Will a clinometer automatically adjust for Slope?

[doctormink]

I want to take advantage of all the knowledge here on this forum. If I have a clinometer that uses a bottom reading and a top reading to calculate tree height does it matter if there is any slope between me and the tree? If so how much slope does it take to seriously affect the height measurement?

[SwampDonkey]

Yes you have to correct for the horizontal shot from your eye to the tree bole. Lets say your slope distance to the tree is 66 feet. Here is a range of corrections every 10 degrees of slope.

Slope%  Horizontal Distance
10                          65.0
15                          63.8
20                          62.0
25                          59.8
30                          57.2
40                          50.6
50                          42.4
60                          33.0

Hor. D= Cos(θ) x Slope Distance

Tangents above and below that horizontal line to the tree (0 % in clino) get you total height. Add tangents if up slope and subtract if down slope of tree, before multiplying by the horizontal distance. Maybe you already know. When the slope is such that you can measure with tape from the ground up to the horizontal position on the tree to which you took a clino shot, you can add the top tangent to the bottom height section below the horizontal line. Always best to be up slope or on the same contour as the tree base. Clinos loose accuracy above 100 % top shot because you can't really see the top too well over 100 %.

[doctormink]

Thanks Donk

[ID4ster]

What type of units does your clinometer have? It should have two scales. They are usually degrees/percent, percent/topogs, and rarely topogs/degrees. You can correct for slope the way that Swampdonkey suggests or you can use another simpler method (In my opinion anyway) Let us know what scales are on your clino and which one you use to measure trees.

[doctormink]

I believe my clinometer has percent and degrees. I also have an electronic one that reads just in feet.

I think there is a mathematical rule that says slope distance is always more than horizontal distance, so why would you add for a downhill slope and subtract for an uphill slope?

[doctormink]

Or why would you add for an uphill slope and subtract for a downhill slope?

[SwampDonkey]

The tangents have nothing to do with slope correction, only the fact you multiply the difference in tangents by the horizontal correction.

The actual formula for tangents is:

Total Height = Horizontal Distance x {tan (top shot) - tan (bottom shot)}

When you are up hill from the tree your bottom shot is negative, a double negative is a positive, so you add. When your down slope the bottom shot is positive, so subtract.

Now what you can do on ground under 20 % slope, where your horizontal eye level is at tree base level or up the bole as far as you can reach, giving 0% slope at that point, is take a short cut. This shortcut says if you stand at a distance from the tree where the top shot is 100 %, all you need to do is measure with your tape from where your standing, level to the spot on the tree where you saw 0% slope in your clino, add that to the height above tree base to this spot on the tree for total height.  This makes a 45 degree angle whose tangent is 1, so horizontal distance is the height above the horizontal line (0 %) of site. With a clino scale of 66 feet or 20 m, the principle is the same, except when your top shot is 66 feet or 20 meters your at the 45 degree angle and your 66 feet or 20 meters from the tree. The shortcut method is convenient if you have no big hillsides, it still uses tangents only there is no need to dig out the calculator. ;D

Another shortcut for real steep ground is to find the spot on the hillside where your making a 45 between bottom and top of tree from your view point on the hill, calculate horizontal correction, from slope distance to tree base, and double it for height. To me it's quicker to just use the old way instead of farting around finding that position on the hill. It's would be less challenge if the hill was constant slope. Most around here round over like a cow bell and/or have several ground breaks up the slope.

[doctormink]

Thanks Donk, all that trigonometry is a little bit much for me, but I get the gist of it.