11.3.2 Roll Test
The data collection for roll has to be over a flat sea floor. One line is surveyed twice, in reciprocal
directions and at survey speed.
When the data, from the two data collections, are looked at in
profile, there will be two seafloors sloped in opposite
directions. Most patch test programs will go through a series of
iterations to determine when the difference between the two
surfaces is the smallest, and this is the roll offset.
Figure 133: Roll data collections
Roll is perhaps the most critical value in the patch test routine
as an error in roll will result in an error in sounding depths.
However, the computation to determine the roll misalignment is usually the easiest and most
consistent.
Graph 1: Depth errors due to incorrect roll alignment
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
-80 -60 -40 -20 0 20 40 60 80
Sounding Error due to +0.5° Roll Error in 20 metres depth
Degrees from Nadir
Depth Error in Metres
Figure 132: Roll data collection
Page 133 of 210
Version 5.0 Rev r002
Date 05-08-2014
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