What is significant number? What is significant digit?

Explanation

Every measurement involve errors. The result of measurement should be reported in a way that indicates the precision of measurement.
Normally the reported result of the measurement is a number that includes all digit in the number that are known reliably plus the first digit that is uncertain.
The reliable digits plus the first uncertain digits are known as significant digits or significant figures.
The digits of significant figure are called significant digit.
In any significant figure the last digit (the right most) is uncertain.
For example, periodic time of simple pendulum is 1.62 sec. In this 1 and 6 are reliable and certain while digit 2 is uncertain.
Thus in this observation there are three significant digit.
When length of object reported aller measurement of 287.5 cm in this measurement 2, 8, 7 are certain while digit 5 is uncertain.
Reporting the result of a measurement that includes more digit than significant digits is superfluous and misleading since it would give a wrong idea about precision of measurement.
A choice of change of different units does not change the number of significant digit or figure in measurement.
For example, in measurement of length 2.308 cm there are 4 significant digit. It can be represented as 0.02308 m or 23.08 mm or `23080 mum`.
In each measurement significant digit are 2,3,0, 8 hence significant digit are 4.
Thus, location of decimal point is of no consequence in determining the number of significant figure.