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Snowshoeing Navigation

Snowshoeing Navigation

by Andy Dappen

Tools

Maps. If you are doing tours following snowed-over roads or are on terrain known to be gentle, then 1: 63,000 or 1:50:000 scale, 15-minute or Green Trail Maps may all be OK. On complex and steep terrain, these scales are adequate as a backup but are not detailed enough as your main map. For steep ground, complex navigation, and 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 baseplate (or protractor) compass. 

 

Photo Credit: The Backpacker's Field Manual (Rick Curtis, www.princeton.edu).

CD statewide map sets are the best $50 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.


The National Geographic TOPO! map set has tools for drawing in routes, tracing trails, calculating bearings, and measuring distances.

Altimeter.
Wrist watches with built-in altimeters work well. The High Gear Alterra has a surprisingly accurate altimeter and is reasonably priced. We think it’s an excellent value.

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.

Maps

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 run-out 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. As a very rough guide remember 32-32 and 16-16...or 1/32nd of an inch is roughly 32 degrees and 1/16th on an inch is roughly 16 degrees.

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.  

Slope Steepness Maps from Caltopo.com (discussed earlier).

Apps for Smartphone like Smart Protractor can measure slope angle.


Compasses

Bearings are a direction of travel (or an angle of direction) relative to either true north or magnetic north. We break down these directions (or angles) into degrees and 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 180 degrees, and west being 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 bezel 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.

Cheap trick for compasses without a built-in declination arrow: use a pen to draw in the declination for your area. To change the declination, use rubbing alcohol to erase the pen mark and start fresh. 

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, re-sight 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, re-sight the bearing, tell the lead person to mark the snow again when they intersect the bearing. Keep repeating.

Here is an excellent map resource with a zoomable world map. Zero in on a place, see it in different ways, and get the current declination. There are links to this in the ‘Map’ area of the WenOut site.

GPS

Excellent dedicated GPS units for trail sports are made by both Garmin and Magellan.

Desirable features for a snowshoeing GPS: High-sensitivity receiver, WAAS enabled, light and compact (3 to 6 ounces), uses AA or AAA batteries, can import waypoints and routes prepared on a computer map set, can export waypoints and tracks recorded in the field into computerized maps, waterproof, well-lit screen for low light conditions, detailed topographic maps that can be loaded into GPS, cost ranging between $200 and $400. Two very good units that offer all this and more are the Garmin eTrex 30 ($300) and eTrex 20 ($200) and Magellan eXplorist 310 ($170).

Waypoints are exact latitude-longitude coordinates with degrees, minutes, and seconds (or digital seconds) noted. If many satellites are locked onto your GPS unit, the margin of error can be plus or minus ten feet.

Tracks. When a GPS is recording your movements, it makes a track (or a string of waypoints) of where you’ve been. This track can later be exported into a computer to show exactly where you went. Out in the field you can use the ‘trackback’ feature and have the GPS follow its track backwards to the start of your walk. This is like following bread crumbs back to the start.

Routes can be prepared on a computer by defining a series of waypoints in the order that you want those waypoints followed. Routes can be imported into a GPS and then the device will lead you to each waypoint in order. Once one waypoint is reached, the GPS leads you to the next waypoint along the route.

Go-to function. This allows you to pick a waypoint stored in the device’s memory. Once chosen, the GPS will lead you directly to that waypoint.

Map Datum. All maps are drawn using specific conventions and reference points. A point on a map using one datum set will have slightly different coordinates than the same point using a different datum set. Some datum sets only cover parts of the globe, like the North American Datum 27 (NAD27). One well-used datum set covering the entire globe is the WGS-84. WGS-84 is currently the map datum for the USGS 7.5-minute maps that we use to explore most places in United States. Good GPS units will let you choose which map datum you want to use. The important thing is to makes sure the GPS is using the map datum that the maps you’re using are drawn to. If the GPS and map aren’t using the same datum set, you’re likely to be confused about where the GPS says you are.

This information about using maps and GPS functions on an iPhone comes from Mike Rolfs: The iPhone is GPS enabled and Mike uses both GPS Kit from Garafa and Trail Maps from National Geographic. GPS Kit is easiest to use and does a good job of tracking one’s route, but Trail Maps uses much higher quality maps -- it uses the same high-quality USGS maps in a 1:24,000 scale that were shown in class. Mike tends to navigate by pinpointing his current using Trail Maps and then navigating with a paper map (which he has along) and compass. Because the map on the phone and the paper map look identical, it is easy to transfer the location seen on the screen over to the paper map. For navigating to a waypoint with the GPS, Mike reports much better results with GPS Kit, but he says the interface between the phone and his PC mapping program is not intuitive so loading in waypoints from the computer takes some thought. Finally Mike reports that navigating to a waypoint with Trail Maps has not worked well for him – he’s had poor luck with the directional arrow.


Additional Resource: See Freedom of the Hills (Mountaineers Books)



Our thanks to Cascade Subaru for helping get more people outdoors in winter and for  sponsoring the series of Self-Sufficient Snowshoeing classes where this information was presented.