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9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 
Echogram measurement no. 
Figure 6 
Relationship between the mean of the first 23 echogram measurements (± stan- 
dard error [SE ] ) for two hauls when part of the second echo is included ( ) 
or excluded ( ) in the QTC (Quester Tangent Corporation, Sidney, British 
Columbia, Canada) IMPACT™ analysis. 
different gain settings and different reference depths. 
Users have also been unaware of the 251 sample require- 
ment, or the effects of repeating the last sound sample 
(padding) in order to fulfill this requirement. Ellingsen 
et al. (2002) mentioned that truncated acoustic reflec- 
tions resulted in a loss of some of the 166 EMs. Perhaps 
results from their field work resulted in modification 
to the QTC IMPACT software so that the last acoustic 
sample was repeated (a process known as padding). 
Correlation of echogram measurements with depth 
The influence of depth on the EMs, and on the resulting 
PCs, may be due to improper echosounder calibration or 
improper depth-correction in QTC IMPACT, rather than 
to true variation in substrate types. It is not possible 
for users to determine the origin of the depth influence. 
Although the reference depth is supposed to compensate 
for the signal-protraction of pings of different depths 
within a data set, none of the QTC IMPACT studies in 
the literature have actually checked to determine if such 
compensation occurs. It has been reported in the litera- 
ture that the QTC IMPACT-generated PCI is correlated 
with depth (Legendre, 2003) and that QTC IMPACT- 
generated substrate classes are sometimes correlated 
with depth (Anderson et al., 2002). As with our findings, 
depth biases were also reported for the El (roughness) 
and E2 (hardness) measurements made by RoxAnn™ 
(Sonavision, Aberdeen, Scotland, U.K.) bottom-typing 
software, as determined by a more thorough study with 
careful seafloor groundtruthing (Kloser et al., 2001). 
Angle of incidence 
Any potential effect due to impact angle of echogram 
reflection, which is a combination of seafloor slope 
and vessel motion, is not widely addressed in the 
literature. Anderson (2001) used QTC VIEW™ to 
distinguish among substrates on steep slopes, some 
of which appear to be as steep as 45° (see Anderson, 
2001, Figs. 4 and 5), whereas von Szalay (1998) and 
von Szalay and McConnaughey (2002) reported that 
