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Georeferencing
How do we make sure all
our data layers line up ?
Georeferencing:
= linking a layer or
dataset with spatial
coordinates
Registration:
= lining up layers with
each other
Rectification:
=The process by which
the geometry of an image
is made planimetric
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)
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Geographic referencing
Latitude: Equator = 0 degrees; Poles are 90 North and South
Longitude: the Prime Meridian = 0, with 180 on the other side
Terrace: 54.5N, 128.6W
[54.5, -128.6]
PG: 54N, 123W
[54, -123]
1. Geographic referencing
is suitable for storing
global datasets, but
involves negative values
south and west of 0, 0
180 E/W
0,0
Brasilia: -16, - 48
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Geographic referencing
2. Geographic is not decimal, it is ‘sexagesimal’ (60s)
1 degree = 60 minutes
1 minute = 60 seconds
Decimal degrees: 58° 30’
= 58.5
30/60 = 0.5
Decimal degrees: 58° 36’
= 58.6
36/60 = 0.6
Decimal degrees: 58°36’36” = 58.61
36/(60*60) = 0.01
Lat/long on Google maps: https://maps.google.ca/
3. The geographic grid is not ‘rectangular’ – the meridians converge
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
1° Longitude is half the distance at 60N
1° Latitude = ~ 111km anywhere
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- 1 degree longitude varies from 0 - 111 km
(the system is not ‘rectangular’ )
…. East-west stretching away from the equator (as a degree is treated uniformly)
‘Geographic’ is good for global data storage
- But not for display and analysis
2007 (OK) –scale is consistent
2008: horizontal scale is almost double
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ELLIPSOIDS AND DATUMS:
the world is ellipsoidal -> ‘geoidal’
Flattening by 1/300
= 0.3%
99.7% sphere
(soccer ball)
0.3% ‘lemon’
(‘football’)
The difference in the major and minor axes has been estimated since ~1830
The latest should be the most accurate, using satellite technology.
Estimated
ellipsoids
Equatorial
Polar
Name
Date
Radius a (metres)
Radius b
(metres)
Flattening
WGS 84
1984
6,378,137
6,356,752
1/298
GRS 80
1980
6,378,137
6.356,752
1/298
International
1924
6,378,388
6,356,912
1/297
Clarke
1866
6,378,206
6,356,584
1/295
Everest
1830
6,377,276
6,356,075
1/301
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Each ellipsoid has a 'Datum'
= "a set of values that serve as a base for mapping“.
For Canada / North America, we have used:
a. North American Datum, NAD27 (1927)
…..
based on Clarke 1866
b. North American Datum, NAD 83 (1983)
…..
based on GRS 1980
NAD27 was the datum for mapping over most of the 20th century
NAD83 is the datum for modern GIS / mapping
The NAD ‘shift’
The differences between NAD27 and NAD83 can be:
Equatorial axis (X): 70 metres
Polar axis (Y):
170 metres
This can matter if you are working at large scales
If your data seem in error by ~ these amounts,
you can suspect it is due to the NAD shift
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The shift:
Greenwich prime meridian
Other datums: e.g. Europe, South America
http://pt.wikipedia.org/wiki/Datum#Caracteriza.C3.A7.C3.A3o_do_Sistema_Geod.C3.A9sico_Brasileiro_SAD69
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ELLIPSOIDS AND DATUMS: as the world is ellipsoidal
MAP PROJECTIONS: because the world is not flat
• Projection [map projections]
A method by which the curved surface of
the earth is portrayed on a flat surface.
Every map projection distorts distance,
area, shape, or direction.
Types of projections
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Geometric model used - Cylindrical
A cylinder is placed over a globe. The cylinder can touch the
globe along a line of latitude (normal case), a line of
longitude (transverse case), or another line (oblique case).
Cylindrical Projections
Mercator – projection kept
lines of compass bearings as
straight lines for navigation…
but distorted areas away from
the equator
Mercator
(16th century)
Transverse Mercator
http://www.progonos.com/furuti/MapProj/Normal/ProjCyl/ProjCEA/projCEA.html
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Universal Transverse Mercator (UTM) System
The UTM system consists of 60 TM projections, 6 º apart
The world is divided into these 6 º longitude zones (1-60)
Each strip (‘zone’) has a ‘Central Meridian’
180W Zone
1 starts at 180W and extends (east) to 174W (centred on 177W)180E
Canada: UTM zones
Each zone has a Central Meridian: e.g. 129W for zone 9, 123W for zone 10
This provides an alternative coordinate system to the graticule – see next lecture
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