Measuring slope angle.

In my Avy I class and in all of the books I have read on avalanches it is mentioned over and over that reading the terrain/topography is one of the most crucial elements in keeping yourself safe in the backcountry. Doing this of course requires being able to accurately measure a slope angle but I have had a hard time finding the most reliable way to do this. The below are all I know of so far:

1. Life-link clinometer
2. Compass with slope angle measurments.
3. Reading terrain maps and math to calculate. (Topo maps, Google Earth, etc)

Curious as to the collectives thoughts on what they use and recommend? Also how you use them, can you measure using the tools from the bottom of a pitch? How about from the side in a safe zone?

thanks

slope angles are subjective and often vary from user to user. i use both the card and a suunto. i carry the suunto on days where knowing the runout is a must. our alpha angle in utah is usually around 22*, if i recall. this year during the january slide cycle i measured a few that went into the teens. the suunto is good but i usually avoid being under the steep slopes that the suunto is so good for. the card is great for measuring the slope from the side as you are approaching or you can sight down it and have a partner read it. the other way is to use a card with a weighted string so you can hold the string on the angle and look at it yourself. but remember that it is the angle of the bedsurface that matters, so don't split hairs when measuring.

APD - Great info, thanks for the reply. Jong question, what does the term "alpha angle" refer to?

While I agree on everything in this thread, remember that using slope angle as a hazard factor is based on statistics. I've found that the mountains don't often respect our statistics and will throw a curve ball at you if you depend on them.

When we are in the feild with the crew I don't usuually measure anything but use a simple guideline that supersteep (55 deg +) is less likely to slab avalanche, mid angle slopes are the most suspect, and low angle slopes (25 deg -) are least likely. I don't need a inclinometer to make those classifications in the field.

So the problem is, is that even though the stats say slides are unlikely on the super steeps and low angle, even in my own personal experience (which is a relatively small statistical sample), I have witnessed avalanches and or had a close calls on both steep and low angles slopes.

While I definitely use angles to mitigate hazard in the field, I rely mostly on scale of terrain (can I run it out to safe area well outside the estimated alpha angle) and islands of safety (can I duck into a safe area) to mitigate my exposure to risk, combined with selecting the terrain based on as much snowpack forecasting data as I can gather.

I'm not dissing slope measurement, just pointing out some caveats. I think it's a good idea to learn to measure and estimate slope angles at some point in your career. And it's an invaluable tool for forcasters tracking long term trends and snow stability. I have just run into a few people that perhaps put a little too much faith in the statistcal probilities of avalanche activity based on slope angle.

I think using the inclinometer for measuring alpha angles (and determining safe zones out of the runout) is the most practical use I've heard yet. I'll probably play around with that some more.

Sarge

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Originally Posted by sarge1957

While I definitely use angles to mitigate hazard in the field, I rely mostly on scale of terrain (can I run it out to safe area well outside the estimated alpha angle) and islands of safety (can I duck into a safe area) to mitigate my exposure to risk, combined with selecting the terrain based on as much snowpack forecasting data as I can gather.

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thanks for the info sarge,


i understand the idea of mitigating risk but it should be mentioned that when most try to mitigate risks they often overestimate their ability while giving less thought and respect to the dragon they are dealing with. it is a little different when you are dealing with pros but when us regular folks are dealing with avy's, imitation is not the most sincere form of flattery.



when the slopes are really unstable i don't trust any slope attached to a steeper slope (ie. with really hard slabs a steep start zone connected to a mellower slope/ridge above can get pulled along with that steep slope). there are no absolutes in snow science, only guidlines at best. it's hard to remember that sometimes when we get so wraped up in our rule based decision making.

[QUOTE=AltaPowderDaze]
i understand the idea of mitigating risk but it should be mentioned that when most try to mitigate risks they often overestimate their ability while giving less thought and respect to the dragon they are dealing with. it is a little different when you are dealing with pros .....
QUOTE]

APD:

I agree with your point. I guess I forgot to mention the most obvious, and even for our crew of pros, the most used risk mitigation technique; that's to just say no.

Typically we are not on slopes if we think there is a chance of it sliding. We merely use "mitigation" techniques as a back-up or plan B. I think this is an important distinction. While there are differences between the pros and the average tourer, no amount of ability can make a significant impact on making you safer in risky avi terrain. The pros ability only gives them a slighty better chance if something does go wrong. I am actually amazed now at the patience of our crew to wait or hit more suitable terrain and not press into an untenable situation.

As for your point on rules, it is a double edged sword. We need to teach rules to the new users, while at the same time teaching all the nuances (and exceptions to the rules) of avi behavior to the more experienced users.

Sarge

Sounds like ducktherope is new to measuring slope angles. Here's a simple method. I like compasses that have an inclinometer and a mirror or as APD described the marked card and string method.

Site something down the hill that is about as tall as you are. This reading gives you an average slope angle from where you are to the object you're sighting. Keep in mind that one slope often varies in steepness, some parts being less steep and some more.

You can do this from on top or below the slope.
http://pic16.picturetrail.com/VOL682.../116514446.jpg

Ducktherope, I'm a little surprised you didn't practice measuring slope angles in your Level1. This skill should have been covered.

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Originally Posted by Oh snow

Ducktherope, I'm a little surprised you didn't practice measuring slope angles in your Level1. This skill should have been covered.

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Oh snow, thanks for the info. Measuring slope angle was covered rather briefly in class and used a clinometer on a ski pole for measurement. I have found this rather impractical in real life and was curious what others were doing. Great discussion all, thanks for the info.

I offer an excellent tool for estimating slope angle in Google Earth - slope angle overlays. The overlays will pick out terrain features down to 30x30ft. This is no substitute for measuring slopes out in the field, of course, but it is very useful for trip planning.

I like to know the steepness of my approach route, ski line, and backup/retreat routes before I attempt them, then I measure and reassess as I go.

Check out my overlays:
www.SlopeAngleMaps.com

I think slope angle's importance cannot be overstated. For example: yesterday I was skinning up an isolated, slightly convex roll, with a few faceted weak layers and crusts in the top of the snowpack. The angle was just about 27 degrees. I knew it was very possible that the layers could collapse and the snow could slide, but I also knew that at this angle there would more likely just be a whumph and some cracks, and that if the slabs slid, they would move slowly. This was very low-consequence terrain, so I was comfortable knowing that there could be a shallow, slow-moving slide.

Sure enough, WHUMPH!, a few cracks shot out, we listened to it propagate around the slope and across to another slope, but that was it. No movement. No slide.

This was a borderline case, where it was just barely not steep enough to move. Had it been 2 or 3 degrees steeper I would not have been there. If you're careful and accurate about how you measure slope, you can predict the behavior and likelihood of a slide and use that to your advantage.

There are a few other considerations that should go into the slope angle discussion. What type of slide are you worried about (Hard Slab, Soft Slab, Wet Slab, Point Release). Each one of these conditions has a different angle of repose. Wet slabs as an example can and will release on slopes of 30 degrees or less.

Slush = Often danger to 20-30 degrees
Wet Grains or Graupel = often danger 30 degrees
Cold, unsintered low density grains = often danger up to 45 degrees
Sintered Grains = often danger upto 55 dgrees

This is why you will often see a high angle of repose in the costal ranges of Alaska where snow is sticking to 55 degree slopes with decent stability just sloughing, whereas in CO you will have hard slabs kicking loose on 36 degree slopes and in Baldy Bowl in Southern CA you will see saturated Wet Slabs releasing on 30 degree slopes.

Keep in mind, any of the these locations could have conditions that are unusual for the location. As an example, Mammoth Mountain had a Continental Snowpack early in the season with very cold, low density snow buried under a crust just rotting away. The results were very large deep slabs releasing to the ground until this condition changed sometime in December.

FWIW there is a mobile phone app called Theodolite Pro that measures slope angles. It needs a phone with built-in gimbals, which is standard on iPhone and most new Android devices.

As with all technology, you still need to understand what you are measuring, how you are doing it, and how to interpret the results.

Quote:

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Originally Posted by Oh snow

Site something down the hill that is about as tall as you are. This reading gives you an average slope angle from where you are to the object you're sighting. Keep in mind that one slope often varies in steepness, some parts being less steep and some more.

You can do this from on top or below the slope.
http://pic16.picturetrail.com/VOL682.../116514446.jpg

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Averaging slope angles from above like this turns out to be a poor practice, because it is the steepest section (usually longer than 20m) is where a slide is most likely to be triggered. In the graphic above, it would be the 35 degree portion. The difference between 30 and 35 degrees can be critical. By averaging the slope and calling it 30 degrees, you are ignoring important information.
Better to carry a compass with a clinometer, a dedicated clinometer or an IPhone with the Mammut app. The guess the angle of many slopes as you are touring, and measure them to see how your guess was. In time you will calibrate your eye. Estimate and verify.

I like to play a game with touring partners in which we both guess slope angles and then see who is closer. After a few days of this, you get really good. I can usually guess angles within the margin of error of my slope meter. The tricky part is doing this in low contrast conditions. It's also a great game to play with people who are less fit than you. They appreciate the quick break.

For a quick measurement at a given location you can use your poles. Adding tick marks and angles to your poles is also and option:

http://www.slidewright.com/Misc/Slope-per-ski-pole.JPG

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Originally Posted by GaperPatroller

Averaging slope angles from above like this turns out to be a poor practice, because it is the steepest section (usually longer than 20m) is where a slide is most likely to be triggered. In the graphic above, it would be the 35 degree portion. The difference between 30 and 35 degrees can be critical. By averaging the slope and calling it 30 degrees, you are ignoring important information.

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Total avy jong here, but isn't that kind of the crux of the matter? If you have higher angle pockets within a generally low angle slope, isn't that entire slope just as prone to slide if triggered in the steep pitch? In practice, how easy is it to pick out a 30m section of 35* + on a "mostly" 30* slope?

I guess at the end of the day it's irrelevant. If your decision criteria for riding a slope is coming down to that minute of a detail, then you probably have bigger problems.

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Originally Posted by adimmen

I like to play a game with touring partners in which we both guess slope angles and then see who is closer. After a few days of this, you get really good. I can usually guess angles within the margin of error of my slope meter. The tricky part is doing this in low contrast conditions. It's also a great game to play with people who are less fit than you. They appreciate the quick break.

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I do this as well - even just by myself when touring. I generally find my margin of error on the steeper side (e.g., I think slopes are 3-5 degrees steeper than I suspected). I just keep my Ranger in my front pocket at all times, and pull it out and bend down and measure the slope angle next to the skin track really quickly to verify my assumption.

I think OGRS says to record the incline of the steepest pitch in the avalanche starting zone (excluding cliffs) averaged over a fall-line distance of 20m.