2. Example

A differential level network is run from BM Ripp through A, TP1, B, and closes back on BM Ripp, Figure C-2.
The elevation of BM Ripp is 820.12 ft.

Figure C-2 Example Level Circuit

The BS and FS numbers shown in Figure C-2 are the Backsight (BS) and Foresight (FS) readings respectively at each instrument set up. These are shown only for example purposes - this is not how readings are recorded in the field. 

The measurement and computation sequence carries an elevation from the BS point through the instrument to the FS point, Figure C-3. The notes are recorded and reduced reflecting this process.  

Figure C-3 Process

Click here for a video of the example. 

 a. Set up notes

(1) Enter column headings on the left plate 

(2) Enter the benchmark information

A closed circuit must begin on a known elevation. Enter the name and elevation of the first point, in this case, BM Ripp. 

b. Run line from BM Ripp to A

(1) Set up the instrument between BM Ripp and A.

Figure C-4

(2) Skip a line in the notes and enter the point name.

(3) Take a BS reading on the benchmark 

A level circuit always begins with a BS reading.

Figure C-5

The BS reading is recorded in the BS(+) column on the row for BM Ripp.

Compute and record EI. The EI is entered in the EI column on the row between BM Ripp and A (the instrument is between the two points).

        EI = 820.12+10.92 = 831.04 

 (4) Take a FS reading on A.

Figure C-6

The FS reading is recorded in the FS(-) column on the row for A.

The elevation of A is computed and recorded in the Elev column on the row for A.

Elev_A = 831.04-3.42 = 827.62

The elevation of A has now been determined so the instrument can be moved.

Notice the data recording and computation flow from BM Ripp to A:

BS reading is added to an elevation to obtain the EI.

FS reading is subtracted from the EI to obtain an elevation.

c. Run line from A to TP1

(1) Set up the instrument between points A and TP1.

Figure C-7

(2) Skip a line in the notes and enter the point name

(3) Take a BS reading on A

Record BS reading in the BS(+) column on the row for A.

Compute and record EI:

827.62+2.73 = 830.35 

(4) Take a FS reading on TP1 

The FS reading is recorded in the FS(-) column on the row for TP1

The elevation of TP1 is computed and recorded in the Elev column on the row for TP1.

Elev_TP1 = 830.35-12.28 = 818.07

The elevation of TP1 has now been determined so the instrument can be moved to its next location.

d. Run line from TP1 to B

(1) Set up the instrument between points TP1 and B.

Figure C-10

(2) Skip a line in the notes and enter the point name. 

(3) Take a BS reading on TP1

Record BS reading in the BS(+) column on the row for A.

Compute and record EI:

818.07+6.34 = 824.41 

(4) Take a FS reading on B

The FS reading is recorded in the FS(-) column on the row for B.

The elevation of B is computed and recorded in the Elev column on the row for B.

Elev_B = 824.41-8.68 = 815.73

B is the final unknown elevation so technically all the elevations have been determined. However, stopping now would result in an open circuit with no opportunity to determine if any errors were made. For that reason, the circuit is closed back on BM Ripp by determining its elevation from B.

e. Run line from B to BM Ripp

(1) Set up the instrument between points TP1 and B.

Figure C-13

(2) Skip a line in the notes and enter the point name

 (3) Take a BS reading on B

Record BS reading in the BS(+) column on the row for B.

Compute and record EI:

815.73+10.39 = 826.12

(4) Take a FS reading on BM Ripp

The FS reading is recorded in the FS(-) column on the row for BM Ripp.

The elevation of BM Ripp is computed and recorded in the Elev column on the row for BM Ripp.

Elev_{BM Ripp} = 826.12-6.04 = 820.08 

Note that the field notes start with a BS on BM Ripp and ends with a FS on Ripp.

The EIs and Elevs were computed as the respective readings were made.

Upon taking the final FS reading, the closing elevation is immediately computed so it can be compared against the known elevation. Although there is a difference between the closing and beginning elevations of BM Ripp, we won’t concern ourselves with the misclosure amount until after we check our math.

f. Perform a page check

The purpose of a page check is to verify the running computations. To arrive at the closing elevation of BM Ripp, we started with its known elevation, added the BS readings, then subtracted the FS readings.

To perform a page check:

• Sum the BS and FS columns,
• Add the BS sum to the beginning elevation,
• Subtract the FS sum

The result should be the end elevation on the page.

If the end elevations differ, that means there's a math mistake somewhere on the page.

The page check should be done in the field immediately. It can be recorded at the bottom of the left plate or, to allow more data recoding room, at the bottom of the right plate.

A page check will only identify a math error - it will not find a reading or recording error. For example, if there is a 5 ft error in a one of the readings, the closing elevation will miss the known elevation by about 5 ft; the page check will also miss by the same amount.