Understanding Coordinates Ellipsoids, Geoids, Datums and Projections

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Presentation transcript:

Understanding Coordinates Ellipsoids, Geoids, Datums and Projections

Important concepts for today What is a Geographic Coordinate System (GCS)? What is a Datum? What is a Projected Coordinate System (PCS)? How do these apply to ArcGIS?

A Geographic Coordinate System Means of measuring spherical coordinates that consists of: Angular unit of measure A prime meridian A Datum

Geodetic Datum: Ellipsoid Surface measurements Reference frame for locations on the surface. Based on: Ellipsoid Surface measurements

Shape of the Earth Sphere Ellipsoid

Axes of the Ellipsoid: Minor Major Axis Semi - Major Semi - Minor

Earth Geoid: http://earthobservatory.nasa.gov/Library/GRACE_Revised/page3.html

There are different ellipsoids that work best at different areas on earth:

Geodetic Datum: Ellipsoid Surface measurements Reference frame for locations on the surface. Based on: Ellipsoid Surface measurements

Local Geodetic Datums

Geocentric Geodetic Datums

Datum conversions:

Importance of Using the Correct Datum Using coordinates from wrong datum can result in position errors of up to 1 kilometer!

What happens when plotted like XY? Geographic Coordinate System

Projected Coordinate Systems:

Different surfaces to Project on:

Distortion Caused by Map Projections Shape Area Distance Direction

Types of Projections Classified by the spatial attribute that they maintain Projection Type Maintains Example Conformal Shape Lambert Conformal Conic Equal Area Area Albers Equal Area Conic Equidistant Distance Equidistant Conic Azimuthal Direction Lambert Equal Area Azimuthal

Cylindrical projection

Conic Projection:

Cylindrical Projection

Planar Projection:

Azimuthal Projections

Homolosine – interrupted projection

Projections How does ArcGIS handle map projections for data? Projecting data creates a new data set on the file system ArcGIS’s projection engine can go in and out of a large number of different projections, coordinate systems, and datums.

In ArcGIS: Exercise 5 3 Situations: Data has no projection > Define Changing the Coordinate System > Project Do not change the data itself > Change the data frame projection

Projecting spatial data sets ArcToolbox is used for converting data sets from one system to another Used for going between projections

Use these tools today