With the use of the sextant for hydrographic surveying came the Snellius problem. That is, finding the position from two angles measured between at least three known points. It is unknown which method MacKenzie Jr used for plotting the horizontal angles for his survey in 1774. It could have been tracing paper [1] but also computation. To compute the position from two angles towards three known positions is called the backwards resection or Snellius measurement after the Dutch geographer and scientist (1580-1626) who first described this method for triangulation on land. Methods to compute a backwards resection date back to 1674 when John Collins described such a method in a paper to the Royal Society.

The method was further described by Edmond Halley in a letter to Sir Robert Southwell. MacKenzie Sr further indicates in his book on surveying that, next to using tracing paper, a ‘station pointer’ could be used [2]. As far as known this is the first reference to a station pointer. The book has, however, some additions by John Campbell and it is unclear whether MacKenzie Sr or John Campbell was the first to describe the method. Campbell further describes a construction method using two circles [3].

The station pointer as described is an instrument with three legs rotating around a common point. Using two scales running from 0° to 180° two angles can be set between the three legs. When the legs are manoeuvred over the three known points the unknown location from where the measurements were taken is in the rotation centre of the three arms and can thus be easily plotted without the need for computations.

MacKenzie Jr’s assistant, Graeme Spence is probably to be credited for the construction of the first station pointer and the first double sextant around 1784 [4]. He probably invented the double sextant (and the station pointer) because MacKenzie Jr was losing his eyesight and thus could not measure himself anymore. When MacKenzie Jr retired in 1788, Spence received (again) two station pointers and two double sextants. The double sextant allows the measurement of two horizontal angles at the same time using two index arms. This is required as the angles change as the vessel moves forward, thus requiring both angles to be taken as close together as possible. With a regular sextant this requires two observers, each with a sextant. This method was still in use until well into the 20^th century.

General use

After 1789 the station pointer is often described in English sources. It is known that George Vancouver used it in 1790[5] . He also probably used a double sextant (‘Hadley quadrant with two clamps’). Until at least 1829 the station pointers from the English Board of Longitude were made by instrument makers George Adams and Edward Throughton [6]. It is known that the famous French hydrographers Beautemps-Beaupre and D’Entrecasteux used horizontal angles (probably from their reflection circles) in their surveys but did not use the station pointer for plotting them [7].

In 1804 one Joseph Huddart claims [8] in an article in Nicholson Journal that he invented the, by then 15 years old, station pointer. Huddart was a friend of Troughton and claimed to have used the instrument from 1786 onwards²⁵. This is however still 7 years after we know that Spence used it in his surveys. Huddart probably copied the instrument from Throughton and claimed it as his own invention.

Station pointer in the Netherlands 

Where the station pointer became (and remained) the preferred method of plotting sextant angles during the survey in many hydrographic services it never became a formal method in the Royal Netherlands Navy Hydrographic Service. Where others used the quick method of plotting positions, the Dutch Hydrographic Service preferred the computation methods using the proportional divider. Although allowance is made in the hydrographic manual of both 1938 [9] and 1952 [10] editions to use the station pointer, sources indicate it was not used in practice for other purposes than coastal navigation [11].

An advantage of computing the positions rather than plotting them using a station pointer is that smaller angles can be laid on the chart and that it is possible to compute the position from two angles to four stations rather than three. But presumably tradition also played a large role in keeping to ‘known’ methods in the same way that the French Hydrographic Service used the reflecting circle until well into the second half of the 20^th century [12].

In commercial surveying the ease of use of the station pointer was preferred over the more tedious (but also more accurate if done correctly) computations. Dredging companies, but also Delft Hydraulics (now Deltares), used large English, American and German station pointers to plot their sextant angles onto the chart. A large advantage of the station pointer over the computational method is that, besides its speed, it is also a more fool proof method that can be performed ‘on the go’.

Combined instruments

Special instruments were designed specifically for hydrography and land survey and often combined the function of a sextant with that of plotting the angles. An example is the Douglas Reflecting Protractor [13] patented in 1811, which combines a protractor with a (single) sextant [14]. A similar instrument is the sextant-transporter by Laporte which was used in the Netherlands by the Topographic Survey and commercial companies such as ‘Bureau Schermerhorn’[15] .