Sometimes on antenna drawings a reference point, or "datum" is specified. Is that what you are talking about? That's up to the individual. I'm not sure what context you are talking about. For a log-periodic antenna I personally might consider the reflector as a reference point, and measure all distances from there. But there's no reason one could not defined the last element, or the start of the boom, the end of the boom... or whatever else one wants to define.
You need to clarify the context of your question for me.