Significance of traversing, accuracy parameters

Significance of traversing

Traversing is a method of control survey in which a series of connected lines are measured in length and direction. It is used because:

• It gives horizontal control for topographic, engineering and cadastral surveys when the area is long and narrow, wooded or built-up, where triangulation is difficult.
• Only adjacent stations need to be intervisible, so the field work is quick and cheap.
• Coordinates of the stations are obtained, and the work can be checked (angle sum, closing error) and balanced.
• It is the basis for plotting details and for setting out works such as roads, canals and buildings.

Accuracy parameters

Horizontal control

• Angular misclosure, limited to $k\sqrt{n}$, where $n$ is the number of angles and $k$ is a constant related to the least count of the instrument (about $10''$ to $1'$, depending on the order of the survey).
• Linear misclosure, given as the relative closing error $e/\Sigma L=1/N$, for example 1 in 10,000 for a major traverse and 1 in 3000 to 1 in 5000 for minor traverses.

Vertical control

• Heights of stations are fixed by levelling (or trigonometric levelling) along the traverse, with the misclosure limited to $C\sqrt{K}$ mm, where $K$ is the length in km and $C$ is about 12 for third-order and 25 for fourth-order or ordinary work, as per the specification adopted.
• Double-run levelling and closure back to the starting benchmark are used as checks.