I do see your point, safety first. But I don't think the lifeguard acted wrongly. The guard did measure her again and saw that it was too close to call from that pole. It was really impossible to say for sure one way or another. Those poles really don't work well, in this case a half-inch off. Plus, with the unlevel grade of the pavement ... You see what I mean, it's possible to use a little common sense at times. And if precision down to the say, 1/8-inch, really was necessary to a child's well-being here, I think they would use a better measuring device than tape on a pole. On Vortex and Beast, precision is necessary, so they have a better measuring device. No one can argue a measurement on those rides because they use those very precise metal platforms.
As a responsible parent (and I realize this has nothing to do with the measurement dispute), I was content because I knew she was a very good swimmer, was actually tall enough (48 1/2 inches at the doctor's office), and was in no danger.
You know what was surprising to me, though, is that my daughter was never measured on any of the other three 48-inch BB rides that day even though she only passed by a half inch. I could see how that could raise some questions. But I guess the lifeguards do this so much they have a very good idea of how tall a kid is and could tell she would pass.