Contour Interval & Slope Angle

[cold_smoke]

I know I learned a really easy way to determine the angle of a slope using a 7.5' USGS map. Some kind of rise over run, I think by counting out 5 lines
(100ft) and comparing that to the distance covered...

can someone refresh my memory? thanks.

[zion zig zag]

I know that (rise/run) x 100 = angle

So just measure out the slope you need (my compass has a ruler on the edge) and count the contour lines for that distance.

Sorry, prolly not the easy trick you were looking for.

[cold_smoke]

Thanks flykdog, with your help I think I figured it out.

if anyone want to know....

1/2in=1000ft. Count how many contours you have in a 1/2in(1000ft) span, multiply that # by 20ft to get vertical drop, divide by 1000ft and multiply by 100.

e.g. 10contours in 1/2in. 10x20=200, 200/1000=.2 x100 = 20*

*** Keep in mind that the slope has to be consistent over the span of your measurement for this method to be accurate and this only works with a 7.5min. (1:24,000) USGS map***

[lemon boy]

my life link inclinometer has a little scale that you can match to the lines that gives approximate angles and is super easy.

[Conundrum]

You can use your blue book which has a scale on one of the reference pages in the back. I can't remember the name of the book but it's the rose stability and snow pit recording book.

[homerjay]

I know that (rise/run) x 100 = angle

So just measure out the slope you need (my compass has a ruler on the edge) and count the contour lines for that distance.

Sorry, prolly not the easy trick you were looking for.

rise/run x 100 = gradient, not angle

100'/100' is a 45 deg. angle but a 100% slope

[zion zig zag]

rise/run x 100 = gradient, not angle

100'/100' is a 45 deg. angle but a 100% slope

Yeah, i typed that too fast, thanks for the correction.

And lemon, my inclinometre has that also, who knew?

[flatNshallow]

am I being dense or couldn't the the formula be simplified:

instead of:
10contours in 1/2in. 10x20=200, 200/1000=.2 x100 = 20*

since (x/1000)*100 = x/10

therefore

10contours in 1/2in. 10x20=200, 200/10 = 20*

was that what you were thinking of?

[Cirquerider]

To calculate slope angle use the inverse tangent of (rise / run) Inv Tan.
For example 200 vertical / 1000 horizontal = .2 (20% slope gradient) Inv Tan = 11.3 degrees.
Or 200/200= 1.0 (100% slope) Inv Tan = 45 degrees.

[cold_smoke]

I knew that SOH CAH TOA shit came into play somewhere....Thanks all for the clarification.

[pde20]

my life link inclinometer has a little scale that you can match to the lines that gives approximate angles and is super easy.

Those things, in theory, are great. I find that the lines are so miniscule and the distances between lines are so tight that you can't really tell the difference well enough for them to be effective.

My question is, particularly for those of you who have NGS Topo or other software. What is the relationship between slope grade and slope angle? I am maths retarded.

[VC]

I knew that SOH CAH TOA shit came into play somewhere....Thanks all for the clarification.

You just gave me some very bad flashbacks

[Cirquerider]

What is the relationship between slope grade and slope angle? I am maths retarded.

You aren't going to like this...Slope angle (degrees) is the inverse tangent of slope grade (percent). Click on your computer's calculator and select the scientific functions (view). The Inv (checkbox) Tan (button) function is right there.

[Telenater]

Not that you guys are really interested, but I wouldn't trust and slope angle calculated from a USGS quad too much. It's probably accurate in a relative sense so that you can tell steeper from less steep, but I wouldn't trust it too far.

Use and inclinometer with a guage, or calculate a guage yourself before going into the field.

Also, be ware, some quads (special series quads rather than formal USGS 7.5 minute topographic quadrangles) can have different contour intervals.

[cj001f]

You just gave me some very bad flashbacks

You learned Sine, Cosine and Tangent via Oscar Had A Hit Of Acid as well?

[TeleAl]

What is the relationship between slope grade and slope angle? I am maths retarded.

Slope angle = InverseTan(Rise/Run)
make sure calculator is in degrees, not radians.

Slope gradient = (Rise/Run) * 100
the answer is then in %.

Note: Rise = delta Y. Run = delta X.

Hope that's right, I am a Math(s) teacher.
And it seems I agree with the above, just saying it differently.

[Hacksaw]

Obviously, maps can be useful for trip/tour planning. But, I don't depend upon them for figuring my slope angles. You can often have avalanche potental terrain (small slopes, but still big enough to kill you) "hide" between 40' contour lines.

Its also a very good idea to look at your maps contour interval. When I went to Baffin Island we looked at our maps the first night in Pangnirturng and picked out some "interesting" terrain to ski. The next day, we flew out to the area and found 300' cliffs in the middle of the bowl we wanted to ski.

No one had noticed that the map had a 500' countiur interval!

Cheers,
Halsted