I'm taking surveying, and I'm going to take statics next semester. Several days ago I looked at the statics book. One of the early problems is to use Table 1 (conversion factors, inside front cover) to express my height in millimeters, my mass in kilograms, and my weight in newtons. My bathroom scale reads in kilograms and the weight of a kilogram in newtons is built into the calculator, so I don't need the table.
The statics book (from what I read of it) and the surveying book both teach that if you are given a problem in a certain measuring system, you calculate in that system. The professor I have for both surveying and engineering teaches that way. The way I learned is that you convert everything to metric and then calculate. To be precise: If a calculation uses a formula in which a constant depends on the measuring system used, write the formula in metric, and convert all quantities to metric before using the formula. Two examples: Soil calculations, which I talked about a few months ago, involve finding how much of the void space in a soil is water. The volume is in cubic feet and the masses are in pounds. The method I used is to convert the cubic feet to liters, using a number that can be read off many measuring tapes, convert the pounds to kilograms, using a number found on many packages, and use 1 for the density of water. The method the prof used uses the density of water in pounds per cubic foot, which I never heard of before. It's fewer numbers, but less familiar ones. In surveying long distances, one needs to correct elevations for curvature and refraction. The correction is proportional to the square of the horizontal distance. The book gives three constants: one for curvature, one for refraction, and one for the sum. Each has three values: one if the distance is in kilofeet, one if it's in miles, and one if it's in kilometers. That's six numbers to memorize (not nine, since three are the sum of two of the other six). To do this problem by converting to metric, you need four numbers: one to convert feet to meters (also used in the soil calculation), one to convert miles to kilometers, the curvature constant, and the refraction constant. The curvature constant is 0.0785, which not coincidentally is close to π/40. So with more and more formulas, the advantage of calculating everything in metric is obvious. You need to remember fewer formula constants. You may need more numbers per calculation when presented with English units, but the extra numbers are conversion factors, which are used in all the calculations when data are in English units. When calculating in English units, you still need conversion factors. Why then do the books and the prof teach doing calculations in English units? Pierre
