The basis for the metric system was not defined by some arbitrary prototype at first, but by measurements everyone on the planet could reproduce using the sun and a bit of water, at least to a very acceptable approximation.
The kilometer was based on 1/10000 the distance between the poles and the equator. The liter was based on the meter, the kilogram was based off the weight of water in a liter, the celcius was based off the freezing and boiling point of water at sea level, etc.
The reference meter was put in place to fix the problem arising from the error margin in measurements, but for practical purposes, the definition was all that was needed for a country to join the metric system.
The kilometer was based on 1/10000 the distance between the poles and the equator. The liter was based on the meter, the kilogram was based off the weight of water in a liter, the celcius was based off the freezing and boiling point of water at sea level, etc.
The reference meter was put in place to fix the problem arising from the error margin in measurements, but for practical purposes, the definition was all that was needed for a country to join the metric system.