Georeferencing Where on earth are we? Critical for importing and combining layers for mapping
1. The Geoid Earth is not a perfect sphere, it is ellipsoidal.. earth is the 'Geoid'. The difference between the length of the two axes = the amount of 'polar flattening' is about 1/300 (0.3%)
Name Official Ellipsoids (from J. Snyder, Map Projections--A Working Manual) Date Equatorial Radius a (metres) Radius b (metres) Polar Flattening WGS 84 1984 6,378,137 6,356,752.3 1/298.257 GRS 80 1980 6,378,137 6,356,752.3 1/298.257 WGS 72 1972 6,378,135 6,356,750.5 1/298.26 International 1924 6,378,388 6,356,911.9 1/297 Clarke 1866 6,378,206.4 6,356,583.9 1/294.98 Everest 1830 6,377,276.3 6,356,075.4 1/300.8 Each ellipsoid has defined a 'Datum' = "a set of values that serve as a base for mapping" For Canadian and North American mapping, the two most important are: a. North American Datum, NAD27 (1927) based on Clarke 1866 b. North American Datum, NAD 83 (1983) based on GRS80 / WGS 1984 NAD27 was the datum for topographic mapping over most of the 20th century; NAD83 is the datum for contemporary digital mapping and GIS data collection. The two can differ by up to 70 metres (x) and 170 metres (y) Topo 101:map datum
2. The Earth's Graticule Latitude and Longitude The graticule is the imaginary grid of lines running east-west (lines of latitude = parallels) and north-south lines of longitude (meridians). The system was first devised by Hipparchus (190-120 BC)
Latitude Latitude = the angle formed between the equatorial plane and a line from the centre of earth to the location e.g. Prince George is at 54 N, Quesnel is at 53 N Latitude is 0 on the equator, 90 (N and S) for the poles. Latitude was measured thousands of years ago using the height of the sun at noon, or by the north star position. 1 degree = 60 (minutes) 1 = 60 (seconds)
Longitude Longitude = based on the arbitrary 'prime meridian' running through Greenwich, England = 0 longitude Longitude ranges from 0 to 180 W and 180 E (the same line). Prince George = 123W Lines of longitude converge at the poles World Time zones are based on longitude one hour zone per 15 degrees longitude (=24 zones/ hours per 360 longitude) Longitude : a book by Dava Sobel
Latitude and Longitude Length of One Degree of Longitude Length of a Degree of Latitude Latitude Kilometres Miles Latitude Kilometres Miles 0º 111.32 69.17 0º 110.57 68.71 10º 109.64 68.13 10º 110.61 68.73 20º 104.65 65.03 20º 110.70 68.79 30º 96.49 59.95 30º 110.85 68.88 40º 85.39 53.06 40º 111.04 68.99 50º 71.70 44.55 50º 111.23 69.12 60º 55.80 34.67 60º 111.41 69.23 70º 38.19 23.73 70º 111.56 69.32 80º 19.39 12.05 80º 111.66 69.38 90º 0.00 0.00 90º 111.69 69.40 degree graticule intersections are logged at confluence.org
The problem with geographic referencing The geographic graticule is used for data storage but is not ideal as a co-ordinate system for digital mapping A. It is not rectangular - 1 degree longitude varies
local example: pg-phonebook 2007 pg-phonebook 2008 Projected Geographic
B. The origin (0,0) creates negative coordinates to South and West C. It is not a decimal system (degrees/minutes/seconds) (degrees, minutes and seconds are sexagessimal ) degrees, minutes, seconds, can also be expressed in decimal degrees e.g. 53d30m36s = 53.51. see http://maps.google.com
3. Universal Transverse Mercator (UTM) System this bit is hard so pay attention The world is divided into 60 x 6 º longitude strips These are numbered 1-60 from 180 degrees West east to 180 degrees East.
Canada: UTM zones the width of each zone varies from 666 km (6 x 111km) at the equator to ~80 km at 80 N/S
UTM coordinates are given in metres Within each zone The Y coordinate Northings (N): from the Equator (0) increase to the n. pole (10,000,000)
The x coordinate - this is the hardest part Eastings (E) for each zone based on the zone Central Meridian at 500,000 the easting value increases to the east, but not above 1,000,000 ~ 300,000-700,000 in BC Coordinates repeat for each zone
Canadian NTS map 1:50,000 including graticule and UTM coordinates UTM zone Eastings are 6 digit, Northings are 7-digit (in Canada)
It may make more sense here : PGMAP: http://pgmap.princegeorge.ca/ (must use Internet Explorer) - Try this in next week s lab The UTM system works well for a local area coordinates in metres
BC: UTM zones How do we deal with multiple UTM zones: Eastings coordinates switch from ~700,000 at the east edge of one zone to ~300,000 at the west edge of the next?
BC Albers coordinate system BC uses UTM for local areas But Albers for the whole province
Summary: BC mapping coordinates Could be one of: 1. Geographic latitude / longitude for reference 2. UTM zones 7-11 - for local mapping 3. BC Albers - for provincial data View all 3 on (maybe!) : http://www.lrdw.ca (imap) Why is it important because we import data from different sources.. and they need to line up with each other.