Eye levels are what the name suggests, a device you can look through which indicates a level plane. The level of accuracy varies but generally they can be relied on in situations where exact leveling is not required. There are two types manufactured.
One has a bubble in it that works like a normal builders level, the other has a degree wheel in it. The degree wheel type are generally more expensive, but they have the added advantage of being able to measure slopes in either degrees or percentages. These measurements can be used to determine the height of objects, like a tree. For an explanation of this click here!
To use a bubble type eye level you look through the eye piece, inside there will be a mirror, which will show a bubble being intersected by a line. When the line intersects the bubble exactly in the middle, the instrument is level and a reference point or benchmark can then be pinpointed on an object nearby.
The easiest way to use the level is to stand on an area that has been determined to be the desired grade of the excavation and locate a reference point from that spot by looking through the level. You can then mark that spot with a paint can or anything that will be easy to see from any point in the excavation. One can then simply walk to various points in the excavation and site back to the marked reference point and quickly see whether they are standing above or below the benchmark, if the mark is lined up with the line in the new spot then the point at which you are standing is level with the point at which you set your benchmark.
It should be pointed out that these type of eye levels will be fairly accurate when used in a relatively small area, like the size of a normal house excavation. It is also good practice to place the benchmark in a central location. This will help to compensate for the margin of error in the eye level if one exists. An eye level way out of adjustment will still give an accurate reading if the two sightings are made at the same distance from the benchmark. part 2