Geographic Information
Systems in Water Science
Unit 4: Module 1, Lecture 2 – Coordinate Systems and Common GIS data formats
Credits…
Material on Coordinate Systems was adapted
from the National Center for Geographic
Information and Analysis (NCGIA) Core
Curriculum in GIScience, an open repository
for curriculum materials related to Geographic
Information Systems
We acknowledge in particularly Peter Dana’s
section on “The Shape of the Earth” and
Anthony Kirvan’s section on “Latitude and
Longitide”
Developed by: Host
Updated: 10.20.04
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Representing Locations on the Earth
 Coordinate systems
 Reference ellipsoids
 The earth isn’t flat, but it’s not round either
 Geodetic datums
 Common reference systems
 Latitude/longitude
 Universal Tranverse Mercator
 Albers Equal Area projections
Developed by: Host
Updated: 10.20.04
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Basic Coordinate Systems
 Coordinate systems represent
points in two-dimensional or
three-dimensional space
 René Decartes "I think,
therefore I am" (1596-1650)
introduced systems of
coordinates based on
orthogonal (right angle) axes.
 These two and threedimensional systems used in
analytic geometry are often
referred to as Cartesian
systems
 Similar systems based on
angles from baselines are
often referred to as polar
systems
Developed by: Host
Updated: 10.20.04
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Problems with mapping the earth
 The earth isn’t round
 Sea level is not level
 Gravity is not uniform across the planet
Developed by: Host
Updated: 10.20.04
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Modeling the shape of the earth
 Earth shapes are represented in
many systems by a sphere
 But earth is not a sphere, it’s
an ellipsoid
 Compressed at the poles
 Wider at the equator
 20 km difference between
poles and the equator
 Precise positioning reference
systems are based on:
 Ellipsoidal models
 Gravity models
 Best models = 100 m
difference between poles and
equator
Developed by: Host
Updated: 10.20.04
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Developed by: Host
Updated: 10.20.04
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Reference Ellipsoids
 There are many
reference
ellipsoids and
gravitational
(geoid) models
used in GIS
 You need to know
which model your
data is based on!
 Global Positioning
Systems are based on
WGS84 (previous slide)
Developed by: Host
Updated: 10.20.04
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Geodetic datums
 Geodetic datums define reference systems that
describe the size and shape of the earth based on
these various models
 Different nations and international agencies use
different datums as the basis for coordinate
systems in geographic information systems,
precise positioning systems, and navigation
systems.
 In the United States, this work is the responsibility
of the National Geodetic Survey
(http://www.ngs.noaa.gov/).
Developed by: Host
Updated: 10.20.04
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Geodetic datums
 Datums define the
relationship between
the physical earth and
horizontal coordinates,
such as latitude or
longitude
 North American Datum of
1927 (NAD27)
 Based on an
elliposoid touching
the earth’s surface
at Meades Ranch
in Kansas
 NAD83
 measured from the
center of the earth
 World Geodetic System
1984 GS84
 describes both
horizontal and
vertical
Developed by: Host
Updated: 10.20.04
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Latitude/Longitude
 A coordinate system
defined by the poles and
the equator
 Prime meridian =
0 longitude
 Equator = 0 latitude
 Other points on earth’s
surface can be located
using lat/long
coordinates
Developed by: Host
Updated: 10.20.04
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Degrees, Minutes and Seconds: Expressing Lat/Long
 Latitude and longitude are expressed
on a sexagesimal scale:
 A circle has 360 degrees, 60
minutes per degree, and 60
seconds per minute.
 There are 3,600 seconds per
degree.
 Example: 45° 33' 22" (45 degrees,
33 minutes, 22 seconds).
 It is often necessary to convert this
conventional angular measurement
into decimal degrees:
 To convert 45° 33' 22", first
multiply 33 minutes by 60, which
equals 1,980 seconds.
 Next add 22 seconds to 1,980:
2,002 total seconds.
 Now compute the ratio:
2,002/3,600 = 0.55.
 Adding this to 45 degrees, the
answer is 45.55°.
Developed by: Host
Updated: 10.20.04
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Latitude/Longitude
Developed by: Host
Updated: 10.20.04
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Universal Transverse Mercator (UTM)
 UTM coordinates define two dimensional,
horizontal, positions.
 High degree of precision for entire globe
 Each UTM zone is identified by a number
 UTM zone numbers designate individual 6°
wide longitudinal strips extending from 80°
South latitude to 84° North latitude.
Developed by: Host
Updated: 10.20.04
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UTM Coordinates
 Locations within a UTM
zone are measured in
meters eastward from
the central meridian and
northward from the
equator. However,
 Eastings increase
eastward from the
central meridian
which is given a false
easting of 500 km so
that only positive
eastings are
measured anywhere
in the zone
 Northings increase
northward from the
equator with the
equator's value
differing in each
hemisphere
Developed by: Host
Updated: 10.20.04
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Using Map Projections
 Lat/Long and UTM coordinates are commonly
used systems for delivering GIS data
 Many GPS systems provide output in Lat/long
or UTM format
 Over large (multi-state) regions, the Albers map
projection
 To be useful in a GIS analysis, data layers in
different projects must be converted to a
common coordinate system
 This is a common GIS operation
Developed by: Host
Updated: 10.20.04
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