Randall Osterhuber Donner Summit Avalanche Seminars
The proper use of a GPS can offer solutions to navigation problems in avalanche terrain that no other tool can.
Backcountry navigation includes figuring out where you are, and how to get from one location to another.
Three tools—all working in concert—can give backcountry travelers a powerful system for on-the-snow navigation: a topographic map, magnetic compass, and a GPS.
A compass or GPS may “point you in the right direction,” but tells you nothing of the terrain you’ll encounter along the way.
Route finding in an around avalanche terrain involves elements of safety, efficiency, observation, timing, exposure, communication with your partners, map reading, and nuances that cannot be taught by any PowerPoint presentation.
While the mechanics of using a topographic map, compass, and GPS can be learned on the couch, these skills do not a route finder make!
Becoming a good route finder in the mountains takes significant experience. Which means dedicating significant time.
Topographic maps give us a cartoon view of the terrain.
Compasses tell us slope aspect. With visibility, a map and compass can also
From maps we can glean terrain elevations, relative slope angles, slope shape, aspect, potential terrain traps, and in some instances information on vegetation, ground cover, and terrain roughness.
be used to triangulate position.
GPSs tell us where we are.
Even though your two-centimeter thick GPS owner’s manual would try to convince you otherwise, your GPS does not know how to get you from A to B; it does not know how to get you back to your starting point; but it does know how to ID your location. Being able to ID our location is not a trivial piece of information.
Your GPS collects location data points, “waypoints”— sometimes lots of them—and then adds, subtracts, and does the elegant trigonometry on them all. Really fast. Your GPS does not know its way around the mountains, but it can compute the relative relationship between all the collected waypoints. That’s what a GPS does.
It’s a big thing—this knowing where we are. Because if we know where we are, and have a good map, we now know the distance and direction to every other point on that map. Like I said: this is not a trivial piece of information!
GPSs that incorporate a graphic display LCD screen showing “maps” are fine, but with practice you’ll find these “maps” are utterly inadequate for navigation. They’re too small, annoying to scroll through; plus, on a -10º C day your GPS can chew through a fresh set of batteries in about 4 hours (no, not the 15 hours your owner’s manual tells you). Dead batteries equal dead map. Lithium batteries will last much longer in the cold. But remember: don’t put lithium batteries in the avalanche transceiver!
Moral of the story? Always carry a paper map.
Maps are produced from surveyed information: datum.
Maps of the Sierra Nevada typically come from one of two datum: NAD 27 (North American Datum, 1927) or WGS 84 (World Geodesic Survey, 1984)
Which datum your map is produced from will be listed in the map’s marginalia.
Your GPS must be set to the same datum as your map. The GPS owner’s manual will tell you how to change datum.
Coordinates are sets of numbers that describe a location. One number describes the latitude (how far north or south of the equator), and one number the longitude (how far east or west of Greenwich, England).
There are four coordinate systems commonly used by backcountry travelers…
(1) Degrees, minutes, seconds Example: 39º 14’ 27” N. This is read as “39 degrees, 14 minutes, 27 seconds north.”
60 seconds equal one minute; 60 minutes equal one degree. This system is born of marine navigation.
(2) Decimal degrees Example: 39.55832º N.
At Tahoe latitudes, using this coordinate system to five significant digits (39.xxxxxº N) describes a ground based resolution of about 1 meter. Most modern handheld GPSs will have a resolution of—depending on signal strength, local topography, etc—between 2 and 10 meters. This coordinate system is the SWAG standard for which to record field observation locations.
(3) Decimal minutes Example: 39º 43.237’ N.
This is read as “39 degrees, 43.237 minutes north.” Most GPSs default to this coordinate system right out of the box. I don’t know why.
39º 29’ 55” N does not equal 39.2955º N which does not equal 39º 29.55’ N
(4) UTMs (Universal Transverse Mercator) is the coordinate system of choice for skiing, riding, and hiking in the mountains. Why? Because it’s the easiest system with which to derive our position on a paper map. Given UTM coordinates, we can easily find that spot on the map; we can also easily describe a position on the map using UTMs.
Three different coordinate systems describing the same location
Decimal degrees
Decimal minutes
UTMs
A pair of coordinates describes a single point—and only a single point—on Earth. Those coordinates, therefore, are embedded with latitude, longitude, elevation, aspect, slope angle, and in many cases geologic and biologic information. If we tag the location with a time stamp, we now know hours of sunlight and dark, time of sun and moon rise and set, and can infer general characteristics of the climate including trends of temperature and precipitation.
The UTM system lays a 1 km2 (1000 m x 1000 m) grid over the terrain. By resolving (estimating) our position within this grid—east and west, north and south—to the nearest tenth of a kilometer (a tenth of the grid), we can get a good fix on our position—with no math, and with our ski gloves on. Your GPS manual will help you change which coordinate system your GPS displays.
UTMs are zone dependent: check your map’s marginalia for which zone you’re in (most of the Tahoe Sierra is Zone 10S or 11S).
On many USGS topographic quadrangles, UTM grid marks are small, light blue tick marks around the map’s margins. Grid labels are also typically printed in light blue.
If you’re printing maps from TOPO! or similar software, choose UTM grid and grid labels in your setup options.
UTM grid labels Example: The summit of Andesite Peak is about 9/10 of the way between 726000 m and 727000 m, and just a little north (30 m or so?) of the 4359000 m mark. So its location would be described as (approximately):
Zone 10S 4359030 m N 726900 m E
Map datum and UTM zone underlined in red
Sharp-witted study that you are, if I were to ask you to find UTM Zone 11S 4354550 m N 246200 m E,
you’d come back quick as ink and say “Ginny Lake.”
Any time outside help is to be contacted, those resources are going to want to know: Where’s the problem? GPS coordinates eliminate ambiguity about the rescue location; especially important when talking with 911 dispatchers far from the mountains.
In low visibility, assessing avalanche hazard is highly problematic. Through our planning and preparation—studying maps, considering various route options, carefully timing our backcountry travel, and being familiar with our equipment—we can begin to manage our risk. Though a GPS alone cannot safely guide us through complex terrain, it is a very sophisticated and unique tool that can fix our position when nothing else can. All winter backcountry travelers should strive to become expert navigators using a map, compass, and GPS. This is but one of many ways we can minimize our exposure and maximize our successful adventure.
Donner Summit Avalanche Seminars Professional, comprehensive avalanche safety education -- since 1998 –
Norden, California
Donner Summit Avalanche Seminars PO Box 83 Norden, California 95724 USA www.DonnerSummitAvalancheSeminars.com (530) 412-3585
Photographs in this presentation by Randall Osterhuber, Andy Anderson, Steve Reynaud, Dirk Schoonmaker, Kenneth Libbrecht, Greg Von Doersten, and Jeff Rieger.
