Datums and Coordinate Systems Lecture 3 cont’d Datums and Coordinate Systems

Reference Datum Is a known and constant surface which can be used to describe the location of unknown points (in x, y and z)

Reference Datums 2 components: Horizontal datum: origin (X,Y) and orientation of a datum defined by the size and shape of an ellipsoid Vertical datum: the reference surface for height E.g. Global Positioning system is based on the World Geodetic System 1984 (WGS-84)

Example: NORTH AMERICAN DATUM OF 1927 based on Clarke 1980 Ellipsoid World Geodetic System (WGS84) based on GRS 1980 Ellipsoid

Datum shifts

Coordinate Systems provide a spatial referencing system to locate points on the earth surface spherical coordinate system coordinates that describe locations on a sphere planar coordinate system coordinates that describe locations within a two dimensional Cartesian space

Planar Coordinate Systems Universal Transverse Mercator (UTM) units in meters transverse cylindrical projection 60 vertical zones cover globe northing in meters from equator easting in meters from false origin 500,000 meters west of zone central meridian

Planar Coordinate Systems United States State Plane Coordinate System units in feet projection in Transverse Mercator or Lambert’s conformal conic tied to U.S. datum one or more zones for each state with a false origin southwest of the zone

Planar Coordinate Systems United States State Plane Coordinate System

Spherical Coordinate Systems latitude and longitude parallel: lines parallel to equator meridian: lines going from pole to pole

Planar Coordinate Systems Prime Meridian 1 ● (x,y) Equator - 1 +

Planar Coordinate Systems rectangular grid with X and Y axes X: easting Y: northing point is defined by X,Y coordinate

W E N S

How does this look in ArcGIS? You have the option of choosing either a pre-defined Datum or an Ellipsoid as your reference surface Recommendation: use a DATUM Example: how to project a coverage in ArcToolbox