Hi there


Standard meridian is the longitude or the meridian which is used for determining standard time of a country.
For example , the standard meridian of India is 82.5 degree east and it passes through Mirzapur in Uttar Pradesh.

Whereas,

The central meridian is imaginary meridian which serves as the starting point for measuring longitude of a celestial body, it is the imaginary NS line bisecting the disk of a planet, satellite, etc., used as a reference, it is the center line of longitude for projection systems.A Central meridian is the center of a map projection where x = 0, and Central meridian can be any longitude line.

Thankyou