Projections and Coordinate Systems
Basic Cartographic Concepts
Jennifer Hackett
Manastash Mapping
www.manastashmapping.com
Projections, DATUM & Coordinate
Systems
What are they and why do they matter?
Mapping a round earth on a flat paper
Where is it?
Coordinates
Surface (Datum)
How do we display a round
Earth on flat paper
Projections
Coordinate Systems, GEOIDs &
Projections
GIS data sets identify the location of each point or pixel using
numbers but the numbers only have meaning in relation to a
coordinate system.
Ellensburg can be identified as:
1628475 606516 (Washington State Plane South)
1711664 -1140 (Washington State Plane North)
686433 5207695 (UTM Zone 10)
120 32 52.85 N 46 59 47.44 W (WGS84 Deg Min Sec)
46.995122 N 120.548014 W (WGS84 Decimal Degrees)
The Coordinate system determines how those values relate to a point on the
Earth. Where do you count from and in what unit.
The Geoid/Ellispoid defines the shape of the Earth.
The projection defines how points on a round Earth will be plotted on a flat
page.
What Happens When you Use the
Wrong Coordinate System?
SEATAC Airport
Coordinate System A
Coordinate System B
Coordinate System C
Coordinate System A
Coordinate System B
These maps show the Location of Airports in Washington State. A has the correct coordinate system and datum
defined (WA State Plane South, NAD83), B has the wrong coordinate system (UTM Zone 10), and C has the
correct coordinate system but the wrong datum (NAD27).
Defining locations
Geocentric coordinate system
XYZ
Origin center of Earth
Used by GPS for internal calculations – not displayed
Spherical or Geographic coordinate systems
Angles relative to the prime meridian and Equator
Expressed as latitude and longitude
Distorts distance and angle.
Cartesian coordinate system:
Flat system placed on earth’s surface
UTM and State Plane
Benefits
Easy to work with
Fairly accurate over small distances
Measurements of length, angle, and area.
Significant distortion of shape over larger areas
http://www.sharpgis.net
Geocentric
State Plane
Edndoc.esri.com
Cartesian
gsd.harvard.edu
hightunnels.org
Geographic System - Lat/Long
UTM
xmswiki.com
Defining the Shape of the Earth
Models of the earth
Real earth: (Mt. Everest 29,029’, Dead Sea shore
-1,385’)
Geoid: “The equipotential surface of the Earth's
gravity field which best fits, in a least squares
sense, global mean sea level” (NOAA)
Ellipsoid - Ellipsoid – a mathematical construct
used to approximate the geoid.
Datum –The center and orientation of an
ellipsoid
www.geod.nrcan.gc.ca/edu/geod/geoid/geoid05_e.php
NAD83 – North American Datum 1983
WGS84 – Global system used by GPS systems
http://www.sharpgis.net
www.sco.wisc.edu/coordsys/docs/WCCS_WSLS_2006.ppt
Drawing a Round Earth on a Flat Paper
A projection represents a 3 dimensional surface in 2-
dimensions.
All projections distort one or more properties – area, shape,
direction, distance and/or scale
Equal area
Conformal projections – preserve the shape of small features and
direction
Equidistant projections preserve distances from one or two points
The larger the area the greater the distortion
visual.merriam-webster.com
Which projection is best?