a mirror

'Filippo [Bruneschelli] . . . could have determined the height of columns or buildings with an upright rod. . . . Or he could have employed a quadrant or, even more simply, a mirror. . . . The surveyor placed the mirror on the ground some distance in front of the object to be measured, then moved himself into a position such that the top of the object appeared in the center of the mirror. The height of the building was then calculated by multiplying the distance between the object and the mirror by the height of the observer divided by his own distance from the glass.'

'Ross King, Brunelleschi's Dome: How a Renaissance Genius Reinvented Architecture, 2000. This method came from Leonardo Fibonacci's Practica geometria, first published in 1220.