The effect of binocular vision disorders on cave surveying accuracy

This article – by Mark Dougherty – was printed in Cave and Karst Science (vol. 33, no.2, pp 51-54). A summary is presented here. 

 

Binocular vision disorders are relatively common and can have a detrimental effect on compass and clinometer readings taken with conventional sighting instruments unless suitable precautions are taken. This article sets out to highlight the effect on survey accuracy, and suggests practical measures that can be taken to mitigate their effects. 

If a person has perfect binocular vision both eyes point in exactly the same direction. A person whose eyes have a tendency to drift relative to one another in the absence of a visual cue to stay aligned has a phoria. This can clearly be a problem when using sighting instruments when one eye is focussed on the target station and the other is focussed on the scale of the instrument. 

The most common phoria is heterophoria in which one of the eyes has a tendency to move in the horizontal plane. The condition is divided into two subclasses: esophoria and exophoria, which refer to the case where the eye drifts inwards or outwards respectively, as illustrated in Figure 1. If this disorder is sufficiently severe it can affect the accuracy of compass readings if it is not identified, but will have no effect on clinometer readings. Although the precise proportion of the population that suffer form this disorder to such a degree is unknown, the numbers appear to be significant. Hyperphoria refers to the analogous (much rarer) disorder in which the eye drifts in the vertical plane. In this case, clinometer readings may be affected whilst compass readings will be correct.

 The author suffers from fairly severe esophoria. When performing a sighting on a tree sighted at a distance of 15m, the difference between compass readings sighted with his right and left eyes was 8°. This example provides an illustration of the potential scope of the problem. It is suggested that cavers should ideally test their eyes for heterophoria and hyperphoria by sighting compass and clinometer on a distant point with each eye in turn and looking for consistent differences between the readings. 

A number of simple methods for coping with the problem present themselves: 

  • •measure each leg twice, once with each eye, and average the results; 
  •  turn your head on its side when reading the instrument that would be erroneously recorded such that the axis in which your eye drifts is always in line with the rotation axis of the instrument; 
  •  sight with one eye over the top of the instrument; 
  •  wait until the reading has fully stabilised before recording it, since for people with a mild phoria the eye eventually stabilises on the direction parallel with the other eye. 

The use of instruments with a built in laser sight that has to be aligned on the distant station also eliminates the problem. 

 

It may be tempting to assume that the issue can be addressed by performing an instrument calibration to determine the magnitude of the problem, then noting the eye with which each reading was taken during the survey and applying the appropriate correction. However, the angle of convergence or divergence is not constant and can vary according to several factors. 

  • The problem is often worse under high illumination levels, so if calibration is performed in bright sunlight the corrections obtained may not be applicable to readings taken underground. It is therefore advisable to perform calibration in lower lighting conditions that more closely approximate those experienced in the cave. 
  •  Tiredness can exacerbate the effect, so that the error increases over the course of a long surveying trip. 
  •  The amount of convergence or divergence between eyes may be variable on the distance to the sighting object. The author found that the 8° difference when sighting on an object 15m distant was reduced to 4° when sighting on a closer target 3m away. 

Overall, it is clear that using one of the methods to mitigate the effect of a phoria is preferable to relying on calibration readings. If it becomes apparent that a someone with a phoria has operated instruments in a cave, it may be possible to correct for the effects if they perform a calibration on multiple targets at different sighting distances under subdued lighting conditions. The correction can only be applied if it is known that the surveyor generally sights with one eye or the other so that the correction can be applied in the right direction. 

The author concludes that binocular vision disorders are potentially a very serious source of errors in cave surveying if they are not identified and their effects mitigated. He suggests that the following clause be added to the notes that supplement the BCRA surveying grades: 

“To obtain grade 5, the surveyor(s) reading the instruments must be either known to not suffer from binocular vision disorders, or active precautions against such errors must be made.”