Why can hand span and arm length not be used as standard units of length?

Hand span and arm length are not suitable as standard units of length for several reasons:

1. Variability: Hand span and arm length can vary significantly among individuals, making them unreliable for standardized measurement. Different people have different hand sizes and arm lengths, leading to inconsistencies and inaccuracies in measurement.

2. Subjectivity: Measuring with hand span or arm length relies on subjective estimation rather than precise measurement. Individuals may interpret hand span or arm length differently, leading to variations in measurement depending on factors such as hand placement or arm position.

3. Lack of Standardization: Hand span and arm length lack standardized units of measurement and clear definitions. There is no universally accepted definition of a hand span or arm length, making it difficult to establish consistent measurement standards across different contexts or populations.

4. Limited Precision: Hand span and arm length are not precise units of measurement, particularly for applications requiring high levels of accuracy or small increments. They lack the precision and resolution necessary for scientific, engineering, or other technical measurements.

5. Incompatibility: Hand span and arm length are not easily compatible with standardized measurement systems such as the metric system or imperial system. They do not have established conversion factors or equivalents to units such as meters, feet, or inches, making it challenging to integrate them into standardized measurement practices.

Overall, while hand span and arm length may be useful for informal or approximate measurements in everyday situations, they are not suitable as standard units of length for scientific, engineering, or other technical applications where precise and standardized measurements are required. Instead, standardized units such as meters, centimeters, feet, or inches are used, providing consistent and reliable measurements that can be universally understood and replicated.