Working from whole to part; accuracy

General principles of surveying

1. Working from whole to part.
2. Locating a new point by at least two independent measurements (distances or angles) from points already fixed.
3. Check every measurement (take it twice or by another method) and keep the accuracy consistent with the purpose.

Why work from whole to part?

In this method a precise control framework is first fixed over the entire area, and the smaller details are then surveyed inside it by less precise methods.

• Errors do not accumulate: each small part is tied to an already correct framework.
• An error made in one part stays inside that part and does not affect the others.
• The big triangles are measured accurately, so the plan fits the ground as a whole.
• If we worked from part to whole, the small errors of each part would add up as we proceed, and the final survey would be distorted.

Example: For a farm, big triangles such as ABC are measured accurately by chain; then the fields and buildings inside are fixed by offsets from the sides of the triangles.

Why accuracy must be followed

• A survey is used to design and set out works; wrong measurements cause a wrong layout, wasted cost and even failure of structures.
• The final plan can only be as good as its measurements. On the other hand, there is no point in high accuracy where the purpose does not need it, so the required accuracy is decided first (e.g. 1 in 1000 for rough work, 1 in 5000 or better for good work).
• Suitable instruments and methods are then selected to reach this accuracy economically.
• Accuracy checks (closing error, repeated measurement) detect blunders before they spread.

Hence, working from whole to part controls the errors, and working to the required accuracy makes the result reliable and economical.