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PostGIS

Introduction
Points
Linestrings
Polygons
Summary

Instruction

Excellent!

You should also know that each geometry has a default SRID stored inside the same geometry column. In order to find out what that default SRID is, you can use the function ST_SRID(geometry). Take a look at how that's used:

SELECT
  ST_SRID(course) 
FROM sf_selected_railways 
WHERE id = 1;

Exercise

Run the template query. You will find out what the default SRID is for our data, and how it differs from the SRID we used to retrive the distance in meters.

As you can see, the default SRID is 4326, also known as WGS84. The main difference from the system with SRID 26910 is that WGS84 uses degrees as the length unit. That's why the resulting length of 0.002 was so different from the roughly 217 meters we got - it is a completely different measure!