Your Turn!

The file here contains the SQL commands to generate and populate a simple table alice which hold the raw chapter contents of the book: Alice in Wonderland. You will need to set up your own tsvector column and index. With your neighbors, see if you can use the data to answer the following:

• In what chapters does the “Cheshire cat” appear?
• In what chapter does the term “mushroom” appear the most? How many times does it appear?

Positional Data

• Location information can be a fundamental aspect of stored data
• Currently, we could store such data, but Postgres has no intelligent methods of working with or analyzing that data
• We’ll focus the next few days on how we can utilize Postgres’s PostGIS extension to unlock the power of location based data
• Don’t overlook the official PostGIS documentation, which can be a helpful resource to accompany these slides and the text

PostGIS

• Unlike the tablefunc extension, PostGIS does not generally come with plain Postgres by default

  • On Windows and Mac however, if you installed Postgres as indicated at the start of the semester, you should already have it on your system
  • If you are missing it in Windows, you can launch the Stack Builder and should be able to add the extension from there

• The PostGIS extension will bring in a lot of extra functions and data types, so you might consider creating a new database to contain GIS type data (maybe analysis_gis)

• Adding the extension to the database is the same as with other extensions:

  CREATE EXTENSION postgis;

Geometries

Well-Known Text

• Most of the new geometries will be constructed by passing in a well-known text string (or WKT)

Coordinate Systems

• To be able to relate and compare locations to one another, a consistent coordinate system needs to be used
  • This covers everything from the mapping projection to whether or not you are working in curved space
• PostGIS (and most GIS applications) let you specify the coordinate system with a Spatial Reference System Identifier, or SRID
• Most commonly used for us will be the most recent World Geodetic System: WGS 84
  • This corresponds to an SRID of 4326
• If grabbing geospatial data from an online source, always check its coordinate system
• All coordinate system information stored in spatial_ref_sys table, so you can query to find necessary SRIDs

New Data Types

• While PostGIS introduces many new spatial geometries, only a few new data types are added to Postgres.
• geography
  • Based on spherical curvature, all calculations take place on a globe
  • More complicated math means fewer functions work, but distances are more precise, especially over large spans
  • Results expressed in meters
• geometry
  • Based on a flat map, where all calculations take place on a plane (the mathematical sort)
  • Math is simpler, but distances are less precise if across large spans
  • Results expressed in units dependent on chosen coordinate system
• Both types can hold all of the spatial geometries mentioned earlier

Creating Spatial Types

• Two main methods of creating geography or geometry types:

  • ST_GeomFromText(WKT, SRID) creates a geometry object to hold the spatial object given by the WKT with the optional given SRID
    • If no SRID is given, it is assumed to be 0 (no SRID at all)
  • ST_GeogFromText(WKT, SRID) creates a geography object to hold the spatial object given by the WKT with the optional given SRID
    • If no SRID is given, WGS 84 (SRID 4326) is the assumed default

• If you look at the output of one of these data types, it is not human-readable

  SELECT ST_GeogFromText('POINT(-75 42)');
  >> 0101000020E61000000000000000C052C00000000000004540
  SELECT ST_GeomFromText('POINT(-75 42)', 4326);

Making Spatial Objects

• Using a WKT to create spatial objects can be clunky if you already have latitude and longitude values in your table as numbers
• PostGIS offers a number of constructor functions for various objects that return geometry data types with no inherent SRID
  • ST_MakePoint(|||long|||, |||lat|||, [|||z|||,|||m|||]) will create a geometric point with optional 3rd or 4th dimensions as well
  • ST_MakeLine(|||point1|||, |||point2|||) will create a line from the first point to the second. There is an array option as well.
  • ST_MakePolygon(|||geometry_linestring|||, [|||cutout_linestring|||]) will create a geometric polygon using the provided linestring with optional cutouts
  • These can have an SRID attached to them and be cast to geography data types as desired
    • ST_SetSRID(|||object|||, |||SRID|||) will attach the given SRID metadata to the object

Adding an Index

• B-Trees are not well suited for indexing coordinate information
  • Which would be “bigger”: (2,0) or (0,2)?
• Instead, PostGIS recommends using the Generalized Search Tree (GiST) index type

CREATE INDEX |||index_name|||
ON |||table|||
USING GIST (|||column|||);

Actual Analysis!

• Now that we’ve gone to all this effort to get the spatial data into a format that Postgres can understand, we can actually do some analysis!
• Two of the most common functions deal with distances:
  • ST_DWithin(|||point1|||, |||point2|||, |||distance|||) returns a True or False depending on whether the two points are within the given distance from one another
    • Remember that geography distances are in meters, whereas geometry distance units depend on the SRID
  • ST_Distance(|||point1|||, |||point2|||) computes the distance between the two points
    • This will be along a curve in geography, or on the flat plane in geometry

Your Turn!

• In the following groups, import in the data here, which is a collection of the small liberal art NW colleges along with their latitude and longitudes. See if you can add a new column with the necessary data type, add an index, and then answer the following questions:
  • What other schools are within 100km of Willamette?
  • What two schools are the closest together?

Today’s Groups

• Group 1: Aurora, Jerrick, Jack
• Group 2: Harleen, AJ, Sergio
• Group 3: Mallory, Evan, Matthew
• Group 4: Marcus, Connor
• Group 5: Tippy, Nick, Jordan
• Group 6: Greg, Sam H, Grace
• Group 7: Dayton, Haley, Sam J
• Group 8: Michael, Hannah, Tiffany