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

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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 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

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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

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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)

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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/ ).

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Geodetic datums



Datums

longitude define the relationship between the physical earth and horizontal coordinates, such as latitude or   

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

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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

°.

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Latitude/Longitude

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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.

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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

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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

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