similar to MySQL & MS SQL Server

What do I need to know about geo-spatial ‘stuff’ to get going?

It depends entirely on what you’re doing :)

The Building Blocks:

Point

LineString

Polygon

With these Blocks, we can ask:

Does this LineString intersect any Polygons?

Which Polygons does a Point exist within?

How much do these Polygons overlap?

What are the closest alternative Points to this Point?

much, much more

SRID?

Spatial Reference ID

Need to understand that when displaying a 3D world on a 2D screen, decisions on how to need to be made

SRID?

Much like a graph needs labeled axis to mean anything, SRIDs provide context to correctly interpret spatial coordinates

There’s a collation of industry standard SRID’s that get setup on your db - look at your shiny new spatial_ref_sys table

Rule of thumb - unless you know better, just use 4326 :)

Spatial Indexes

Greatly improves the speed of querying complex spatial datasets

Pulling this all into Ruby Land!

RGeo provides the spatial ‘building blocks’ for Ruby

ActiveRecord PostGIS Adapter provides the migration support & RGeo type casting layer to ActiveRecord/Rails

RGeo

https://github.com/rgeo/rgeo

‘An implementation of the industry standard OGC Simple Features Specification, which provides data representations of geometric objects such as points, lines and polygons, along with a set of geometric analysis operations.’

What’s missing?

A nice abstraction layer allowing scoped spatial queriesi.e.Hotel.near(location).with_wifiRegion.within(country) country.outside?(region)

Common Spatial Commands

ST_IntersectsDoes a line or polygon intersect with another line or polygon? Useful for ‘areas travelled through’ or ‘area union’ kind of questions

ST_DWithinLets us ask whether a geographic object is within a certain distance of another geographic object

ST_ContainsDo all given points lie within a given polygon?

ST_OverlapsDo 2 different geometries share space?

Common Spatial Commands

Plenty more, check out the API!

So, what is this good for?

Criminally underused in the web dev world

Incredibly powerful for providing context to applications that utilise location related data(in the Rails world, think mobile APIs…)

Great for data analysis & strategically targeting markets - think growth hacking!

So, what is this good for?

Working with existing spatial datasets

ESRI Shapefile

A popular format for storing geometric location information & associated metadata

Used heavily by the Australian Government- e.g. municipal boundaries, ABS data & census data

Most of our friendly geo-spatial software will happily find a way to work with this data format

Some Gotchas

Make sure your schema includes the postgis extension

Don’t delete the spatial_ref_sys table!

Pay close attention to your SRIDs, and unless you’re sure, stick to using 4326 on your db

When importing data, convert it to 4326 if a data source is in a different SRID

Some Gotchas

Rails likes to use SELECT table.* - beware that this can be painful to performance if your table contains lots of intricate spatial data. Consider using a join to store complex data in a separate table & only call when necessary.

Even with spatial indexing, large datasets can become a burden on the database. Consider using traditional id based relationships as a cache for complex geospatial result sets.

Alternatives

MySQL Spatial Extensions

Not as mature as PostGIS

Only MyISAM supports spatial indexes - ok if you’re not into data retention :)

SQL Server

More mature than MySQL & not a bad choice if you’re a MS Shop

Alternatives

Oracle Spatial

I’ve heard it’s pretty good if you have the wallet for it!

I’ve also been informed that there’s a free version that may cut it for many people :)

MongoDB

Great for basic geospatial functionality

Not as fully featured as PostGIS, may leave you wanting in certain areas

Getting Started in Rails

First step: Get a copy of PostgreSQL & PostGIS

Easiest way on OS/X is to grab Postgres.app from http://postgresapp.com/

Grab a copy of QGIS while you’re at it - http://www.kyngchaos.com/software/qgis for Mac users

Getting Started in Rails

rails new spatial_project -d postgresql!

gem ‘activerecord-postgis-adapter’ Gemfile!

adapter: postgis config/database.yml!

rake db:create!

rake db:gis:setup

Getting Started in Rails

You can now run migrations like these:create_table :spatials, do |t| t.string :name, t.multi_polygon, :area, srid: 4326, t.point, :position, srid: 4326, t.line_string, :path, srid: 4326 end

Getting Started in Rails

And create spatial indexes like this:add_index :spatials, :area, spatial: true

Resources

http://gis.stackexchange.com/

http://postgis.net/documentation/

http://daniel-azuma.com/articles/georails

https://www.qgis.org/