To: Mounted search and rescue <msar-riders@lists.ibiblio.org>
Subject: Re: [MSAR] GPS and field navigation
Date: Tue, 04 Jul 2006 11:11:56 -0400
When we have roads, power lines, heights of land, mountains, rivers
and lakes, these are often sufficient along with the map and compass for
us to establish with a reasonable degree of accuracy, where we are on
the map. In those featureless areas however, where all looks the same
for miles and miles of slowly rolling bushland, grassland, dessert,
tundra, swamp etc., the GPS takes over and will give a definitive
location which then may be plotted on a map (please note that "map"
remains an important tool).
One thing that we all must keep in mind is the use of GPS units are an
enhancement tool for field navigation and not a replacement tool.
Technology is no where near the point for GPS units to replace the map
and compass. Because of satellite coverage, unit placement and blocked
signals there will connectivity flaws with reception and in many
situations a person may simply need to move several meters to restore
satellite signals. The major down side to GPS Units is the need for a
power source, but that can be corrected by carrying a few extra
batteries (generally AA or AAA), which will extend the units operations
for another 12 to 24 hours. Even a week's worth of batteries can be
easily carried in something that takes up less space than a pack of
cigarettes.
The Global Positioning System is composed of twenty-four satellites
20,200 km (12,500 miles or 10,900 nautical miles) above the earth. The
satellites are spaced in orbit so that at any time a minimum of six
satellites will be in view to users anywhere in the world. The
satellites continuously broadcast position and time data to users
throughout the world.
Using a portable or handheld receiver unit that receives data from the
closest satellites, the GPS unit triangulates the data to determine the
unit's exact location (typically in latitude and longitude), elevation,
speed, and time. This information is available around-the-clock anywhere
in the world and is not dependent on weather. The "Selective
Availability," which made the public GPS less accurate than the military
GPS, was turned off on May 1, 2000. Thus, the GPS unit you can buy over
the counter at many retailers is as accurate as those used by the
military today.
Many over-the-counter handheld GPS units contain base maps of a region
of the earth but most can be hooked up to a computer to download
additional data for specific locales, which is a feature quickly is
becoming a valuable tool to planning section personnel when developing
incident action plans. UTM is becoming the standard with most search
operations in the United States, however, since UTM deals with meters
many americans do not understand the mathematical conversions required
for field application. On the newest 1:25,000 US topos, 1 mm. would
represent 25 meters on the ground. Even the most common Silva compasses
have a mm. scale on one side and this may be used to measure east from a
grid line and north from the E-W grid lines.
Map and compass skills will and must always be a part of field
navigation for SAR personnel. However, the use of GPS units have
quickly become equally important and with basic GIS functionality
software programs, these systems being integrated into search planning,
high-tech downloadable data is of utmost importance to enhance POD
efforts. If you remember, it just a few years ago when laptop
computers began to make an appearance at ICP's and have now become the
norm for progressive and/or highly skilled SAR teams.
Tomi`
TROTSAR Mounted Team
Maryland