One of the previous posts on this website details the device I have assembled in the hope to speed up underwater cave surveying and at the same time make it more accurate than using the traditional divers compass, depth gauge and slate.
Using the Adafruit BN0055 ‘9 DoF IMU’ inside a waterproof housing as the tilt compensated compass should give a reasonable degree of accuracy but just how accurate is it going to be ?
To find out I ran some tests using a DistoX2 for comparison.
A small wooden jig was constructed that allowed easy foresight and backsight alignment of the home built device and the DistoX2 so that comparable shots could be easily collected.
The sizing of the recess in the wood is such that when the box is rotated for the foresight/ backsights and pushed up against the right and left hand edges the sensor of the BN0055 is in approximate alignment with the plastic pegs used to align the DistoX2, the BN0055 is mounted around 90° out from the long axis of the box so its raw reported bearings is around 90° different.
Forty comparable foresight and backsight shots were taken with both devices and the data entered into a spreadsheet. The first task was determining the average difference between the DistoX2 data and the box data (I should come up with a decent name for this device…) The average difference between the two was 89.56°.
The Raw data from the box was then corrected by 89.56° and re-compared to the DistoX2 data. Average difference to Distox2 and Standard deviation values were calculated.
The foresight and backsight differences were also calculated to give a quality check for the shots as the jig wasn’t moved until foresight/ backsights were taken with both devices. The DistoX2 foresight/ backsight differences were far smaller than those calculated for my home made device.
These tests were conducted on a flat surface so further tilted tests will be done to assess this aspect, overall I am happy with the results so far, I have been able to buy an off the shelf sensor and without any complicated calibrations or maths have a sensor that is able to report magnetic bearing to within a few degrees of a DistoX2.
Assuming the tilted performance isn’t much worse then any large errors underwater will come from the ferrous metal equipment carried by the diver (or in the sump) and the ability of the diver to align the device with the dive line which is another challenge itself which needs thinking about.