lbl landprep

Recording survey measurements

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All measurements should be recorded in a field book that can be easily carried in a shirt or pants pocket. There are a number of ways to record data in a field book.

1. Tabular Format

The standard surveyor’s field book is set up as:

BS I.S. FS Rise Fall R.L. Distance Remarks
1              
2              
3              
4              
5              
6              
7              
8              

Where: B.S. = Back Sight,  I.S. = Intermediate Sight,  F.S.= Fore Sight,  R.L. = Reduced Level

  • Back sights.(BS) The first measurement in a survey should be taken from some permanent structure that is easy to find. This point is referred to as a benchmark and should be clearly noted in the field book. This will always be recorded as a back sight.
  • Intermediate sight (IS) is any reading taken from the one surveying station after recording the first reading at that station (BS) and the last survey reading (FS) before shifting the level to its next station.
  • A  Fore sight  (FS) is the last reading before shifting the level or finishing a survey.
  • Reduced level is its relative height to some datum point. Large numbers are height ground and small numbers are low ground. Reduced means that the heights usually had to be reduced or calculated before they were recorded as heights. Most points are reduced or calculated relative to a benchmark.
  • Mathematical checks

Mathematical checks must be done for each page of the survey book if using more than one page. This check will only work if each page begins with a back sight (BS) and ends with a foresight (FS). For this to occur if the page normally ends with a intermediate sight (IS) it is recorded as a foresight on that page and a BS on the first line of the next page.  When many readings are taken this ensures that error location as identified by mathematical checks are restricted to searching single pages.

Mathematical check:

Sum BS -Sum FS = Sum Rises-Sum Falls = Last RL - First RL

These answers could be all positive or all negative, but they must be equal.

This is a check of the mathematics only not of the survey accuracy

Survey Accuracy

To check the accuracy of the survey, the survey must be closed. A survey is closed by completing new set-ups and readings, until we return to the first point surveyed. It will be impossible for the first reading and the last reading taken during the survey to be identical. The level of variation will depend on the equipment being used and the skill of the operator. A practical limit is given for allowable error depending on the distance from the origin to the most distant point.

Allowable error over a distant measured one way:

Steep ground = (plus or minus 0.024 x square root (kilometers) meters)

Flat ground = (plus or minus 0.012 x square root (kilometers) meters)

Example of a field book and checks

BS I.S. FS Rise Fall R.L. Distance Remarks
1 3.70         10.0 0 Benchmark 1
2   1.85   1.85   11.85 15  
3   2.55     0.70 11.15 26  
4 2.92   2.78   0.23 10.92 50 Chang station
5   2.34   0.58   11.50 75  
6   3.47     1.13 10.37 100  
7   2.8   0.67   11.04 125  
8 2.43   1.28 1.52   12.56 150 Change station
9   1.83   0.50   13.06 174  
10     4.99   3.06 10.0 348 Benchmark 1

The reduced level (RL) can then be plotted onto a field plan and contour lines drawn between points of the same height.