Tag Archives: PostGIS

Geo-Rails part 9: The PostGIS spatial_ref_sys Table and You

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When you create a spatial database using PostGIS, you may notice that PostGIS automatically installs a table called “spatial_ref_sys”. This is a standard table for spatial databases, as required by the Open Geospatial Consortium’s specification. It defines which SRIDs are allowed in your geometries, and provides information about the corresponding coordinate systems.

In this article, we’ll take a brief look at the spatial_ref_sys table and how you can use it in your application. We’ll cover:

  • What’s useful about the spatial_ref_sys table
  • Where the spatial_ref_sys data comes from, and how you can populate your own custom data.
  • Accessing spatial_ref_sys data from Ruby using RGeo’s SRSDatabase

This is part 9 of my continuing series of articles on geospatial programming in Ruby and Rails. For a list of the other installments, please visit http://www.daniel-azuma.com/blog/archives/category/tech/georails.

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Geo-Rails part 8: ZCTA Lookup, A Worked Example

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This week we’ll put together what we’ve covered so far in this series by implementing a simple but usable service: looking up the Zip Code Tabulation Area (ZCTA) for a location. This is an actual task I had to do for my job at Pirq, and while I will pare it down for this article, we’ll go through some of the actual trade-offs and optimization decisions I made in our implementation.

In this article, we will cover:

  • The goals for the service, and what is a ZCTA anyway
  • Obtaining ZCTA data from the U.S. Census
  • Developing our own ZCTA database
  • Querying the database
  • Improving performance using polygon segmentation

This is part 8 of my continuing series of articles on geospatial programming in Ruby and Rails. For a list of the other installments, please visit http://www.daniel-azuma.com/blog/archives/category/tech/georails.

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Geo-Rails part 7: Geometry vs. Geography, or, How I Learned To Stop Worrying And Love Projections

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This week we’re going to look at how to choose a coordinate system for your database. In PostGIS, this includes the choice of geometry vs geography columns, as well as which projection (if any) to use, and how to interact with it from Rails.

In this article, we’ll:

  • Review geographic and projected coordinate systems
  • Discuss the pros and cons of using the PostGIS geographic type
  • See why I typically store data in a projection
  • Look at some specific projections I recommend using (or avoiding)
  • Learn how to handle projected data in Rails

My original series plan for this week called for a worked example of a location-based web service, bringing together much of the material that we’ve covered so far. But as I was writing it, I realized there was one more topic we probably ought to cover first. So I’ll publish the example next week.

This is part 7 of my continuing series of articles on geospatial programming in Ruby and Rails. For a list of the other installments, please visit http://www.daniel-azuma.com/blog/archives/category/tech/georails.

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Geo-Rails part 6: Scaling Spatial Applications

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Scaling, scaling, scaling. Can Rails really scale? It’s been a source of FUD and the butt of running jokes. But scaling is a serious matter when it comes to large data sets, and it’s something we need to pay attention to in the geospatial realm where big data is commonplace.

In this week’s article, I’ll go over the basic issues every geospatial programmer should know about scaling, and provide tips for writing your geospatial Rails application so it doesn’t fall over when you go national. We will cover:

  • The bottom line regarding scaling
  • Building spatial indexes for your database
  • Writing queries to take advantage of indexes
  • Simplification and segmentation of large objects

This is part 6 of my continuing series of articles on geospatial programming in Ruby and Rails. For a list of the other installments, please visit http://www.daniel-azuma.com/blog/archives/category/tech/georails.

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Geo-Rails part 5: Spatial Data Formats

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The location revolution is a revolution of data. Ubiquitous data, from mobile GPS and user input as well as from census and other datasets, is what makes location-aware applications possible. And so the first task of many geospatial projects is to determine how to find and utilize (and, in some cases, produce) external data.

In this article, we will survey some of the important spatial data formats, including serialization, file formats, and api-oriented formats. Specifically, we will look at:

  • Basic serialization using WKT and WKB
  • Variants on WKT and WKB
  • Reading public datasets from shapefiles
  • Web service oriented formats such as GeoJSON
  • XML-based formats commonly used in web services

We will also go over a few quick examples using Ruby and RGeo. This will be a fairly high-level overview and we won’t go into a lot of detail. We’ll take deeper looks at some of these formats in future articles.

This is part 5 of my continuing series of articles on geospatial programming in Ruby and Rails. For a list of the other installments, please visit http://www.daniel-azuma.com/blog/archives/category/tech/georails.

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Geo-Rails part 2: Setting Up a Geospatial Rails App

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Before going in depth into any particular topic, I thought it would be useful to write a getting-started tutorial, walking through setting up and working with a simple example Rails app using RGeo. In this tutorial, we will:

  • Install the main software components we need for a geospatial application, including a spatial database and the needed Ruby libraries.
  • Set up a new Rails application configured to use the spatial database.
  • Create an ActiveRecord model with a spatial attribute.
  • Experiment with location data in the model.
  • Perform simple spatial queries.

This should help you get started writing basic location features in Rails, giving you a feel for what the tools are and how they fit together.

This is part 2 of my series of articles on geospatial programming in Ruby and Rails. For a list of the other installments, please visit http://www.daniel-azuma.com/blog/archives/category/tech/georails.

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