It would seem that more accurate data would result in more accurate assessment. However, it is questionable whether the cost in time and effort to gather absolutely accurate data actually increases the value of the outcome. The longer it takes to produce the assessment the greater the missed opportunities to communicate your message. Along with that reality, you should consider that few disasters exactly replicate the previous event, even if the event is of the same type. The natural variability of events within the same class of disasters makes a reasonable estimate, based on a level of detail that assures you have probably bracketed the problem, as good as a perfect assessment.