Mapping coordinate systems

Transcription

1 Mapping coordinate systems 1. 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 ( BC) 1

2 Latitude Latitude = the vertical angle from the centre of earth to the location e.g. Prince George is at 54 N Quesnel is at 53 N 90N Latitude is 0 on the equator, 90 (N and S) for the poles. 1 degree = 60 (minutes) 1 = 60 (seconds) Longitude Longitude = the angle formed between line from centre of earth to the (arbitrary) 'prime meridian' running through Greenwich, England and the local meridian. The 0 location is arbitrary. Longitude ranges from W / 180 E (the same line) Prince George = 123 W 2

3 We identify locations by latitude, longitude e.g. UNBC campus agora See: In decimal degrees: , (N, W) Note that longitude is negative In degrees, minutes, seconds: (N) (W) In degrees and decimal minutes (e.g. GPS) Longitude The prime meridian location is arbitrary it could have been through Paris. The Paris meridian has a longer history now at longitude east 3

4 Putting the Prime Meridian in England is just one example of: World Time zones one hour zone per 15 longitude (=24 zones/ hours per 360 longitude) Polar areas use GMT zulu time 4

5 1 degree longitude varies from ~111 km at equator to 0 km at poles Longitude (~ half the distance at 60 degrees latitude) It is not rectangular - 1 degree longitude varies This means the geographic graticule is used for data storage but is not ideal as a co-ordinate system for digital mapping Latitude and Longitude Length of One Degree of Longitude Length of a Degree of Latitude Latitude Kilometres Miles Latitude Kilometres Miles 0º º º º º º º º º º º º º º º º º º º º

6 2. The Geoid Earth is not a perfect sphere, it is ellipsoidal.. 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) Polar Radius b (metres) WGS ,378,137 6,356,752 1/298 GRS ,378,137 6,356,752 1/298 WGS ,378,135 6,356,750 1/298 International ,378,388 6,356,912 1/297 Clarke ,378,206 6,356,584 1/295 Everest ,377,276 6,356,075 1/301 Polar Flattening Each ellipsoid includes 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, NAD83 (1983) based on GRS80 / WGS

7 Datums a. North American Datum, NAD27 (1927) based on Clarke 1866 b. North American Datum, NAD83 based on GRS80/WGS > NAD27 was the datum for topographic mapping over most of the 20th century -> NAD83 is the datum for digital mapping / GIS data -> The two can differ by up to 100 metres (x) and 200 metres (y) Topo 101:map datum 7

8 3. UTM map coordinates Critical for importing and combining layers for mapping Universal Transverse Mercator (UTM) System this bit is hard so pay attention The world is divided into 60 x 6 º longitude (vertical) strips numbered 1-60 from 180 degrees West east to 180 degrees East 8

9 Canada: UTM zones Zone Central Meridian the width of each zone varies from 666 km (6 x 111km) at the equator.. to ~120 km (6 x 20 km) at 80 N/S 10,000,000 UTM coordinates are in metres Within each zone 0 The Y coordinate Northings (N): measured from the Equator (0) to the north pole (10,000,000) in metres 9

10 UTM coordinates 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 the easting value decreases to the west but not below zero BC range= 300, ,000 Zone must be given as Coordinates repeat for each zone UTM : Eastings are 6 digit, Northings are 7-digit (in Canada) Blue grid squares in this map are 1000m = 1km 10

11 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? Canadian NTS map 1:50,000 including graticule and UTM coordinates 11

12 It may make more sense here : PGMAP: BC IMAP: View these also in Google Earth The UTM system works well for a local area BC Albers coordinate system BC uses UTM for local areas But Albers for the whole province 12

13 Summary: BC mapping coordinates Could be one of: 1. Geographic latitude / longitude for general reference 2. UTM zones for local /regional mapping 3. BC Albers - for BC provincial data Why is it important because we import data from different sources.. and they need to line up with each other. Web references Natural Resources Canada UTM grid UTM system BC Ministry of Forests ArcGIS resources earth graticule degree graticule intersections : confluence.org 13