on November 9, 2016
Our team flew an aerial survey for cetaceans around Iceland in July 2016, using four observers split between two platforms on a DeHavilland Twin Otter aircraft. This is the second year we have used the devices, after attempting the same survey in 2015, which was unfortunately unsuccessful due to poor weather. In 2016 we flew a total of 3266 nautical miles and 35 hrs on effort during the survey. Each observing station had a bubble window. Our observers used geometers, all connected to a single laptop that was periodically monitored by the flight leader. Observers also had access to an analogue inclinometer in case of geometer failure, but they were not required.
Our observer protocol incorporated the geometers for the measurement of all observation times, locations and declination angles. The latter were collected as the sighting when the sighting was abeam of the aircraft. The observers pressed the geometer button every time they recorded a vocal observation, even if that observation did not require a declination angle. These included the initial sighting, comments made after the sighting, and any other comments about environmental conditions or survey effort. This gives every observation a valid time stamp that is easily incorporated into a dataset, saving a tremendous amount of data transcription time and effort.
Our geometers, which were prototype devices, performed well over the course of the survey. I “sighted in” the devices prior to the survey, which involves sighting at targets, in this case coins, from a known height and distance, and again midway through the survey. We noted no calibration drift in the declination measurements over time, and measurements were reproducible within about half a degree.
The observers enjoyed using the geometers and preferred using them over the analogue inclinometers. The software included an option of changing the orientation of the devices so they could be used vertically rather than horizontally: this proved to be a big advantage on the Twin Otter, which has quite small windows.
We noted the following advantages of the geometer over analogue inclinometers:
- No chance of observer error in reading or transcribing the declination measurement. No chance of “rounding” readings by observers;
- More accurate time stamps on all observations;
- Faster to use in areas of high density sightings. The observer simply sights the group and presses the button, rather than having to read the measurement off a scale;
- The observer can use both eyes to make the sighting, allowing better observation effort to be maintained. With analogue inclinometers, the sighting can only be seen with one eye when using the instrument;
- Data are recorded into a file that is easily incorporated into a data entry template, saving transcription time;
- Recording of GPS data at each button press saves time in constructing the survey dataset.
The main remaining issue with the devices was that we were unable to obtain valid yaw measurements. Despite considerable effort in trying to calibrate the devices in flight, the yaw measurements obtained were not accurate enough for use. We therefore took declination measurements at the time the observer estimated the sighting was abeam of the aircraft, which is standard practice on aerial surveys, but this is subject to some error and possibly bias in estimating the abeam angle. If the devices produced valid yaw measurements, the observers could collect sighting angles anywhere in the viewing field, rather than just abeam, which would be a real advantage in some situations.
In summary, our experience with using the geometers in two surveys has been quite positive. Distance measurements are of course the primary data for any survey, and in my opinion the geometer increases the ease and accuracy of collecting these data. We will use them in future surveys.
October 27th 2016
Daniel Pike, NAMMCO