The Global Positioning System (GPS) is a group of Earth satellites carrying highly accurate atomic clocks and transmitting precise timing signals. Your GPS receiver uses these timing signals to calculate its distance from each satellite (trilateration), and thus determine its position on the globe.
The position of the 32 satellites is constantly changing as they orbit the Earth every twelve hours. At least six satellites (space vehicles) should be visible at any time. The signals from at least three satellites are required to determine a latitude and longitude. Four or more satellites will produce a more accurate position.
Most GPS receivers are used for automobile navigation, guiding a vehicle to particular address. The default setup for most units is not the best for mapping trails.
Be aware of the many situations in which GPS data may be of poor quality.
Other sources of error include:
One of the largest errors may come from the satellites themselves. Occasionally one of the atomic clocks may drift off time. Also the orbit of a satellite may be off track. If your GPS is using one these signals to calculate position, your track data may be poor. The Wide Area Augmentation System (WAAS) reports satellite errors, and may help your receiver avoid these errors.
Research by the USFS and the University of Minnesota indicates that resource-grade and recreational-grade receivers with WAAS enabled produced comparable results in open terrain under a clear sky. However, leaf canopy in the forest will significantly degrade GPS performance, especially in consumer-grade receivers.
Check your GPS receiver's firmware to be sure it is the latest version. Downloading a free firmware upgrade from the manufacturer�s website may improve your GPS and give the device additional features.
Read the manual and become familiar with your machine's menus and options.
Default settings unsually include a "power saving" function in which the receiver sleeps for short periods to save energy. This saves battery power, but interferes with recieving WAAS signals. Selecting "Normal" mode instead of "Battery Saver" mode will improve your GPS track.
Since most GPS units are used while driving on public roads, default settings include a function that will try link your location to a nearby road. Since you are mapping trails and not roads, turn this function off.
Set receiver options:
The season of the year and the time of day can be important factors in the quality of your GPS tracks. Mapping in late fall and winter will produce much better data because the bare branches allow a better view of the sky.
Using an external antenna mounted above head height will greatly improve the accuracy of your GPS tracks. Even the cheap ones make a big difference. If you do not have an antenna, mount the GPS as high as possible: on top of your backpack, clipped to a shoulder strap, or even on your hat. An external antenna also allows you to free up your hands to write things down or place the GPS receiver in your pocket or pack.
Install fresh batteries for each trip. Options like antennas and WAAS will require fully charged batteries. As the batteries grow weak, performance will degrade.
Take your time. Ride slowly. This will give the receiver time to calculate an accurate position for every turn of the trail. Riding at fast pace may reduce the accuracy of the track. Consider walking, instead of riding, when you are collecting GPS data for a map.
Consider using UTM coordinates to make measuring distance easier.
A new program called DNRGPS will be released soon.
View your saved shapefiles with ArcExplorer, a free GIS viewer with access to up-to-date aerial photography. This program provides a rich user interface with navigation tools similar to Google Earth.
Download it here:
http://www.esri.com/software/arcgis/explorer/index.html
Send your GPS data to the GPS coordinator for your ranger district (yet to be named):
�>Shapefiles
A "shapefile" is actually a set of files: