The best reference point for dimensioning on pressure vessels is the:

The datum line is the best reference point for dimensioning on pressure vessels because it establishes a consistent and reliable framework from which all measurements can be taken. By using a datum line, welders and fabricators are able to ensure that dimensions are uniform and that various components align correctly. This consistency is vital in pressure vessels, where precision is essential for structural integrity and safety.

The datum line serves as a fixed reference point, eliminating ambiguity in measurements and reducing the possibility of errors that could arise from using less stable reference points. It provides a baseline for all other dimensions, which is particularly important in complex designs where multiple measurements need to be coordinated with one another.

In contrast, other reference points such as the center line, base line, and end line may not provide the same level of reliability or may be more subject to variations in fabrication processes or material properties, potentially leading to inconsistencies in the final product. Therefore, the datum line's role in providing a stable and universally accepted reference makes it the correct choice for dimensioning on pressure vessels.