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

2. 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
Designing better MAPS: A Guide for GIS Users
Cynthia A. Brewer
ISBN
ISBN

3. Datum
A mathematical model that represents an approximation of the earth.
Datum resource information -

4. 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

5. Alexander Clarke
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

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

7. 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 -

8. 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 -
Lat & Long lectures Dr. Rodriguez
Geographic Grid - In Image folder
Latitude - In Image folder
Longitude - In Image folder

10. 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 -
State Plane Info -

12. 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.

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

16. 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

18. 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.

20. 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) = Miles (101.2 Feet)
1 Degree Longitude = 69 miles * Cos(Latitude)

21. 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

22. Decimal Degrees
Increment between lines of latitude or longitude are represented as the decimal fraction between them.
lat: N long: W or
lat: long:
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.

23. 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 = degrees
16 seconds = 16/3600 = degrees
2. Add up the degrees to get the answer:
51° ° ° = DD
Convert DMS to DDD - Document in Image Folder