Profile Leveling

Profile leveling is used to establish changes in elevation along a line. Common lines requiring surveying are drains, roads, fences, and retaining walls. When this information is plotted on a graph, it will give a profile of the line and will enable one to establish grades, find high or low spots, and make estimates of depths of cuts and many other decisions. The following sections will illustrate the procedure for profile leveling and the preferred way to record the data.

Before a profile can be made, the surveying crew establishes the stations by setting a stake or flag where the rod readings are to be taken. When the terrain is uniform, the stakes can be set a fixed distance apart (25, 50, or 100 ft). When the terrain is not uniform, or when there are addition stations that need to be recorded, such as a sidewalk, the survey crew reconnoiters the line and establishes a station

TABLE 15.2. Data for profile leveling.

STA

BS

HI

FS

ELEV

BM1

6.02

106.02

100.00

0.00

7.34

98.68

100

5.76

100.26

225

3.67

102.35

290

1.72

107.40

(0.34)

105.68

340

4.03

103.37

400

6.65

100.75

460

4.00

107.28

(4.12)

103.28

TP2

3.34

105.27

(5.35)

101.93

BM1

(5.25)

100.02

EBS

15.08

EFS 15.06

Acceptable error = 0.10 {EQ\r(920 ^ 5280)} = 0.04 0.02 < 0.04 Closure error acceptable

Acceptable error = 0.10 {EQ\r(920 ^ 5280)} = 0.04 0.02 < 0.04 Closure error acceptable at each important point. Because the purpose of the profile is to show the true slope of the ground, the irregularity of the terrain will largely determine where the stations should be established. When there is a definite change in the slope of the ground, the crew should set a stake to determine the elevation, even if it does not fall at a uniform distance.

The major difference between profile and differential surveying is the addition of additional foresights that are used to define the profile of the terrain. These additional foresights are called intermediate foresights and they are treated differently during the error checks on the notes. Table 15.2 shows that a common way to record the intermediate foresights is to identity the true foresights with parenthesis. An additional method is to add a column to the table for the intermediate foresights.

Once the centerline of the ditch, terrace outlet, channel, road, or other line to be profiled is established, the distance from the starting point to each station is accurately measured. For higher level surveys, 2 x 2-inch stakes may be driven flush with the ground surface, and rod readings taken on the tops of the stakes. On less important surveys, the foresights may be taken with the rod set directly on the ground. Next, the level is set up, readings are taken, and elevations are established for each staked point along the line. Turning points are used as needed to complete the survey. Finally, a closing circuit of readings must be made to check the accuracy of the survey. This is done by running a line of differential levels back to the bench mark where the survey began. If no turning points are used, a sight is taken at the benchmark used to establish the height of instrument, and compared with the original backsight. Figure 15.10 illustrates the steps for conducting a profile leveling survey. The data for this survey is in Table 15.2.

Www Profile Leveling

0.00 ft

FIGURE 15.10. Profile example.

0.00 ft

FIGURE 15.10. Profile example.

With the level set up near the line to be profiled, the rod is held on the bench mark, and a reading of 6.02 ft is recorded. This establishes the height of the instrument as 106.02 ft (100.00 + 6.02). Next the rod is held at stations 000, 100, 225, and 290, where rod readings of 7.34, 5.76, 3.67, and 0.34 ft are recorded. At station 290 a stake is set flush with the surface so it can be used to record the elevation and also be used as a turning point. The elevation of each point is calculated by subtracting the rod reading from the height of instrument. Notice that the same HI is used for all stations up to and including 290, as all readings are made from the same instrument setup.

Because the surveying crew anticipated in advance the need for a turning point, the profile leveling exercise is continued by moving the instrument to a new location to provide a view of the remaining stations. A backsight of 1.72 ft is observed on station 290, establishing the new height of instrument as 107.40 ft. The survey continues by recording the rod readings for stations 340, 400, and 460. These readings are 4.03, 6.65, and 4.12 ft.

The profile leveling survey is completed by closing the circuit. The instrument is moved, and a backsight of 4.00 ft is observed on station 460. A stake is set at TP2, and a foresight of 5.35 ft is observed. The procedure is repeated to complete the survey from TP2 to BM1.

Table 15.2 contains a set of notes for this survey. Note the similarities and differences between these notes and Table 15.1 (data for differential leveling). At the completion of the survey, it is noted that an error of 0.02 ft has accumulated during the survey.

15.11.1. Error Control

Because profile leveling notes usually have more foresights than backsights, the method of checking for arithmetic mistakes is slightly different from that for differential leveling. The only foresights to be included when calculating the sum of the foresights (SFS) are those taken on turning points, including bench marks if they were used as TPs. In the sample notes, Table 15.2, the foresight readings that are to be used for checking are shown in parentheses. The data in Table 15.2 indicate there were no arithmetic errors in the survey, and the closure error was acceptable. Note: This procedure provides a check on the turning points only—not the intermediate foresights. Any errors in the rod readings for the intermediate foresights will not be detected using the standard checks for error. Additional methods such as multiple reading or plunging the telescope must be used. Extreme care must be used to prevent mistakes in reading and recording the rod readings and in calculating the elevations at all stations.

+1 0

Responses

  • ari
    Is levelling important in agricultural engineering?
    5 years ago
  • Terttu
    What are the steps profile leveling?
    3 months ago

Post a comment