The girth ( circumference) of a tree is – or at least should be – much easier to measure than the height, as it is a simple matter of stretching a tape round the trunk, and pulling it taut to find the circumference.
As a general standard, tree girth is taken at 'breast height'; this is defined differently in different situations, with most foresters measuring girth at 1.3 m above ground, while ornamental tree measurers usually measure at 1.5 m above ground; in most cases this makes little difference to the measured girth. On sloping ground, the "above ground" reference point is usually taken as the highest point on the ground touching the trunk, but some use the average between the highest and lowest points of ground.
Some of the inflated old measurements may have been taken at ground level. Some past exaggerated measurements also result from measuring the complete next-to-bark measurement, pushing the tape in and out over every crevice and buttress.
Modern trends are to cite the tree's diameter rather than the circumference; this is obtained by dividing the measured circumference by Pi; it assumes the trunk is circular in cross-section (an oval or irregular cross-section would result in a mean diameter slightly greater than the assumed circle). This is cited as dbh (diameter at breast height) in tree literature.
From Pocket Wikipedia, http://www.free-soft.ro/pocket-wikipedia/