This was originally posted in December 14, 2011. Backcountry navigation was the topic for a backcountry ski class. Listed below are the notes from the session. The links in these notes have good explanations of compass use and techniques. The link to the zoomable world map of the world is also very useful — particularly in United States and Canada — where you can look at and make screen-sized prints of any topographic map.
Backcountry Skiing – Navigation
By Andy Dappen
Maps. If doing tours following snowed-over or are on terrain known to be gentle, then 1: 63,000 or 1:50:000 scale, 15-minute or Green Trail Maps may be OK. On complex and steep terrain, these scales are adequate as a back up but are not detailed enough as your main map. For steep ground, complex navigation, true-cross country travel, 7.5-minute maps (1:24,000 scale) are the norm. These give you the topographic data you need to better interpret avalanche hazard and to travel cross-country efficiently.
Compass. Use a compass with a baseplate or protractor (orienteering compass).
CD statewide map sets are the best $60 you can spend on staying found and are extremely useful for preplanning. We use the National Geographic Topo! CD maps for making our maps at WenatcheeOutdoors. The ‘shading’ feature available for these maps helps you see the terrain better.
Altimeter. Wrist watches with built-in altimeters work well. The Highgear Alterra has a surprisingly accurate altimeter and is reasonably priced. We think it’s an excellent value. Casio makes a number of altimeter watches and models we’ve tested in the past that were very accurate and affordable.
Pocket map. Keeping a map in the pocket allows you to check it often. Quick and easy access to your map is really important in not getting lost. Keeping the map in a Ziplock bag keeps it from getting soggy.
What does it mean when we say a 7.5-minute map? A ‘minute’ is a 60th of a degree. A minute of latitude is the same as a nautical mile. A minute of longitude at the equator is also a nautical mile. A nautical mile is 1.15 statute miles or about 6,000 feet. A 7.5-minute map is a map showing 7.5 minutes of latitude.
A minute on a map can be divided into 60 seconds. Each second is, therefore, about 100 feet apart. For GPS use, we usually use digital seconds which divides minutes into a thousandth. Each digital second is therefore, about 6-feet apart.
Contour lines: a V pointing uphill equals a gulley, a V pointing downhill is a ridge.
We can use maps to look for terrain features (e.g. ridges), vegetation (forests), and slope steepness (calculated through contour lines) to greatly reduce our exposure to avalanches.
Steepness, Avalanches, and Skiability
- 55 degrees plus (frequent sloughs hugely reduce number of larger slides. Extreme skiing, roped techniques)
- 45 to 55 degrees (smaller frequent slabs and sloughs. Fewer big slides. Extreme skiing)
- 35 to 45 degrees (prime slide steepness, 38 degrees is the bull’s eye. Black diamond to double-diamond skiing.)
- 35 to 30 degrees (slabs in very unstable conditions. Blue square / black diamond skiing)
- 30 to 25 degrees (slabs in extremely unstable conditions. Blue runs)
- 25 degrees and below (slush flows possible and runout zones can be overrun. Green runs )
Map Trick. On a 7.5-minute map, if the contour lines are closer than 1/32 of an inch, then the slope is steeper than 33 degrees – this is the starting point for really hazardous avalanche terrain.
Map Trick. On 7.5- minute maps (1 to 24,000 scale) each quarter-inch represents 500 horizontal feet. Given standard contour intervals (40 feet), 6 contours is about 26 degrees ( 240 vertical feet per 500 horizontal feet); 7 contours is 30 degrees (280 vertical feet per 500 horizontal feet); 8 contours is 33 degrees, 9 contours is 36 degrees; 10 contours is 39 degrees; 11 contours is 42 degrees, 12 contours is 44 degrees, 13 contours is 46 degrees, 14 contours is 48 degrees, 15 contours is 50 degrees, 16 contours is 52 degrees; 17 contours is 54 degrees; 18 contours is 56 degrees. The largest number of human triggered slab avalanches occur on slopes whose steepness is between 35 degrees (8.5 contours per quarter inch) and 45 degrees (12.5 contours per quarter inch) with 38 degrees (around 10 contours per quarter inch) being the most dangerous slopes of all.
A bearing is a direction relative to either magnetic or true north. We break down these directions into degrees and we usually use 360 degrees to cover all the different directions that could be traveled with north being a true bearing of 0 (or 360) degrees, east being a true bearing of 90 degrees, south being a true bearing of 180 degrees, and west being a true bearing of 270 degrees.
Compasses with a baseplate (or protractor) allow you to easily measure the true bearing between two points on a map (line up the edge of your compass’ baseplate between the two object in question; then line up the north-south lines on the compass bezel with north-south lines on the map and read the bearing). Should you want to follow this bearing on land, you’ll need to compensate for declination. More details about this.
Compasses also allow us to easily take a magnetic bearing from the land (point the compass at an object in question, line up the north/south lines on the compass bezle with the magnetic needle, and read the bearing). Remember this is magnetic bearing and, if you want to draw this bearing onto your map, you’ll need to compensate for the declination. More about this.
Map and compass use is complicated by declination. Declination is the degree difference between true north or geographic north (which is the reference used for drawing maps) and magnetic north (which is what our compass needle actually points at). Around Central Washington, declination is about 16 degrees East, meaning the magnetic needle points 16 degrees east of true north. More about declination.
Declination changes over time. In our part of the Cascades the declination from when many of the maps were first prepared in 1965 until today has decreased from 22 degrees east to 16 degrees East or roughly 1 degree every 8 years.
Practice taking true and magnetic bearings — on land and from a map.
Also, practice following bearings on land.
- Tips for following a bearing without continually looking at compass. Use a faraway tree, a distant peak, the relative position of the sun, or the relative position of your shadow to travel in the right direction without continually looking at the compass..
- To follow a bearing in a whiteout, use a person in front of you to mark the snow when he intersects the bearing you’re following. Go to that point, resight the bearing, redirect the front person right or left, remark the snow when that person intersects the bearing again. Keep repeating.
- To follow a bearing up a hill, take a back bearing to something below. Or have a person switchbacking up the slope ahead of you. Have them mark the spot when they intersect the bearing. Move to that spot, resight the bearing, tell the lead person to mark the snow again when they intersect the bearing. Keep repeating.
Excellent map resource: http://www.mappingsupport.com/p/gmap4.php. Zoomable world map. Zero in on place, see in different ways, get current declination. Will be putting on at WenOut site.
Calculating the time required for a trip is important to determine how early to start a trip or whether you better have a headlight along.
Learn your pace. Pace depends on snow conditions, group size, size of your pack… A small group with really fit people on a day trip in moderately firm conditions might use an uphill formula like this to determine how quickly they’ll cover ground: 3 miles per hour plus 1500 vertical feet per hour plus 5 transitions per hour.
With a group of more than 5 people, or with a group of average fitness skiers, or with fit people breaking lots of new snow, or with fit people carrying multi-day packs, this uphill formula is safer: 2 miles per hour plus 1000 vertical feet per hour, plus 4 to 5 transitions per hour.
For the downhill legs of a ski trip, figure it will take you about a third of the time needed to climb those legs.
Resources that help explain navigation, map and compass use, route finding, route selection:
- Freedom of the Hills (Mountaineers Books)
- Backcountry Skiing: Skills for Ski touring and Ski Mountaineering (Mountaineers Books)