The developments of the nautical sextant were mainly related to astronomical positioning (latitude and longitude from heavenly bodies). Parallel to this development the sextant found employ in hydrographic surveying. Where early hydrography was mainly concerned with coastlines and discoveries, the science came into its own right in the 18^th century. In that century scientific methods were developed to enhance the accuracy of charts and add more depth measurements to allow safe navigation.

A good example is the founding of the French ‘Depot des cartes et plans de la Marine’ in 1720 and the English ‘Hydrographic Office’ in 1795. Of course, companies such as the Dutch and English East India Companies had been charting for much longer. However, their charts were often kept secret or, when published, not updated. To enhance the safety of navigation, more detailed and up to date charts were required. For this purpose, many depth measurements had to be taken resulting in formalized hydrographic surveys as we still know them today.

Early surveys were often performed as ‘running surveys’ where the start- and endpoint of the survey was determined astronomically and the positions in between from dead reckoning using the compass heading and (estimated) distance travelled. This method did not give very accurate results for the increasing requirements from navigation.

Early sextants for horizontal positioning 

In 1765 John Mitchell wrote that the Octant was to be preferred above the compass [1] for determining horizontal angles. In 1771 Alexandre Dalrymple (the first British Hydrographer) stated [2] that the Hadley Quadrant was to be preferred over the compass in terms of accuracy and ease of use. The downside of the Hadley Quadrant was however still, that it could not measure large angles. In 1774 Murdoch Mackenzie (Sr, 1712–1797) writes that a brass sextant of 9” radius (120°, ca 23 cm) is more suitable for this task next to an 18” radius octant [3]. As far as can be determined his cousin, (Lt. Murdoch MacKenzie Jr) first applied this method during a survey of the Thames river in 1774. Earlier surveys in 1772 and 1773 show significantly less depth measurements, an indication that position determination was harder than at the later Thames survey [4].

The first ‘true’ survey sextants appear during second half of the 19^th century (apart from the double sextants as described) [5]. These differ from their astronomic family members in that they usually do not have filters to block out the sun, have relatively large mirrors and a large telescope to make as much use as possible of any light. Another design item that often sets them apart is their second set of ‘legs’ allowing the user to place the instrument ‘upside down’ to allow quicker changes between notating angles and measuring them. Technically most of these are quintants (140°) rather than sextants, allowing the known points or beacons to be placed further apart. Most of the early survey sextants are of British origin, whereas in France and Italy the reflecting sextant is commonly found for the same purpose. There are very few early survey sextants designed by other nations known.

Later sextants for horizontal positioning

During the 20th century the various hydrographic services started to use specialized sextants more often although even then nautical sextants were used for the same purpose. An important addition on many later survey sextants was a 90 degree prism, allowing the sextant to theoretically measure angles up and over to 200 degrees. The placement of these prisms was usually in such a place that when looking over the horizon mirror to what is normally the horizon, the image is reflected 90 degrees 'left' in the prism. A survey sextant by Hughes and Son has a different arrangement with the prism just in front of the telescope. Until today the survey sextant can be bought including a prism [6] but in practice they are no longer used other than in training as most GNSS devices will give similar results without the need to set out three points for measurement..