GPS for Patrollers by Monica Spicker Mt. Spokane Ski Patrol All figures are copyrighted and used with the kind permission of Dr. Peter H. Dana, The Geographer’s Craft Project, Dept of Geography, Univ. of Colo., Boulder.

TOPICS  GPS System explained  Limitations of the System  What is a Map Datum?  Review of Grid Systems and Maps  Initializing your GPS Receiver  Getting a “Fix” (this one’s legal!)  Waypoints  Navigation

Parts of a GPS System

Control System

A Satellite

The Orbits

How a position is “fixed”

Sources of Errors  Satellite Geometry (largest source)  Reflections from Objects (Multipath)  Antenna Quality  Atmospheric Interference  System errors (time and orbital)  Receiver noise Geometric Dilution of Precision (GDOP) measures the error of position over time.

What Gives a “Good” Position One satellite directly above Three low and spread around

GPS cannot “see” through objects!

Initializing the receiver  Setup  Map Datum  Grid System  Distance units  Other features or options

Map Datums  Earth model to reference and compute coordinates  GPS based on World Geodetic System 1984 (WGS 84)  USGS quad maps based on North American Datum 1927 (NAD27)  Future maps will be based on NAD83  Other countries have their own datums  GPS Receiver set up must match map to be “in sync.”

Map Projections Converting a spherical Earth to a flat map.

Latitude and Longitude

Universal Transverse Mercator (UTM)  NATO system  Metric units (kilometers or.62 miles)  World is split into 60, 6º degree zones  Zone 1 starts at West longitude 180º  Distorted above North 84º, and South 80º, not applied above these latitudes.  Smallest grid unit on USGS Quad maps is 1 km.  Final units are meters east and then north from SW starting point.

Averaging  If possible use averaged positions before plotting or setting waypoints.  Averaging helps reduce position errors.  Average for at least 3 to 5 minutes.

Waypoints  Input coordinates of a desired point  Saved coordinates of a field point  Used during navigation to travel along a preset route.

Navigation  Traveling in a straight line from waypoint to waypoint (route).  Feedback is distance and direction - you may need a compass!  Can save waypoints along the way and follow back (reverse route).  XTE = cross track error, distance at a right angle to the right or left of the route line.  Straight line may not be safest - use a map!

Rule To Remember! NOT A REPLACEMENT  A GPS receiver is an accessory, NOT A REPLACEMENT for a map and compass!