Home' Position : Position Aug Spt 2015 Contents calculations. Once the raw EDM calibra-
tion data has been checked, it can be pro-
cessed to determine the additive constant,
scale factor and cyclic error (if required)
using available software tools, e.g. EDM-
CAL, Baseline or calibration spreadsheets.
The EDM calibration result should
be checked to ensure that it reflects
expectations and the quality of the
instrument tested. As a general rule,
the instrument correction should
approximate the precision to which
the instrument is capable of measuring
distances, as stated by the manufacturer.
If the calibration result significantly
exceeds the manufacturer's specifications,
the following may have occurred and
appropriate action should be considered:
• The instrument may not be in good
working order and in need to be
serviced and then re-calibrated.
• The observation procedures may not
have been followed to a satisfactory
standard, commonly caused by poor
meteorological observations and/or
low precision instruments, and taking
shortcuts to save time.
• The verified baseline values may no
longer be accurate. This is unlikely to
occur if the baseline has recently been
verified, but can occur if it has been con-
firmed to be subject to pillar movement.
All field notes and calculations relating
to the EDM calibration are to be retained
by the surveyor in order to maintain legal
traceability of distance measurements.
Records should be kept indefinitely,
because measurements made by any
instrument (and at any point in time) may
The cyclic error is generally insignificant
in modern instruments, and consequently
not applied to measured field distances. If
the cyclic error is found to be significant,
it should be applied as a correction
• Measure at least five individual slope
distances to the same single reflector,
re-pointing the EDM (only) after
each measurement. This will allow
the instrument to go through the
initialisation procedure and reset the
signal strength for each measurement.
The instrument should not be set
to display the mean of a set of five
measurements in lieu of five individual
readings, unless this procedure is
repeated five times independently.
• Record the temperature and
atmospheric pressure at both the
instrument and the reflector to an
accuracy of at least 0.5°C and 1
millibar (mb) respectively, using
calibrated thermometers and
barometers. Temperatures should be
measured at the height of instrument
and reflector to minimise the effect
of radiated heat from the ground.
Pressure may be measured at the
instrument only, provided the baseline
is not located in steep terrain.
• Ensure that the instrument is kept
shaded from direct sunlight when
transported between pillars.
• Compare all other reflectors. Once
all inter-pillar distances have been
measured to the one uniquely
numbered reflector, compare this
reflector with the remaining reflectors
by measuring to each in turn. This
should be carried out on the shortest
line by comparing slope distances.
However, if the reflectors vary in
height, measurements should be
reduced to the horizontal before the
comparison is made. This comparison
is important when using different
makes of reflector but can also be
significant when different reflector
holders of the same make are used
(e.g. single reflector holders versus
triple reflector holders). Where found
to be significant, variations should be
applied as corrections to the additive
constant for each reflector concerned.
The accurate observation of
meteorological data is essential for a
reliable EDM calibration. An error in the
measurement of 1ºC in temperature or
3mb in atmospheric pressure will cause
a corresponding error in the reduced
distance of approximately 1ppm.
If possible, relative humidity (%) should
also be observed (once for each inter-pillar
distance), although its effect is minimal.
Handheld met sensors currently available on
the market are affordable and can provide
temperature, atmospheric pressure and
relative humidity in the one compact unit.
Upon completion of the field work,
surveyors are required to restore any
baseline security measures to their
Figure 3. Observation sequence
for 4-pillar baselines.
Figure 2. Observation sequence
for 7-pillar baselines.
original state, e.g. replacing any protective
caps and bolts securely to minimise
damage to pillars caused by vandalism.
Damaged or missing caps and bolts
should be reported to LPI immediately, so
repairs can be undertaken promptly.
LPI strongly recommends using its EDM
calibration booking form. All data entry
fields should be completed:
• Make, model and serial number of the
instrument and reflector.
• Make, serial number and correction to
the thermometers and barometers used.
• Weather at the baseline, including
cloud cover, wind speed and direction,
and the presence of heat shimmer, fog
or rain if applicable.
• 'From' and 'to' pillar numbers (each
pillar has a unique PM number).
• Instrument height and reflector heights
above the pillar plate, read to an
accuracy of 1mm.
• Temperature and atmospheric pressure
as read. The correction to each
reading is to be applied when reducing
observations. It is advised to also
record relative humidity.
• At least five slope distance measure-
ments for each inter-pillar distance.
• Observations should be dated and
signed by the observer.
In the event of booking errors, each
mistake should be crossed out (not erased
or made illegible) and the correct value
entered alongside. All such alterations
should be dated and signed or initialled
by the person making the correction.
Electronically recording observations
onto the instrument's memory card is a
welcome backup and check.
Data processing should commence as soon
as possible once field work has been com-
pleted. Firstly, the field observations must
be checked against any electronic data
recorded in the field. Booking sheets must
be complete, checked and include all mean
32 position August/September 2015
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