They were omitted from the detection function analysis, three of the four having in any case no sighting angle
recorded; encounter rate analyses for the stratum were carried out both with and without these sightings.
Data recorded included horizontal sighting angle
, downward angle to sighting (in reticles), species, group size, orientation of the animals when first sighted, depth, Beaufort sea state, swell height, glare, GPS fix, date, and time.
In a line-transect aerial survey of narwhals in Scoresby Sund, in which the survey plane had flat windows, there were poor sighting rates out to a sighting angle
of about 40 [degrees] from the vertical (Larsen et al., 1994) and in an aerial survey of cetaceans in the Gulf of St Lawrence, in which the survey aircraft had shallow bubble windows, maximum detection was not achieved until 35 [degrees] from the vertical (Kingsley and Reeves 1998).
However, properly adjusting the sighting angles
of the cameras' placement corrects those problems.
Sightings were considered to be duplicates if they were made less than one minute apart by observers on the same side of the aircraft and had similar recorded sighting angles
. Two such sightings were eliminated from the analysis (see Table 2).