Coordinate systems & projections

CS 128/ES 228 - Lecture 3b 1

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” 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.


Where is the Library ( )?According to

Dave ( ), it is at (3, -1)

According to Ted ( ), it is at (1, 1)


Lesson?

Where you place the origin matters

Not so much to the data, but definitely in how you refer to it


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


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


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


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.


World Wide UTM Zones

www.ems-i.com


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


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


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)


State Plane Coordinate Zones


An interesting concept of North?


North?Which way is north on this map?

Does this make sense?


What is North? There are three

“kinds” of north

Magnetic North Grid North True North


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


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”


So what’s the difference?

Declination Computation

http://www.ngdc.noaa.gov/geomagmodels/struts/calcDeclination


Who went out of their way?

Choice of projection has a great influence on perception of distance!


Conclusions Designing a map is like writing an

essay – you can only do a decent job if you know What you wish to convey, and Who your audience is

With these in mind, you choose Your projections/coordinate systems

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Coordinate systems & projections

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