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%)
Official Ellipsoids
(from J. Snyder, Map Projections--A Working Manual)
Equatorial
Name
Date
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.