Leveling instruments

Leveling instruments

Older instruments:

The wye level is the oldest and bulkiest of the older style optical instruments. A low-powered telescope is placed in a pair of clamp mounts, and the instrument then leveled using a spirit level, which is mounted parallel to the main telescope.

The dumpy level was developed by English civil engineer William Gravatt, while surveying the route of a proposed railway line from London to Dover. More compact and hence both more robust and easier to transport, it is commonly believed that dumpy leveling is less accurate than other types of leveling, but such is not the case. Dumpy leveling requires shorter and therefore more numerous sights, but this fault is compensated by the practice of making foresights and back sights 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.

Automatic level

Automatic levels make use of a compensator that ensures that the line of sight remains horizontal once the operator has roughly leveled the instrument (to within maybe 0.05 degree). The surveyor sets the instrument up quickly and doesn't have to relive 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.

1)  Gun sight

2)  Circular level (pond bubble)

3)  Leveling Screw

4)  Base Plate

5)  Objective Lens

6)  Focusing Knob

7)  Horizontal fine motion screw

8)  Horizontal circle window

9)  Horizontal circle setting ring

10)  Reticle adjusting screw cover

11)  Eyepiece

Setting up an automatic level

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 centralized. 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.

Sighting

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 leveling staff

Reading the staff

The staff starts at zero, on the ground. Every 10 cm is a number, showing (in meters to one decimal) the height of the bottom of what appears to be a stylized E (even numbers) or 3 (odd numbers), 5 cm high. The stems of the E or 3 and the gaps between them are each 10mm high. These 10mm increments continue up to the next 10 cm mark.

To read the staff, take the number shown below the reticle. Count the number of whole 10mm increments between the whole number and the reticle. Then estimate the number of mm between the last whole 10mm block and the center of the reticle. The diagram above shows 4 readings: - 1.950, 2.000, 2.035 and 2.087.

The person holding the staff should endeavor to hold it as straight as possible. The leveler can easily see if it is tilted to the left or right, and should correct the staff-holder. However, it cannot easily be seen that the staff is tilted towards or away from the leveler. In order to combat this possible source of error, the staff should be slowly rocked towards and away from the leveler. When viewing the staff, the reading will thus vary between a high and low point. The correct reading is the lowest value.

Digital levels electronically read a bar-coded scale on the staff. These instruments usually include data recording capability. The automation removes the requirement for the operator to read a scale and write down the value, and so reduces blunders. It may also compute and apply refraction and curvature corrections.

Laser level

Laser levels project a beam which is visible and/or detectable by a sensor on the leveling rod. This style is widely used in construction work but not for more precise control work. An advantage is that one person can perform the leveling independently, whereas other types require one person at the instrument and one holding the rod.