The person in charge of the leveling instrument should give some thought to where it will be positioned before setting it up. The tripod should be set on firm ground so that the person using it has secure footing and can conduct the level circuit.
The level and tripod are placed in a desired location with the tripod legs spread well apart and firmly pressed into the ground by standing on the feet at the base of the tripod. The tripod head should be nearly level with the telescope and at a convenient height for sighting. On hillsides, place one leg of the tripod uphill and two downhill for better stability. If the tripod has a domed head, the instrument can be shifted so that the circular bubble is near level. Coarse level adjustment can be obtained by using the foot-screws on the base so that the circular bubble can be carefully centered in the middle of its setting circle.
The level must first be adjusted so the crosshairs are in focus for the operator, which requires the following steps. Point the telescope towards a uniformly light-coloured surface, or a sheet of white paper. Turn the telescope eyepiece until the reticule crosshairs appear sharp and absolutely black. Turn the eyepiece slowly until the image starts to go out of focus. A small rotation in the opposite direction will refocus the hairs correctly. This setting corresponds to the technician's eyesight; it is constant, but individual to each technician.
The telescope can be pointed roughly at the leveling rod, by looking along the open sight and turning the instrument by hand. Set the vertical hair of the reticule cross along the middle of the rod image by turning the horizontal adjustment screw. Turn the focusing knob until the image of the rod graduations are sharp. The observer should move his/her eye up and down and from side to side behind the eyepiece, to ensure parallax does not exist (there should be no movement between the staff image and the reticule cross when viewed in slightly different positions). If such parallax is observed, the instrument must be refocused, as described in the previous paragraph. In the case of self-leveling instruments, the line of sight is now horizontal and the rod is ready to be read.
With a tilting level, the line of sight must be set horizontally using the tubular bubble. Position the fixed reflector for optimum illumination of the tubular bubble as seen through the bubble viewing eyepiece. The split bubble image seen in that eyepiece must be set to coincidence, by turning the tilting knob below the eyepiece. When the bubble ends are far from coincidence. an arrow in the bubble image indicates the proper direction to turn the tilting screw to obtain coincidence. The split bubble should be checked before each reading of the rod.
The rod is positioned properly by placing it vertically on the point of a stable object. The rod must then be held plumb. The instrument person can tell if the rod is plumb in one direction by observing if the rod is parallel to the vertical cross-hair, but he/she can not tell whether it is tipped forward or backward. Therefore, the rodperson should use a rod level to plumb the rod in this direction. If the rodperson does not have a rod level, plumbing of the rod can be accomplished by balancing the rod between two fingers (if there is no wind). Waving the rod slowly towards and away from the instrument and observing the lowest reading on the rod is another method of ensuring that the rod is vertical, as in Figure E-2.
Figure E-2. Positioning the rod.
Errors created by not holding the rod plumb are much greater for readings taken near the top of the rod than for those taken at the bottom.
When the instrument is properly leveled and the rod is in a vertical position, the technician observes and records the reading indicated by the horizontal crosshair on the rod as seen through the telescope. To verify the accuracy of the reading, observe the split bubble and the rod reading again.
The telescope reticule has stadia lines, one above and one below the centre crosshair. The average of the readings of stadia lines on the rod represents the actual horizontal reading. The difference of the two stadia readings multiplied by 100 represents the distance between the instrument and the rod (Figure E-3).
Figure E-3. Reading the rod height 3.456 m, distance 29.2 m, (Source: Automatic Level, Wild NAO, Instructions for Use, Manual G2 106e - IX.80, p.9).
Normally, rod readings to 0.002 m are sufficient. Reading to 0.001 m implies a degree of accuracy that does not exist and is generally inconsistent with the sensitivity of the equipment used to gather other stage data.
Recording accurate and complete field notes is the most important part of the leveling operation. Notes and sketches constitute a permanent record of the survey, and it should be possible for them to be interpreted with ease by anyone having a knowledge of leveling. Non-standard notes that can be interpreted only by the note keeper are unacceptable.
The format for recording level notes during routine level checks of gauges and normal line or differential leveling between bench marks (with backsights and foresights approximately balanced) is shown in Figure E-4.
Figure E-4. Level notes between two bench marks, Muddy River at new gauging station.
Hydrometric level note forms are divided into columns for recording observations and computing elevations. The lower part of the level sheet, or an additional sheet, is used for recording the necessary descriptive notes that must accompany the observation sheet.
When recording notes, enter the elevation of the bench mark or reference mark on the top line in the column headed Elevation. The bench mark identification number is entered in the column headed Station, with any descriptive information recorded in the space at the right. Should it be necessary, an additional description sheet can be attached to the notes. On the top line, in the column headed BS (backsight), the reading obtained with the leveling rod held on the benchmark or point of known elevation is entered as the backsight. The value for the column height of instrument HT.INST. or HI is computed by adding the backsight value to the known bench mark elevation. One line down, in the next column, headed foresight (FS), the foresight reading for the point for which an elevation is to be determined is observed and recorded. Commonly called a turning point (TP), this value is then subtracted from the HI and the result is the elevation of the foresight point. This is entered in the elevation column and on the same line as the foresight just observed. When the instrument is moved, the new height of instrument is determined by a backsight on the TP. The observation and notes are continued in this manner until the circuit is closed by leveling back to the original bench mark.
Level notes may be recorded in a notebook or on any sheet of paper, as long as the above method is applied. Using this format of note keeping, the information on each horizontal line pertains to the bench mark or turning point noted in the station column. A typical set of level notes between two bench marks should be recorded as illustrated in Figure E-4, and for direct water level in Figures E-5 and E-6. Note that Figure E-5 is a special case (no surge in water level during the level run, and the elevation of the turning point rock being identical to the water elevation). It is preferable to use the approach shown in Figure E-3 with the cut on the rod being indicated.
Note: Something solid (e.g., a section of pipe) can be driven into the stream of the river bed for a turning point, if a suitable natural turning point can not be found.
Note: When measuring the water level with a rod and level, the rod can be set on a fixed point under the water surface. The water level can be read on the rod like on a staff gauge, using care to observe any water level surging and to read the mean water level correctly.
Figure E-5a. Level notes for direct water level, Clear River at Little Bend.
Figure E-5b. Level notes for direct water level (submerged TP), Clear River at Little Bend.
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Figure E-6a. Example 1: Level notes for staff gauge A, Clear River at Little Bend. |
Figure E-6b. Example 2: Level notes for staff gauge B, Clear River at Little Bend. |
Figure E-6c. Illustration of corrections to staff gauges.
