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Coordinate systems & projections. Today’s dilemma – What coordinate system shall we use?. Coordinate systems (ways to find things). Once a projection is chosen, the map needs a coordinate grid to measure location. Common systems: UTM State Plane Coordinates. About “LatLong”. - PowerPoint PPT Presentation
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CS 128/ES 228 - Lecture 3b 3
Coordinate systems (ways to find things)Once a projection is
chosen, the map needs a coordinate grid to measure location.
Common systems:
UTM State Plane
Coordinates
CS 128/ES 228 - Lecture 3b 4
About “LatLong” Latitude and Longitude are two
coordinates used to describe a point in 3-dimensional space.
They are NOT planar Thus, they are not easy to use on a
(planar) map Ergo, graticules1 are useful, but not
for maps1a network of parallels and meridians on a map or chart.
CS 128/ES 228 - Lecture 3b 5
Where is the Library ( )?According to
Dave ( ), it is at (3, -1)
According to Ted ( ), it is at (1, 1)
CS 128/ES 228 - Lecture 3b 6
Lesson?
Where you place the origin matters
Not so much to the data, but definitely in how you refer to it
CS 128/ES 228 - Lecture 3b 7
Cylindrical projections Least distortion
along line of contact
In this case equator
But nobody lives there!
http://ioc.unesco.org/oceanteacher/resourcekit/Module2/GIS/Module/Module_c/module_c4.html
CS 128/ES 228 - Lecture 3b 8
Moving the Line of ContactIf we turn the cylinder
transversely and place the line of contact through Olean, then there will be little error near Bonas!
(and all points due north or south of here)
www-atlas.usgs.gov
CS 128/ES 228 - Lecture 3b 9
The Politics of Mapmaking Problem: We might like Bonas to be
the most accurate portion of the map, but folks in California may differ.
Solution: Let them use a different projection!
Create “Zones” for each region of interest
CS 128/ES 228 - Lecture 3b 10
Universal Transverse Mercator system Based on a set of
cylindrical projections running along meridians
Distortion minimized in a N – S “strip” (zone)
Zones are 8o wide but overlap by 1o on each side. 60 world wide.
CS 128/ES 228 - Lecture 3b 12
UTM coordinates Coordinates are based
on an arbitrary origin at equator and 500,000 m west of central meridian
E-W position: “easting”N-S position: “northing”
NYS has 3 zones – most state-wide datasets for New York use zone 18
CS 128/ES 228 - Lecture 3b 13
Where is Our Origin?
We “live” in UTM zone 17. Its origin is at 0˚ latitude (the equator) and 84˚ W longitude
That’s just west of Detroit, but on the equator (just off the coast of Ecuador)
If we use UTM zone 18, the boundary would be near Belmont and the origin in a suburb of Quito
CS 128/ES 228 - Lecture 3b 14
State Plane Coordinate System Older system – usually based on Clarke 1866
ellipsoid and NAD 27 datum
Goal: distortion < 1 part in 10,000
Each state divided into either E-W or N-S zones, depending on its orientation. Most use either Transverse Mercator or Lambert Conformal projections (Alaska, New York, and Florida use both)
Only exception: Alaska panhandle (uses Oblique Transverse Mercator)
CS 128/ES 228 - Lecture 3b 18
What is North? There are three
“kinds” of north
Magnetic North Grid North True North
CS 128/ES 228 - Lecture 3b 19
Magnetic North Pole Lines that point to
“magnetic north” point to the magnetic north pole on earth.
This location “moves”
Currently about 1000 miles off at 81.3°N,110.8°W http://geo.phys.uit.no/articl/roadto.html
CS 128/ES 228 - Lecture 3b 20
Geographic North Pole Always found at same place (90° North
Latitude) Famous as home of Santa Claus.
Lines on a map that point to the geographic North pole represent “True North”
Lines on a map parallel to the central meridian represent “Grid North”
CS 128/ES 228 - Lecture 3b 21
So what’s the difference?
Declination Computation
CS 128/ES 228 - Lecture 3b 22
Who went out of their way?
Choice of projection has a great influence on perception of distance!