Measurement of Length: Ruler,Measuring Tape, Vernier Caliper, Screw Gauge

Here you can some common tools for measuring length.

Longest lengths are measured through measuring tape, then goes the ruler scale and then the vernier caliper and screw gauge for small to minute as well as accurate measurement. 

However, physical measurement involves error and every physical measurement is an approximation.

Accuracy and Precision
The most meaningful way of interpreting a measurement is as an interval with a lower bound and an upper bound. Imagine that we have measured a line segment, using a ruler divided into centimeters, and found the length to be 5 cm. 
To be more precise, we can state the measure as an interval -- either in words, 5 cm to the nearest 0.5 cm, or using notation, such as 5 cm±0.5 cm (5±  0.5 implies a lower bound of 4.5 and an upper bound of 5.5). We can also state that the maximum possible error for this measure is 0.5 cm.the precision of a measurement depends on the size of the smallest measuring unit.. The smaller the interval, the more we have "narrowed it down," and thus the more precise the measurement.

Scientific Notation and Prefixes for SI Units

The numerical values of basic or derived in SI units maybe too big or too small in comparison with each other. They are well expressed using Scientific notation, also known as standard form. This is expressed as:

                                                                   M x 10ⁿ

^where

•   M is the coefficient  1<M<10
•  10 is the base
•  n is the exponent or power of 10

For example when we say that a star is 8 x 10⁹km from Earth means, it is 8000,000,000 km from Earth.

SI Unit Prefixes for Length

NAME	PREFIX	SYMBOL	FACTOR
gigameter	giga	Gm	109
megameter	mega	Mm	106
kilometer	kilo	km	103
decimeter	deci	dm	10-1
centimeter	centi	cm	10-2
millimeter	milli	mm	10-3
micrometer	micro	μm	10-6
nanometer	nano	nm	10-9
picometer	pico	pm	10-12