Triton Isis - Manual (Page 142)

June 2004 Isis® Sonar User's Manual, Volume1 128
Table 8-3. The six downsampling methods in Isis
Method What It Does
(if 20 samples were used)
Pros and Cons
Average This setting takes 20 samples,
sums them and divides by 20. In
the case of 16-bit data, the math
happens as signed arithmetic.
The Average method is a good,
general-purpose downsampling
technique; it does not emphasize
data in any particular way. Average
is the third slowest (fourth fastest)
method.
Max Of the 20 samples, Isis uses the
maximum value to display. In
the case of 16-bit data, the
maximum magnitude (value
farthest from zero) is displayed
to preserve the signing of the
data. The Max technique is good to use for
a small-object search, since it is
guaranteed that every strong echo
(or “hit”) will be displayed. However,
in conditions where there is a lot of
background noise or clutter, this also
has the undesirable effect of
increasing the gain of the whole
picture and “washing out” the small
objects. Max is the third fastest
(fourth slowest) method.
RMS RMS = “Root-Mean-Square.”
Isis computes the square of all
20 samples, sums them, divides
the result by 20, and then takes
the square-root of the result. If
the first of the 20 samples was
negative, then the result is
negative. This has the effect of
preserving the general signing
of the data (negative or
positive). The RMS technique does not
guarantee that every strong sonar
return will be displayed on the
screen, but in areas where there are
at least several strong samples, RMS
will have a geometrically greater
chance of displaying the strong
returns. Two strong samples next to
each other will show up much
stronger than just one strong sample
by itself. RMS downsampling gives
the most accurate overall image of
the sea floor. RMS is the slowest
downsampling method. (However,
when 8-bit data are being
downsampled, RMS is faster than
Min/Max.)
Chapter 8: Using the View Menu