Common Terms Used in Leveling

Common terms used in leveling (see fig. 1-21) are bench mark, turning point, backsight, foresight, and height of instrument.

A bench markis a point of known or assumed permanent elevation. Such points may be marked with a brass pin or a cap set in concrete, a cross or square mark cut on concrete, a lone metal stake driven into the ground, a specifically located point on a concrete bridge, culvert, or foundation, or similar objects that are not likely to be disturbed.

Temporary bench marks (TBM) are points of known or established elevation usually provided for convenient reference in the course of surveys and construction when permanent bench marks are too far away or are inconveniently located. Such bench marks may be established on wooden stakes set near a construction project or on nails driven into

trees. Bench marks on trees will have more permanence if set near the ground line where they will remain on the stump if the tree is cut and removed. Federal, state, and municipal agencies and private and public utility companies have established bench marks. These bench marks are located in nearly all major cities in the United States and at scattered points in less populated areas. They are generally bronze caps securely set in stone or concrete with elevations referenced to mean sea level. Their primary purpose is to provide control points for topographic mapping. They are also useful as points from which other bench marks may be established for public or private projects. Such bench marks should be used, when convenient, for the more important surveys. Caution should be exercised

in using existing bench marks in areas of subsidence due to mineral or water removal.

is determined in the process of leveling, but which is no longer needed after necessary readings have been taken. A turning point should be located on a firm object, whose elevation will not change while moving the instrument setup. A small stone, fence

post, temporary stake, or axe head driven firmly into the ground usually is satisfactory.

A backsightis a rod reading taken on a point of known elevation. It is the first reading taken on a bench mark or turning point, and is taken immediately after the initial or new setup.

A foresightis a rod reading taken on any point on which an elevation is to be determined. Only one backsight is taken during each setup; all other rod readings are foresights.

of sight. It is determined by adding the backsight rod reading to the known elevation of the point on which the backsight was taken.

A turning pointis a point on which the elevation

Height of instrumentis the elevation of the line

Lesson 18

Setting Up the Level

Before attempting to set up the level, be sure that the tripod wing nuts have been tightened so that when held horizontally each leg will barely fall under its own weight. Next, holding two tripod legs, one in each hand, place the third leg on the ground.

Using the third leg as a pivot, move the held legs until the footplate is nearly horizontal. Then, without altering the horizontal position of the footplate, lower the two legs to the ground. Apply pressure to the legs to ensure a stable setup. Be sure that the tripod legs are spread at such an angle that the tripod is stable and that objects may be viewed through the telescope from a convenient posture.

For a level with four leveling screws, line up the telescope over one pair ofleveling screws and center the bubble approximately. The process should be repeated with the telescope over the other pair.

Continue this procedure until the bubble remains centered, or nearly so, for any position of the telescope. Final centering of the bubble is usually easier if only one screw is turned rather than trying to adjust two opposite screws at the same time. The leveling screws should be tightened only enough to secure a firm bearing. For self-leveling levels with three leveling screws, turn the telescope until it is parallel with two of the screws and bring the bubble to the center using both the screws. Then with the third screw, bring the bubble to the center of the circle. When you rotate the telescope, the bubble should remain

centered in any position. If it doesn't, the bubble needs adjustment. (Refer to adjustment section.) Before attempting to take sights, focus the crosshairs with the eyepiece. Point the telescope at some light surface such as a white building or the sky and turn the eyepiece slowly in or out until the most distinct appearance of the crosshairs is obtained.

Then focus the telescope (by means of the focusing screw) on a level rod, held at some point about 30m (100 ft) from the instrument. Then move your eye slowly up and down to observe if the crosshair apparently moves over the face of the rod. If the crosshair does not appear to move, it is properly focused. If you detect movement, further adjustment

is necessary. Once the eyepiece has been adjusted, no further adjustments are necessary so

long as the same individual uses the instrument. After focusing on the rod, center the bubble exactly in the level vial before taking the rod reading. Be sure that the reading is taken with the rod in avertical position, and that no foreign material prevents clear contact between the rod and point to be read. On bench marks and for more precision, some surveyors ask the person holding the rod to move the rod back and forth over the

center, using its base as a pivot. A rod level could also be used for this purpose. The minimum reading thus observed is the true vertical reading. Be sure not to lean on the instrument or step close to the tripod legs, because this may throw the instrument off level.

Lesson 19

Profiles and Cross Sections

The object of profile leveling is to determine the elevation of the ground at measured distances along a selected line. These elevations can then be plotted on profile paper at selected scales so that studies can be made of grades, depths, and high and low

spots, and so that estimates can be made of quantities of cuts and fills. Cross sections are simply profiles usually taken at curved line. The procedure for running traverse right angles to a base line such as the center of a surveys is as follows: road, ditch, gully, or other selected base line. Cross sections may be run along with profile levels, or

they may be run after the profile line has been staked and profiles have been taken.

On many projects it is customary to stake out a traverse line with a transit and tape before running the levels for profile and cross sections. The traverse line may be the centerline of a drainage ditch, dam, irrigation ditch, or an offset line. It maybe a continuous straight line, a broken line

1. The procedure in running a profile, and recording field notes, is essentially the same as in running bench levels, except that rod readings are taken on the ground at field stations and at major breaks in slope between stations. Distances between readings are measured and recorded by full or plus stations. Normally, a line on which a profile is to be

run is located and stationed before or during the time profile levels are taken.

2. Whenever possible, a BM should be set near the starting point stake. If this cannot be done, it will be necessary to run levels from the nearest BM to the starting point. Location of the starting point stake is described in the notes so that it can be relocated if it is pulled out or otherwise lost. The start of the profile should be a full station. It may be 0 + 00 or any other selected full station. Frequently, it is desirable to use a higher station such

as 3 + 00, if in meters, or 10 + 00, if in feet, where it may be necessary to run the profile both ways from the starting point. This avoids having to record minus stationing, which is always confusing. Normally, the starting station for surveys involving streams, waterways, irrigation canals or ditches, and gullies should be located at the upstream end and proceed in the direction of flow. In some cases, however, topography and the system to be surveyed might be such that the survey could be done quicker and easier by locating

the starting point at the downstream end and proceeding upstream. This is especially true for drainage surveys. In all surveys that might involve computations for water surface profiles by computers, the stationing numbering should proceed progressively downstream. The sample field survey notes (fig. 1-23) illustrate the use of a station other

than 0 + 00 for the starting point.

3. After establishing a starting point, the instrument handler sets up the level and reads a

backsight on the BM to determine the HI and then observes a rod reading with the rod held on the ground at the starting point. A ground rod reading is taken at each full station and at such plus stations as are necessary. Ground rod readings are taken to the nearest 0.01m (0.1 ft),and ground elevations are computed only to the nearest 0.01ft).

4. When the rodholder has moved to about 90to 100m (300to 325 ft) away from the instrument, a TP is taken on a solid object. The instrument handler then moves ahead, sets up, takes a backsight on the TP, computes a new HI, and continues as before. Rod readings and elevations on TP's and BM's should be read and recorded to 0.001m (0.01ft).

5.If a transit line has not been run previously, the instrument handler should draw a sketch in the field book to indicate changes in direction of the profile and its relationship to nearby landmarks. Stations should always be measured and recorded at all important points along the profile line, such as at branch ditches, subsurface drain laterals,

gullies, overfalls, culverts, bridges, roads, fence lines, headgates or takeouts, drops, checks, and similar features. It is useful to have this information in studying the profile for design and later in staking construction work on the ground.

6. Frequently, it will be desirable to set a hub stake driven flush with the ground every 150 m(500 ft) or less in order to "tie in" or relate other survey work to the profile.

7. The sample survey notes (fig. 1-23, 1-23(a)) indicate the method of recording cross-sectional notes when cross sections are run at the same time as the profile. Stations are selected where cross sections are wanted. The stationed line or transit line is used as the base line from which measurements are taken to both sides at right angles to the base line.

Sometimes an offset line is used, and cross sections are taken only to one side or both sides as is deemed necessary to obtain the information desired. A rod reading is taken on the stationed line at the station and recorded on the right-hand page opposite the appropriate station. For example, station 9 + 00 (fig. 1-23) or 30 + 00 (fig. 1-23(a)). Since this reading was taken on the base line, 3.41 m (11.2 ft)is recorded directly on the centerline of the righthand sheet of the field book.The rodholder then moves out at right angles to the base line with the rod and one end of the tape to the first major break

in ground slope. The rear chainhandler stands at the base line, reads and calls off the distance from this point to the rodholder. The instrument handler reads the rod and records the distance and rod reading either to the right or left of the centerline of the right-hand sheet, depending on which side of the line the shot was taken. The rod reading and

distance are recorded as T or v ,the top number being the rod reading and the bottom number the distance.

The process is continued until the cross section is run out as far as necessary in one direction. The rodholder then returns to the base line, and a similar process is repeated in the opposite direction.

Elevations along the cross-section line generally are not computed in the field unless they will be plotted in the field. This work is usually done in the office. It is not essential that the zero of the cross section be the centerline of the gully, ditch, or stream. In

some cases, the profile line may be along the bank of a ditch. In any case, the zero of the cross section is on the base line. The instrument handler must indicate in the field notes the direction of the cross section so that it will be clear. It is standard practice to refer to "right" and "left" when one faces the direction of progressive stationing of the profile

line. A cross section taken at a proposed structure site should be located so that this line may be reproduced later if necessary. This can be done by setting a hub stake at the zero point of the cross section and one or more additional stakes on the cross-sectional line 10to 30m (30to 100ft) from the zero point. These stakes should be driven nearly

flush with the ground so that they will not be disturbed, and guard stakes should be driven beside them for protection and ease in finding.

Lesson 20

Use of Grade Rod

In surveys for construction layout and construction checking, the use of grade rod readings, determined from the established HI, eliminates the need for converting rod readings to elevations at each layout or checkpoint.

Grade rod is the reading that would be obtained from the present instrument position if the rod were placed at the planned grade. (Grade rod = HI planned grade elevation). When the HI is above grade elevation, the grade rod has aplus value and is so marked in the notes, such as +1.92m(+6.3ft). If the HI is below grade elevation, the grade rod has a minus value and is so marked, such as - 2.53m (- 8.3ft).

To find the cut or fill in construction layout surveys, the actual ground rod reading is subtracted from the grade rod. If the result has a minus value, a fill is indicated. If the result has a plus value, a cut is indicated. For example: If the HI is 75.99 m

(249.3 ft) and the planned grade elevation is 74.07 m (243.0 ft), the grade rod would equal 75.99m (249.3 ft) minus 74.07 m (243.0 ft) or 1.92 m (6.3ft). If the foresight of the point were 2.99 m (9.8 ft), then 1.92 m (6.3 ft) minus 2.99 m (5.8 ft) would equal - 1.07 m (- 3.5 ft), indicating a fill. If the foresight were 1.55 m (5.1 ft), then 1.92 -1.55 = +0.37 m (6.3 - 5.1 = +1.2 R), indicating a cut (figs.1-24 and 1-24(a)).

Figures 1-25 and 1-25(a) illustrate survey notes for design, construction layout, and construction check of a small field ditch project. Afew prior random shots with a level indicated the ditch could be constructed to provide the necessary surface drainage on a 0.005 m per meter (ft per foot) bottom grade with a 0.30-m (1-ft) drop at its outlet to the bottom of the main ditch.

Lesson 21

Setting Up Slope Stakes

Slope stakes are set as part of a layout procedure widely used to guide and check earthwork construction. The procedure given can be applied to excavation of ditches, diversions, spillways, and other embankment work, such as levees and dikes, that

requires construction to specified slopes. Figures 1-26 and 1-26(a) show an example of the use of the grade rod method to set slope stakes for an earth dam. In the example, assume that the earth fill will be staked to the following dimensions: top width, 2.4 m (8 ft); upstream side slope, 3:l; downstream side slope, 2:l; elevation at top of settled fill, 31.08

m (101.9 ft). Also, assume location and elevation of bench mark and centerline of dam have been previously staked, and proceed as follows:

1. Run levels from BM to point of convenience at damsite and compute HI = 31.24 m (102.42 ft). (See figures 1-27 and 1-27(a)).

2. Compute difference between HI elevation and top of fill: 31.24 - 31.08 = 0.16 m (102.42 - 101.9 =0.5 ft). This is grade rod at station 0+30 m (1+00ft).

3. With centerline rod reading of 1.28 m (4.2 ft)at station 0 + 30 m (1 + 00 ft), compute elevation of ground surface, then compute fill height, 31.08 -29.96 = 1.12 m (101.9 - 98.2 = 3.7 ft).

This can be done also by merely subtracting the difference between HI and top of fill from the centerline rod reading, 1.28 - 0.16 = 1.12 m (4.2 -0.5 = 3.7 ft).

4. The centerline fill has now been determined. To set the slope stakes at right angles to the centerline you must keep in mind side slope; top width; berm width, if any; and fill (or cut) at the centerline. The distance out from the centerline at which a slope stake is correctly set is given by the relationship:

d = + hs (without berm) or d = f+ b + hs (with berm)

where d = distance in meters or feet, w = top width in meters or feet, h = fill (or cut) in meters or feet, s = side slope, and b = berm width in meters or feet.

5. For the first trial at setting a slope stake on the upstream side of the dam at station 0+30 m or 1+00 ft, measure a distance at right angles to the centerline stake of d = + 3(1.28 - 0.16) =4.56 m [d = + 3(4.2 - 0.5) = 15.1 ft]. Take a rod reading at this point. In the example the rod reading was 1.13 m (3.7 ft),which gave a distance of d = + 3U.13 - 0.16) = 4.11 m [d = # +3(3.7 - 0.5) = 13.6 ft]. When the rod was moved from 4.56 to 4.11 m (15.1 to 13.6 ft) from the centerline, the rod reading again was 1.13 m (3.7ft), so the stake was put in and marked (figs. 1-27,1-27(a)). Allowance for settlement should be marked

on the slope stake. The distance, rod reading, and elevation are recorded in the field book.

6. Slope staking is a trial and error method. Practice will develop judgment in making distance adjustments so that a minimum of trials will be necessary to find the location for the slope stake. The same procedure is used to find the downstream slope stake at station 0+30 m (1+00 ft) and the stakes at the other stations.

Lesson 22