Geographic Information Systems in Water Science

1
Geographic Information Systems in Water Science

• Unit 4 Module 1, Lecture 2 Coordinate Systems
  and Common GIS data formats

2
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 Danas
  section on The Shape of the Earth and Anthony
  Kirvans section on Latitude and Longitide

3
Representing Locations on the Earth

• Coordinate systems
• Reference ellipsoids
• The earth isnt flat, but its not round either
• Geodetic datums
• Common reference systems
• Latitude/longitude
• Universal Tranverse Mercator
• Albers Equal Area projections

4
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 three-dimensional 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

5
Problems with mapping the earth

• The earth isnt round
• Sea level is not level
• Gravity is not uniform across the planet

6
Modeling the shape of the earth

• Earth shapes are represented in many systems by a
  sphere
• But earth is not a sphere, its 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

7
(No Transcript)
8
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)

9
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/).

10
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 earths
  surface at Meades Ranch in Kansas
• NAD83
• measured from the center of the earth
• World Geodetic System 1984 GS84
• describes both horizontal and vertical

11
Latitude/Longitude

• A coordinate system defined by the poles and the
  equator
• Prime meridian
• 0 longitude
• Equator 0 latitude
• Other points on earths surface can be located
  using lat/long coordinates

12
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.

13
Latitude/Longitude
14
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.

15
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

16
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