Working with georeferenced data Copyright 2000 2004 ESRI

Lesson 8 overview u Georeferencing u Coordinate systems u Datums u Projections and distortion

What is georeferencing? u Data is referenced to a location on the earth’s surface

Coordinate systems Y Data - X +Y +X +Y usually here X -X -Y

Datums and datum conversion u A frame of reference for measuring locations on the

Referencing locations u Locations on the earth are referenced to the datum u Different

Map projections u Map projections convert curved surface to flat surface Cylinder Copyright ©

Projection distortion u Distortions make geographers SADD 3 D Earth 2 D Map Shape

Types of projections u Generally classified by the spatial attribute they preserve Projection type

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What is georeferencing? u Data is referenced to a location on the earth’s surface u Geographic u Projected coordinate systems Copyright © 2000– 2004 ESRI. All rights reserved. Introduction to Arc. GIS I 3

Coordinate systems Y Data - X +Y +X +Y usually here X -X -Y +X - Y u Geographic coordinate system u Cartesian coordinate system u Latitude and longitude are not uniform across the earth’s surface u Measures of length and angle are uniform Copyright © 2000– 2004 ESRI. All rights reserved. Introduction to Arc. GIS I 4

Datums and datum conversion u A frame of reference for measuring locations on the surface of the earth u Measurements are referenced to a geodetic origin and an ellipsoid model Local datum NAD 27 Ellipsoid CLARKE 1866 Meades Ranch Kansas + + Earth’s surface Copyright © 2000– 2004 ESRI. All rights reserved. Earth-centered datum NAD 83 Ellipsoid GRS 80 Earth’s center Introduction to Arc. GIS I 5

Referencing locations u Locations on the earth are referenced to the datum u Different datums have different coordinate values for the same location UTM NAD 83 I-10 through Redlands, CA UTM NAD 27 Copyright © 2000– 2004 ESRI. All rights reserved. Introduction to Arc. GIS I 6

Types of projections u Generally classified by the spatial attribute they preserve Projection type Maintains Example Conformal Shape Lambert Conformal Conic Equal area Albers Equal Area Conic Equidistant Distance Equidistant Conic Azimuthal Direction Lambert Equal Area Azimuthal Copyright © 2000– 2004 ESRI. All rights reserved. Introduction to Arc. GIS I 9