19. Geography Exercises

Here’s a reminder of all the functions we have seen so far. They should be useful for the exercises!

  • Sum(number) adds up all the numbers in the result set

  • ST_GeogFromText(text) returns a geography

  • ST_Distance(geography, geography) returns the distance between geographies

  • ST_Transform(geometry, srid) returns geometry, in the new projection

  • ST_Length(geography) returns the length of the line

  • ST_Intersects(geometry, geometry) returns true if the objects are not disjoint in planar space

  • ST_Intersects(geography, geography) returns true if the objects are not disjoint in spheroidal space

Also remember the tables we have available:

  • nyc_streets

    • name, type, geom

  • nyc_neighborhoods

    • name, boroname, geom

19.1. Exercises

  • How far is New York from Seattle? What are the units of the answer?


    New York = POINT(-74.0064 40.7142) and Seattle = POINT(-122.3331 47.6097)

    SELECT ST_Distance(
      'POINT(-74.0064 40.7142)'::geography,
      'POINT(-122.3331 47.6097)'::geography
  • What is the total length of all streets in New York, calculated on the spheroid?

    SELECT Sum(
    FROM nyc_streets;


    The length calculated in the planar “UTM Zone 18” projection is 10418904.717, 0.02% different. UTM is good at preserving area and distance, within the zone boundaries.

  • Does ‘POINT(1 2.0001)’ intersect with ‘POLYGON((0 0, 0 2, 2 2, 2 0, 0 0))’ in geography? In geometry? Why the difference?

    SELECT ST_Intersects(
      'POINT(1 2.0001)'::geography,
      'POLYGON((0 0,0 2,2 2,2 0,0 0))'::geography
    SELECT ST_Intersects(
      'POINT(1 2.0001)'::geometry,
      'POLYGON((0 0,0 2,2 2,2 0,0 0))'::geometry
    true and false


    The upper edge of the square is a straight line in geometry, and passes below the point, so the square does not contain the point. The upper edge of the square is a great circle in geography, and passes above the point, so the square does contain the point.