Coordinate systems and projections Laura Johnson Cherie Aukland

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Module Outline Methods of determining location Spatial Reference Coordinate system Datum Projection Module name and number

What Is Our Location? The question: Where am I on the earth? Answers: Blacksburg, VA zip code 24060 mile marker 118, I-81 Worsham field: Latitude: 32.21997, Longitude: -80.41873

Where am I? All of the previous answered the question, but which one will help me the most? All are forms of georeferencing, or communicating location on the earth’s surface

Spatial Reference A way to communicate location that has up to three components: Coordinate system (X, Y) Datum (accounts for shape of the earth) Projection (minimizes 3D -> 2D distortion in some capacity)

Coordinate Systems Measure X, Y from an origin point Origin and units vary based on the coordinate system Common systems: UTM, Universal Transverse Mercator Public Land Survey System (PLSS) coordinates Latitude and Longitude (WGS84)

Public Survey Public Land Survey System (PLSS) used to survey most of the Western US and Western Canada Used to determine ownership Doesn’t account for the curvature of the earth Divides areas into townships, ranges, sections, and quarter sections Coordinate systems typically in meters or feet

Latitude and Longitude Geographic coordinates Starts at the equator and the prime meridian Units of measurement are degrees, minutes, and seconds 60 minutes in a degree 60 seconds in a minute

Spatial Reference Coordinate system Datum Projection

What is a datum? Model of the shape of the earth’s surface The earth is not a perfect sphere Spheroid = equation attempting to model the actual shape of the earth’s surface Spheroid Earth’s Surface Origins

What is a Datum? Datums define: shape of the earth location of latitude and longitude of origin

Datums Earth’s Surface Datum

Common Datums NAD27 NAD83 WGS84 What do you think will happen if you use the wrong datum?

Spatial Reference Coordinate system Datum Projection

Projections Take a 3-D world and make it 2-D

Projections Important to know what projection you are using to minimize measurement and display errors Projections are a way of taking a 3-D object (earth) and making a 2-D object (map)

Errors Projections usually try to hold one of the following constant: Conformal (keep shapes the same) Equal area (keep areas the same) Equidistant (keep distances the same) No projection can achieve all three Some use a compromise of each

Projections Three ways of projecting Cylindrical Conic Planar Earth Wherever the projection surface “touches” the earth, there will be the least amount of distortion. Earth Earth

Common Spatial References Albers Equal-area Projection, good for large areas where area measures are important Lambert conformal conic Also good for large areas where visualization is important

Common Spatial References Universal Transverse Mercator (UTM) One of the most common coordinate systems and projections Measures in meters Has “Zones” Great for local areas Geographic or Cylindrical Equidistant No projection or datum Just has coordinates assigned Directions true, All else distorted

Common Spatial References State Plane system Varies by state Specifies projection, coordinates, and datum Good for state-wide analyses Some states have multiple coordinate systems For example, Texas Can use UTM, Lambert, Albers, or own projection Good for surveying

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