Geographical perl modules

1. Geographical perl modules

2. Some etymology

3. Some etymology • geo'graphy • Drawing the Earth

4. Some etymology • geo'graphy • Drawing the Earth • geo'metry • Measuring the Earth

5. The interest in geography

6. The interest in geography • The Open Guide to London • Geocache, MUD-London and other web based mapping ideas

7. The interest in geography • 'Grubstreet' had map links with OS grid coordinates (www.streetmap.co.uk) • We can use the X and Y to plot a map.

8. The interest in geography Y X

9. Find by distance • We know the location of A (X1, Y1) • We know the location of B (X2, Y2) • The distance between them is: Sqrt ( (X2 - X1)^2 + (Y2 - Y1)^2) • And: OS eastings and northings work in Metres

10. BUT: everyone else uses Latitude and Longitude • The standard for GPS • Works worldwide

11. The problem • The world is flat

12. The problem • The world is flat round

13. Latitude and Longitude are angles

14. Mercator's Projection

15. Mercator's Projection • Was designed for nautical use • Preserves angles (azimuth, heading) • Distorts large distances • Works well over short range distances • The Mercator projection is geared to temperate latitudes (e.g. Europe)

16. Transverse Mercator

17. Ordnance Survey Grid • Is a transverse Mercator, with false (offset) Easting and Northing. • A perl module exists to convert between OS Grid and Lat/Long • Geography::NationalGrid

18. Geography::NationalGrid • Object Orientated interface • Each object is a location • As parameters to new specify one of the following: • Lat/Long • OS Grid reference e.g. TQ 123456 • 6 digit Easting and Northing (i.e. X and Y)

19. Geography::NationalGrid • Method calls include: • latitude, longitude • gridReference • easting, northing • deg2string( $degrees ) • Converts an angle to degrees, minutes and seconds

20. Geography::NationalGrid • Subclassable • Subclasses are used to implement grids for individual countries. • The module comes with: • Geography::NationalGrid::GB • Geography::NationalGrid::IE

21. Back to OpenGuides • Location and find_by_distance are based on the Ordnance Survey grid

22. Back to OpenGuides • Location and find_by_distance are based on the Ordnance Survey grid • The OS charge £££ licence fees to use their data and maps

23. Back to OpenGuides • Location and find_by_distance are based on the Ordnance Survey grid • The OS charge £££ licence fees to use their data and maps • We want a system that will work outside the UK and Ireland

24. Why don't we do it ourselves? Radius of a circle of parallel R = E cos A where E = radius of Earth A = latitude

25. The radian approximation • For small θ sin θ < θ < tan θ θ must be in RADIANS

26. The radian approximation • For small θ sin θ < θ < tan θ θ must be in RADIANS • For a small distance on the ground • The conversion between lat/long and X/Y is linear

27. Transverse Mercator revisited • There is an emerging standard, UTM • Universal Transverse Mercator • It is not UK-centric

28. Problem #2 • The world is flat round squashed

29. The Earth is an oblate spheroid • More like the shape of an apple than a ball • Instead of projecting onto a cylinder, we project on to an ellipsoid

30. To use UTM • You need to specify a datum • This includes an ellipsoid and offsets (false easting and false northing)

31. Geo::Coordinates::UTM • Takes an ellipsoid, not a datum • Hence no internal facility for false easting or false northing. • Non OO interface.

32. latlon_to_utm my ($zone,$east,$north) = latlon_to_utm ( 'clarke 1866', 98.251, 2.562);

33. utm_to_latlon my ($lat,$long) = utm_to_latlon ('clarke 1866', '30V', 12554, 41562);

34. Plug-ins for CGI::Wiki and OpenGuides • CGI::Wiki::Plugin::Locator::UK • CGI::Wiki::Plugin::Locator::UTM