Using Transits, Laser Levels and Optical Builder’s Levels
Using Transits, Laser Levels and Optical Builder's Levels
Can you imagine how you might go about trying to determine the differences in elevation between two points which are approximately 100 feet apart? Using a four foot wood or aluminum level would be very impractical. A water level might also be tough to use, as the 100 plus feet of tubing would be very tough to maneuver. Surely, one would think, there has got to be an easier way.
Well, as you might expect, there is a much easier way to determine elevation differences. You simply use a transit or a builder's level. These highly sensitive and accurate instruments (that's what they are often called in the field - instruments) are the only way to go. They are easy to set up and can be used in any weather that you can stand being in.
What is the difference between a level and a transit, you might ask? A level basically does just one thing - it establishes a level line when set up on a tripod. A transit can do a little more. It not only can establish a level line, but also a plumb line when looking through the scope or sight. Also, a transit has the full 360 degrees of a circle marked on its base. This means that when you rotate the sight around on the tripod you can do basic surveying.
How Do They Work?
Have you ever seen one of those old World War II submarine movies? Or, how about the movie Hunt for Red October? In just about every submarine movie or show, there always seems to be a scene where you are looking through the periscope. That is what it looks like when you peer through an optical level or transit. The only difference is that the crosshairs in levels and transits don't have all the markings that you sometimes see in the periscopes. In fact, when I'm in a good mood and using my transit, I'm often heard saying "Flood torpedo tubes 1 & 4! Bearing 245 degrees! Range 3,000 yards!"
Back to reality. Optical levels and transits operate in a very easy manner. Once they are placed on a tripod and adjusted (made level themselves), they project a level line as you look through the instrument. This means that any spot that you look at which is right in the middle of the crosshairs is at the same elevation as the thing you just looked at a moment before.
OK, so how can they tell the difference between two points? No problem! When used in conjunction with a grade pole (a stick or pole that is marked in feet/inches or metric units), a level or transit enables you to determine elevations with great accuracy. The method is very easy.
Here is the challenge. Let's say you want to measure the difference in elevation between two points on a hill. The first thing to do is set up the level or transit in a location so that after it is adjusted, the line of sight through the crosshairs is higher in elevation than the two points you are trying to measure.
Now, you look through the instrument while a second person takes the grade pole and places it on one of the two locations. Making sure they are holding the pole in a plumb position, take a reading. For our example, let's say the first reading is 2 feet 6 inches. Now, send the person to the second spot and take a reading. Let's say that the crosshair hits the grade pole at 10 feet 9 inches at this location. OK, what is the difference in elevation between the two spots? The difference between the two points is 8 feet 3 inches. Which spot is lower than the other? Obviously, the second location is lower in elevation than the first (assuming that the markings on the grade pole start at zero on the bottom of the grade pole.)
Turning in a Circle
The neat thing about optical levels and transits is their versatility. When placed on a tripod, these devices can rotate a full 360 degrees on top of the tripod, just like the submarine periscopes. This function allows you to set up a level and take readings in any direction. The speed of operation depends only on two things: how quickly the person with the grade pole can move from location to location; and how quickly you can locate the grade pole in the cross hairs, focus and take a reading.
Durable but Delicate
Levels and transits are made to withstand dusty construction site conditions. They can withstand getting wet. Heat and cold conditions do not bother them. However, if you drop one, you are in trouble.
These instruments are delicate with respect to impacts. The optical lenses can be knocked out of adjustment, as well as the leveling screws which are used to level the instrument on the tripod. Only professional service centers can realign an instrument.
Using a level or transit takes some practice. The method of setting up the tripod is somewhat tricky, especially on sloped surfaces. Muddy or unstable soil can be a problem. You may adjust the level and begin using it, but several minutes later, because of your moving around the tripod, the soil may move slightly. The level may now be out of adjustment and who knows how many readings might be in error.
There are some tricks that will help you achieve highly accurate readings. For example, let's say that you need to take measurements for points all in a straight line. The best place to set up the level or transit is at one end of the line, not in the middle. Levels can begin to make slight errors, if slightly out of adjustment, when you begin to rotate them on the tripod. If you are at one end of the line of points, you shouldn't have to rotate the level. All that you will need to do is focus the lens as the grade pole moves.
Checking for Errors
Checking a transit or level that is suspected of being out of adjustment is fairly easy. Locate a small 1/4 or 1/2 acre pond on a windless day. Set up your level and ask the grade pole individual to go around the pond and set the bottom of the pole right where the water hits the shore. Since the water in the pond is level, all the readings should be the same.
Let's say that you need to do some of the things we talked about, but can't find that necessary person to hold the grade pole. Once again, no problem. Within the past five years, laser levels and transits have become reliable and affordable.
These devices are great because they are designed for one person use. You set up the instrument in virtually the same fashion as an optical instrument. The difference lies in the fact that you simply turn on the laser and BINGO, a fine, thin, red beam of light begins rotating.
You take the special grade pole, which has a sliding target, and move around from point to point. The only thing you need to do is slide the target up and down the grade pole and take the readings when the laser hits the center of the target. It's that easy.
Does any of this sound like fun? If so, just about every tool rental store rents levels and transits. Go get one for an afternoon and sink some ships! Happy hunting!