Friday, 2 January 2015

Finding contour on uneven land with an A-frame level

One of the things that permaculture advises in order to ensure water, or rainfall, stays on your land and doesn't flow out, is to dig swales. Swales are ditches that run on contour, and the idea is that these fill up, and the water soaks in and flows down into the ground under your land, rather than out onto your neighbours.

The simplest way to work out the contour of your land, is with an a-frame level. This is simply two poles (the vertical strokes of the A) crossed with another at equal angles on both sides (the horizontal stroke of the A). A weight is hung from the top (and center) of the frame and if the two pieces of land the legs are sitting on are level, then the weight sits in the center of the horizontal pole. Moving a leg around when the weight is not in the center, might find the point at which contour continues (it has a level slope).

The problem is that in practice it just isn't a simple task where you swing the legs around, find contour, swing the next leg around, and repeat all the way across the field. Land, unless it has already been unnaturally cultivated, is bumpy. I've made an a-frame level, and having gone out in my paddock, the bumps simply don't suit it. And I'm not the only person who wonders if they are doing it wrong.

After a few confusing non-answers from people which clarified nothing, Dan Grubbs came along with the definitive answer.

Maybe I'm missing something, but if I understand both your and Susanne's problem is based on the A-frame level being at a fixed distance and not able to span across a small dip or bump. Some of my suggestions will depend on the scale we're dealing with.

I would suggest if you are wanting to peg out a contour line on very "bumpy" ground I would use a bunyip with at least 20 feet of tube between the stakes or use a laser level. This way you can vary the distance between the contour pegs you put in the ground to accommodate (go past) the bumps and tire tracks.

Remember, contour lines aren't something that go where you want them to be, there are what they are, imaginary lines made up of points along that line that are at the exact same elevation. With this understanding, bumps aren't something that aren't in your way, they form part of the landscape and cause your contour line to curve. Now, as JC suggests, you can level them out. But, if you can put in your contour pegs at different distances, say every 10 feet until you see that the bump would cause the level to swing to what appears to be down hill, you can stretch your bunyip level or laser post farther than the bump or tire track (maybe 15 feet) and find that contour point on the other side of it and then you simply have a contour line that you can imagine going though the bump that you will come back and level out as JC suggests.

Susanne: As I mentioned above, I would simply stretch my bunyip across your beds and find where the contour line continues on the other side. If you only have an A-frame level, go ahead and simply clean out the soil in your bed only big enough to accommodate where the leg of the A-frame level will reach and only deep enough to allow the tool to indicate level. Make your best guess where it would naturally fall if there were no raised bed. Don't worry, you're going to find the level on the other side. Then, flip your A-frame level to get to the other side of the bed and mark the next contour peg on the other side of the bed.

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