PostGIS

1. Tudor Pavel
1 / 27

2. Requirement:
Suggest parking areas close to
user's location.
2 / 27

3. 3 / 27

4. 4 / 27

5. Calculating distances between
points ain't that easy.
5 / 27

6. Of course, for small cities...
we can ignore the fact the planet is spherical
and see it as flat.
6 / 27

7. But that's still a problem unless we
properly convert from degrees to
meters/kilometers.
7 / 27

8. So we used this table...
8 / 27

9. And this table...
9 / 27

10. To implement a distance converter module
moduleDistanceConverter
#...
#ConvertLatitudedistancefromKilometerstoDegrees
defto_deg_lat(km:,lat:45)
km/ONE_LAT_DEGREE[deg_approx_lat(lat)]
end
#ConvertLatitudedistancefromDegreestoKilometers
defto_km_lat(deg:,lat:45)
deg*ONE_LAT_DEGREE[deg_approx_lat(lat)]
end
#ConvertLongitudedistancefromKilometerstoDegrees
defto_deg_lng(km:,lat:45)
km/ONE_LNG_DEGREE[deg_approx_lat(lat)]
end
#ConvertLongitudedistancefromDegreestoKilometers
defto_km_lng(deg:,lat:45)
deg*ONE_LNG_DEGREE[deg_approx_lat(lat)]
end
private
#...
end
10 / 27

11. All is well, except...
This converter is Ruby. What if we need to
calculate distances in an SQL query?
11 / 27

12. A naive version
SELECT(sqrt((43.1234-latitude)^2+(23.1234-longitude)^2))ASdist
FROMareas;
That's just the linear distance between two points in a plane, so it's correct
with our assumtion when in a small city?
But the problem is it is using degrees which is a bigger problem the closer
we are to the poles.
One longitude degree will be worth less kilometers than a latitude degree.
So, what now?
12 / 27

13. "Here I come to save the day!"
PostGIS
13 / 27

14. 14 / 27

15. 15 / 27

16. This is a driving distance 5.7 km.
16 / 27

17. PostGIS version of the query
SELECTST_Distance_Sphere(
ST_MakePoint(43.1234,23.1234),
ST_MakePoint(latitude,longitude)
)ASdist
FROMareas;
For the example, the straight line distance with our Ruby converter: 4.5 km
PostGIS result: 6.5 km
Wat
17 / 27

18. Mister StackOverflow, please help
18 / 27

19. After some research involving...
19 / 27

20. 20 / 27

21. Being reminded about projection
distortion
https://gmaps-samples.googlecode.com/svn/trunk/poly/puzzledrag.html
21 / 27

22. Even starting to doubt the Earth is
actually a spheroid.
Why would they have more coordinate
systems??
22 / 27

23. After a while...
23 / 27

24. I looked closer at the docs
24 / 27

25. Did you miss it?
25 / 27

26. 26 / 27

27. Conclusions
Read the docs.
Think about the simplest possibilities first.
27 / 27