Center for Operational Oceanographic Products and Services staff member conducts tide station levelling in support of the US Army Corp of Engineers in Richmond, Maine. The spirit level is on a tripod with sight lines to the two points whose height difference is to be determined. Subtracting the “back” and “surveying and levelling pdf free download” value provides the height difference.
If the instrument is placed equidistant from the two points to be measured, any small errors in its adjustment and the effects of earth curvature and refraction will tend to cancel out. A rod or staff is held vertical on that point and the instrument is used manually or automatically to read the rod scale. The procedure is repeated until the destination point is reached. It is usual practice to perform either a complete loop back to the starting point or else close the traverse on a second point whose elevation is already known. The closure check guards against blunders in the operation, and allows residual error to be distributed in the most likely manner among the stations. In double-levelling, a surveyor takes two foresights and two backsights and makes sure the difference between the foresights and the difference between the backsights are equal, thereby reducing the amount of error. Double-levelling costs twice as much as single-levelling.
The curvature of the earth means that a line of sight that is horizontal at the instrument will be higher and higher above a spheroid at greater distances. The effect may be significant for some work at distances under 100 meters. The line of sight is horizontal at the instrument, but is not a straight line because of refraction in the air. The change of air density with elevation causes the line of sight to bend toward the earth. For precise work these effects need to be calculated and corrections applied. For most work it is sufficient to keep the foresight and backsight distances approximately equal so that the refraction and curvature effects cancel out. Refraction is generally the greatest source of error in leveling.
For precise leveling networks on a national scale, the latter formula should always be used. The wye level is the oldest and bulkiest of the older style optical instruments. More compact and hence both more robust and easier to transport, it is commonly believed that dumpy levelling is less accurate than other types of levelling, but such is not the case. Dumpy levelling requires shorter and therefore more numerous sights, but this fault is compensated by the practice of making foresights and backsights equal. Precise level designs were often used for large leveling projects where utmost accuracy was required.
They differ from other levels in having a very precise spirit level tube and a micrometer adjustment to raise or lower the line of sight so that the crosshair can be made to coincide with a line on the rod scale and no interpolation is required. The purpose of Wikipedia is to present facts, not to train. The surveyor sets the instrument up quickly and doesn’t have to relevel it carefully each time he sights on a rod on another point. It also reduces the effect of minor settling of the tripod to the actual amount of motion instead of leveraging the tilt over the sight distance. Three level screws are used to level the instrument. Set up the tripod at just above chest height. Make sure it is stable, and mount the level on the top.
Adjust the leveling screws until the pond bubble is centralised. As long as the pond bubble is central, the automatic compensators are able to finely level the instrument. To ensure this is the case, whilst looking through the scope, gently tap the level. The view will waver for a few moments before steadying. If this does not happen, the instrument is not level enough for the compensators to cope, and needs adjustment. Sight towards the staff using the gun sight. Look through the eyepiece and focus the reticle by gradually turning the reticle focusing ring anti-clockwise.
Turn the focusing knob to focus on the staff. Turn the fine motion screw to centre the staff in the field of view. Turn the focusing knob to eliminate parallax between the staff and reticle. The staff starts at zero, on the ground. The stems of the E or 3 and the gaps between them are each 10mm high.