How much wire mesh is needed to fence a plot?
The Sunday Nation
Nairobi,
15 December 2013
James Wainaina is trying to figure out how much wire mesh he needs for his “small 2-acre shamba”. He writes; “I need to fence off my land to stop neighbours from encroach into it. However, it is very far away and I will not be able to supervise the work personally. Can you calculate for me the length of fencing wire mesh I should buy?”
Unfortunately, James, I cannot help you! The reason is that there is no mathematical formula relating the area of a surface and its perimeter. To work it out, we also need to know the shape of your shamba, and its inclination, that is, the amount of slope.
But, I must explain why I am not able to help you. I will use a typical residential plot in a town. Most of these are surveyed as 50 feet by 100 feet – the so-called “one-eighth of an acre” (even though it is actually about 8% smaller than eighth!). To fence this plot, you would need 100ft + 50ft + 100ft + 50ft of wire mesh. That is 300feet.
Now suppose you had two such plots lying side-by-side with the 100-foot lengths touching. You have double the size of land (a “quarter acre”), but do you need double the length of fencing wire?
Your plot is now a square measuring 100ft by 100ft, therefore you need only 400ft of fencing wire (100 + 100 + 100 + 100). This is much less than double the length required for one plot – 300 x 2 = 600ft.
And things can get even more interesting: suppose that the two plots were lying with the short (50ft) sides touching. The combine length is now 200ft and the width is 50ft. So you would need 200 + 50 + 200 + 50 = 500ft of fencing wire.
But when land is surveyed into so-called quarter acres, the surveyors do it in a way that makes the length double the width. Thus the plots come out as 70ft by 140ft. In that case, the fencing wire required will be 420ft long (70 + 140 + 70 + 140)
So, for a quarter-acre you might need 420ft or 500ft or 600ft of fencing wire depending of the shape of the plot. And that is not the end of it…
If your “small 2-acre shamba” is lying on a slope, matters become a lot more complicated. As explained in this column in September last year [2012], land areas are measured on a horizontal plane. Thus if the plot is on a hill, it will have longer measurements than another of same area on flat ground; but their acreage will be exactly equal!
To find out what the lengths are, we need to know the gradient of the slope, then we apply Pythagoras Theorem to calculate the distance along the ground and…. That’s too complicated; isn’t it easier to just go to the land with a roll of sisal thread and measure it out?
The long and short of it is that anyone telling you that they can calculate the perimeter without visiting the site will be misleading you!
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