Geographic Information Systems in Water Science

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

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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”

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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

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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

Modeling the shape of the earth

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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 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

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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

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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