How To Navigate Along a Contour Line on a TOPO Map
By Brian O'Connor
From time to time, we're asked to search along a contour line or two ("out" on one, "back" on the other).
That sounds easy enough, but it's a little more complicated than one might suspect. For example, even the best GPS barometric altimeter (like the one in Garmin's 60CSx) is accurate only to ± 70 feet, which is almost two 40-foot contour intervals. And if you're on a gentle slope, and it takes 200 feet of horizontal travel to climb 40 vertical feet, you could be way, way off. (The altitude computed by a standard GPS unit, which calculates elevation from satellite transmissions is even less accurate [± 150 feet].)
Then there's the problem of GPS lock. If you're under dense cover and don't have 4 satellites locked up, you won't (by definition) have a 3D position fix (including altitude), and you may not even have enough satellites for a 2D fix.
Jim McNutt and I were talking about such things, and how a Type 15 compass might be used to follow a contour line using the compass's pendulum clinometer, and Jim suggested shooting what amounts to an elevation bearing in much the same way you'd shoot a regular compass bearing.
I thought about that, and did some tests, and here's what works:
1. Ignore declination offset, and set the bezel to 360 degrees.
2. Hold it like you were going to take a bearing (mirror open and on top, needle reflected, etc.).
3. Rotate the compass and mirror 90 degrees (clockwise, if you're right handed) so that the sighting notch is 90 degrees from where it normally would be (at 3:00 o'clock, not 12:00 o'clock).
4. Now — look at the inclinometer in the mirror, and align its line with the 90 degree line of your declination scale, using the mirror's sighting line just the way you'd shoot a bearing! (You can ignore the needle and the lines etched in the capsule floor.)
5. Find an object through the sighting notch, and you've just found your contour line (you can turn your body right or left, to find a convenient target, which you then walk to)!
Caption: Notice that the compass is on it's right side; where the sighting notch is, and that the clinometer is aligned with 90 degrees on the declination scale. (In this photo, the sighting line on the mirror is a little off-off center — it should pass directly over the needle's pivot point — because it was impossible to get the camera perfectly oriented.)
Hiker Hell "This blog is about learning from other people's mistakes, so you don't make the same ones."
Many stories of how people on hikes got into trouble- the kind of trouble that leads to searches or recoveries.