By eye, any estimation could produce important errors as it usually happens. Fortunately, the current state of the art permits to establish accurate results. In this kind of measurement trigonometry can be a great helper, but devices based on lasers are easier to use.
So, what tree? Where? With a strange name, hyperion (the name was used in a 2011 movie called Immortals for the bad guy), this tree has an impressive height of 115 meters. It can be found in a country where epic natural monuments and distances are pretty normal. In California, in the Redwood National Park, these record breakers stay for having a good time climbing them.
Nevertheless, that feat could be more difficult than expected as Wikipedia informs about that the exact location is not widely known. This has relationship with the interest of protecting the ecosystem in which this great tree lives, supposedly not too long from thousand years of age!