Hopton's 'topographical glass' was a form of altazimuth theodolite, but the division and configuration of the vertical circle, the mounting of the alidade and the scales on the horizontal plate were all different from Digges's topographical instrument (catalogue nos 45 and 46). Hopton was fully aware of the relationship between his design and that of Digges and, before giving an account of its use, felt it necessary to explain the advantages of his 'glass' over the earlier instrument. The examples he then offers of its applications include range-finding and military surveying.
One of the cited advantages of the topographical glass was the space in the horizontal plate for a larger central compass. The compass needle was pivoted over a printed card with a degree scale and, in addition, a form of sundial making use of the relation between solar declination, solar azimuth, and time at a particular latitude. When using the compass and degree scale the instrument, as Hopton says, acts as a circumferentor. He offers the reader assistance in supplying this part of his instrument: ' and because I will not trouble you ouerlong with the making, behold the figure which I will cause to be printed in a voyd paper to saue labour in drawing the same'. The woodcut of the compass card is indeed reproduced on an extra page at the end of the volume, which also carries advertisements for the London mathematical instrument makers John Thompson and Elias Allen.