Datum & Coordinate Systems Lecture 6 Materials: Map Examples Activities: Latitude & Longitude Activity Google Earth Activity Produced by Tim Towler Revised 10/27/08

Representing Geography Maps Maps are a 2-D representation of Reality Legend Scale (ratio of map distance: real world distance) Directional arrow As a 3-D world is represented in 2-D, the transformation Always involves distortion All maps involve distortion to represent a 3-D surface in 2-D Resource: How to Lie with Maps Mark Monmonier ISBN 0-226-53421-9 Designing better MAPS: A Guide for GIS Users Cynthia A. Brewer ISBN-10 1-58948-089-9 ISBN-13 978-1-58948-089-9

Datum A mathematical model that represents an approximation of the earth. Datum resource information - http://www2.arcwebservices.com/v2006/solutions/awx.jsp

Datum are errorless in Abstract Once you have a mathematical model of the size and shape of the earth, you can apply a coordinate system to in. Coordinates without a datum are vague Datum provides an origin that gives meaning to the coordinates Datum are errorless in Abstract When a datum is applied to reality, error is introduced

Alexander Clarke - 1866 Calculated the shape of the earth based on surveying measurements Established the Clarke 1866 Ellipsoid Standard reference for mapping Used to create NAD27 North American Datum of 1927 NAD83 North American Datum of 1983 Uses GRS 80 Ellipsoid as it’s Base Model

Different Datum are more accurate for different parts of the world. Notice where NAD27 and the European Datum of 1950 are accurate and inaccurate.

GPS has contributed to Datum Development over time WGS 84 - World Geodetic System 1984 the reference frame upon which all geospatial-intelligence is based Latest update 2004 NGS - National Geodetic Service Brass markers are placed to pinpoint locations based on Datum WGS 84 information - http://earth-info.nga.mil/GandG/wgs84/

Coordinate Systems Systems used to pinpoint a location as related to an origin Cartesian Coordinate System X, Y axis Based on an origin Divides an area into 4 Quadrants Upper right (+, +) Lower left (-, -) Coordinate System Overview - http://www.colorado.edu/geography/gcraft/notes/coordsys/coordsys.html Lat & Long lectures Dr. Rodriguez Geographic Grid - In Image folder Latitude - In Image folder Longitude - In Image folder

UTM & State Plane UTM The National Imagery and Mapping Agency (NIMA) (formerly the Defense Mapping Agency) adopted a special grid for military use throughout the world called the Universal Transverse Mercator (UTM) grid. In this grid, the world is divided into 60 north-south zones, each covering a strip 6 degrees wide in longitude. These zones are numbered consecutively beginning with Zone 1. In each zone, coordinates are measured north and east in meters. UTM Info - http://erg.usgs.gov/isb/pubs/factsheets/fs07701.html State Plane Info - http://geology.isu.edu/geostac/Field_Exercise/topomaps/state_plane.htm

State Plane The state plane coordinate system (SPCS) is similar to the UTM system in many respects, but it does have some important differences. In the SPCS, each state has its own zone(s).The number of zones in a state is determined by the area the state covers and ranges from one for a small state such as Rhode Island to as many as five.

Geographic Grid Coordinate System Degrees, Minutes, Seconds DD MM SSS Decimal Degrees DD.MMMMMMMM Origin Equator Prime Meridian

Latitude Measured as the internal angle from an origin in the center of the earth. 0 degrees - Equator 90 degrees - Poles North is Positive South is Negative

Longitude Measured as a portion of the circumference of the earth. Longitude lines are all Great Circles Range 0 – 180 degrees East is Positive West is Negative Great Circle - largest circle that can be made from a sphere. Longitude - all lines of longitude are Great Circles Latitude - Only the Equator is a Great Circle, as you go N or S from the Equator, the circle is smaller.

Degrees, Minutes, Seconds 1 Degree Latitude - 69 Miles 1 Degree Longitude - 69 Miles at the Equator Converges at the Poles In San Diego, 1 Degree Longitude = 58.5 Miles 1 Degree = 69 Miles 1 Minute (1/60 of a Degree) = 1.15 Miles 1 Second (1/60 of a Minute) = 0.01917 Miles (101.2 Feet) 1 Degree Longitude = 69 miles * Cos(Latitude)

Where is San Diego? 32o 28’ 18” N (Latitude) 117o 48’ 53” W (Longitude) Also represented as 32o 28’ 18” -117o 48’ 53” North of the Equator is Positive East of the Prime Meridian is Positive San Diego is North of the Equator and West of the Prime Meridian

Decimal Degrees Increment between lines of latitude or longitude are represented as the decimal fraction between them. lat: 32.720939 N long: 117.151969 W or lat: 32.720939 long: -117.151969 The more decimal places, the more accurate the location. This is the format that will be used to collect data using the GPS units and used in the GIS software.

Converting from DMS to DDD The latitude of London expressed in DMS is 51° 29' 16" North. To convert this location to DD, follow these steps: 1. Divide each value by the number of minutes (60) or seconds (3600) in a degree: 29 minutes = 29/60 = 0.4833 degrees 16 seconds = 16/3600 = 0.0044 degrees 2. Add up the degrees to get the answer: 51° + 0.4833 ° + 0.0044 ° = 51.4877 DD Convert DMS to DDD - Document in Image Folder