How do you shoot elevation with a transit?

Answer 1

Grade rods are graduated in feet, tenths, and hundredths of feet. The rod is used to determine differences between ground elevations at two points. A level of some kind is necessary.

You start from a point of known elevation or height, for example, the point along a road under construction where a cut slope will intersect the ground surface. That point will be shown on the plans and cross sections, and will be established by surveyors in the field. Usually they will pound a stake in the ground and the actual catch point will be a nail with whiskers. To check the elevation difference between that point and some other point (let's say, the point on the ground directly above where the road centerline will be) you need to occupy the lowest point with the grade rod. What you do is sight along a level line (usually with a hand level, held against the grade rod) to the point that is higher. You move up and down the grade rod until the level line of sight hits the point of known elevation -- or a point of known distance above the known point (which you can establish with a piece of survey lath, a measuring tape and a magic marker).

Let's say you are on the side of a hill and the cut slope stake is up there someplace, indicating what the vertical distance (cut) will be between that point and centerline. You don't make the vertical cut there, of course, you make the cut above centerline. But at centerline the ground surface is lower so in order to know what to cut, you need to know how much lower the ground surface is. You occupy the centerline with the grade rod, holding it vertical. You get out your hand level and, holding it against the grade rod, find the cut slope stake at ground level (or the point of known vertical distance you marked on the lath above it). You move the hand level up and down the grade rod, maintaining it level, until you are sighting that point at the cut stake. Then the number opposite the hand level on the grade rod is the elevation difference between the point you are occupying at centerline and the point you are shooting to. If you were at 6.3 ft on the grade rod and you shot to a point 2.5 ft above the catch point nail, the elevation difference between the ground where you were and the catch point is 6.3-2.5=3.8 ft. So whatever the stake says the cut to centerline is, you can subtract 3.8 ft because the ground above centerline is 3.8 ft less than it is where the stake is.

You occupy the low point with the grade rod, because you have to shoot along a level line to a higher point. If the point you are shooting to is to high to get to in one shot, you do it in steps and add them up.