Thus there are two aspects regarding measurements. One is actual MEASUREMENT of objects or happenings and the second is expressing that in terms of NUMBERS.
Physics is an exact science. It deals with accurate measurements. But the question comes up 'How much accuracy do we need to make our measurements accurate?'The answer depends upon the purpose for which measurements are made. To know exact time we take to reach the school we may need to know the number of minutes we take to reach the school. But Olympic records need timings in fractions of seconds! For us the year is equal to 365 or 365.25 days. And it is acceptable. But ask an astronomer. He will say that it is equal to 365.242195 days! But apart from such astronomers, for whom is such accuracy important?
Estimation
It is easy to measure the length of your page but if you are asked to find the height of a high rise building how would you do it? It is of course possible to do that by going to the terrace of the building, hanging a rope… etc. Not very convenient!
On the other hand one can estimate approximate height of a floor and then do some simple arithmetic to arrive at a figure which, though not accurate from the point of view of an architect, may serve our purpose. Thus even if accurate measurement is necessary it is always advisable to estimate. This would help; you to avoid silly mistakes that frequently take place while calculating.For better estimations, specially for large numbers, comparison is easier to make. For example in the above calculations it would be easier to compare the height of the high rise building in terms of a building with two or three storeys.
Approximation
In measurements approximation also plays an important role. In day to day practice we always use approximation. You must have heard people saying "It is approximately five minutes walk from the station" or "Both of them are more or less of the same height" etc. Approximations also play an important role in science. To give you some idea about the size of an atom your chemistry teacher would say, "In a centimetre, there might be approximately 100 million atoms lying side by side!"
