Checksums and Data Validation
Checksums A checksum is the sum of a 0- and a 180-degree reading at the same
depth. Ideally, the sum should be zero since the readings have oppo-
site signs. In practice, variations in casing grooves, the positioning
of the probe, and the zero-offset of the probe itself contribute to
non-zero checksums.
You can use checksum statistics to evaluate the quality of your data.
Slope Indicator’s Digitilt DataMate generates checksum statistics to
help you detect possible reading errors while you are still on site and
able to make corrections. If on-site validation is not possible,
checksum statistics can be generated on your PC using inclinometer
data reduction software such as Slope Indicator’s “DigiPro” pro-
gram, which provides a graph of checksums as well as a full set of
statistics.
Standard Deviation of
Checksums
The standard deviation provides the surest and easiest way of vali-
dating the data set. You can compare the standard deviation for the
current data set to a typical standard deviation established for that
installation.
It is good practice to make several surveys of the casing to establish
a good initial (datum) data set. The standard deviation of checksums
for the initial set can be used as “typical” for the installation. Note
that the “typical” is likely to be different for each installation.
After a survey, generate the standard deviation for checksums. (With
the DataMate, use the “Validate” command.) If the standard devia-
tion is within 3 to 5 units of typical, the data is probably good. For
example, if the typical standard deviation is 4, then acceptable stan-
dard deviations for subsequent data sets could range as high as 7 or
9. Narrower limits may be appropriate for deeper installations and
critical measurements. Wider limits may be appropriate for shal-
lower installations or for poorly-installed casing.
The DataMate also calculates checksum statistics for groups (zones)
of ten depths. This is especially useful in deeper installations, where
the large number of readings may make a single bad reading statisti-
cally “invisible.”
Inclinometers
Rembco Geotechnical Contractors specializes in
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Rembco techniques apply when adding new structures: especially
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