Accuracy, precision, trueness and limitations
One of the most popular questions consultants and engineers ask… ‘how accurate is it?’
?How accurate is what?
Firstly, we have an issue, do we all understand accuracy? Do we know the difference between accuracy and precision? Measurement systems can be accurate but not precise, precise but not accurate, neither, or both.
Some people who ask the question haven’t even thought about it. Most professionals think about it and probably understand it. To a surveyor, this question poses problems.
- I can’t tell you how accurate or precise a survey is until the survey is complete, I can talk to you about limitations and expectations. I can even talk to you about quoted accuracies in technical specifications. However, they won’t mean much, my experience tells me how accurate (or precise) I expect my observations and calculations to be.
- It’s not what a surveyor wants to hear. We measure anything and everything, accurately, we don’t measure things half-heartedly.
- Every single survey contains errors, always, there is never an exception to this rule. See my blog on errors.
- If I give you a figure, I’m opening myself up for criticism or problems further down the line.
- The working environment is a major contributor on our operations. The working environment is almost always out of our control.
- There are so many factors which impact on accuracy…
a. The working environment
b. The weather (nature)
c. People (operators & public)
d. Calibration & Adjustments
f. Object being observed
h. Grids & Scales