A macrometer is an instrument for measuring the size and distance of distant objects. Distant in this sense means a length that can not be readily measured by a calibrated

Calibration

Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....

 length. The optical version of this instrument used two mirrors on a common sextant

Sextant

A sextant is an instrument used to measure the angle between any two visible objects. Its primary use is to determine the angle between a celestial object and the horizon which is known as the altitude. Making this measurement is known as sighting the object, shooting the object, or taking a sight...

. By aligning the object on the mirrors using a precise vernier

Vernier scale

A vernier scale is an additional scale which allows a distance or angle measurement to be read more precisely than directly reading a uniformly-divided straight or circular measurement scale...

, the position of the mirrors could be used to compute the range to the object. The distance and the angular size of the object would then yield the actual size.

The Macrometer Interferometric Surveyor is a commercial GPS-based system for performing geodetic

Geodesy

Geodesy , also named geodetics, a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space. Geodesists also study geodynamical phenomena such as crustal...

measurements.