A manual tank gauge stick must be marked to the nearest:

The requirement for a manual tank gauge stick to be marked to the nearest 1/8 inch is intended to ensure accurate measurement of the liquid level in underground storage tanks. This level of precision is sufficient to provide reliable data for inventory management and regulatory compliance, as it allows operators to detect changes in liquid levels that may indicate leaks or other issues.

Marking the gauge stick to the nearest 1/8 inch strikes a balance between accuracy and practicality. It allows operators to obtain a measurement that is detailed enough to assess inventory levels effectively while also being straightforward for users to read and interpret easily. This is particularly important in maintaining operational safety and ensuring environmental protection.

More granular measurements such as 1/16 inch can lead to unnecessary complexity, while coarser measurements like 1/4 inch or 1/2 inch would not provide sufficient detail for effective monitoring, potentially missing important variations in tank levels. Thus, the 1/8 inch marking is deemed the most appropriate for manual tank gauging.