Brown's inaccurate survey drawing, on which the canal line
appears to have been sketched in freehand, could only have
provided a rough guide for establishing a line on the ground
be eye and personal judgement.
Centre-line pegs, the tops of which were levelled to the water
surface, were driven in probably at two to three chain intervals,
or closer where the canal was on an embankment or in a cutting.
For high banks, very long stakes were often used in cuttings,
holes were first dug before pegs were set in them at the correct
level. Alignment between pegs was left to the overlookers
to decide as work proceeded, a practice which in places led
to short meanders, which are common enough on most waterways,
although unnecessary and avoidable.
It would have been a simple matter to calculate and then
mark out toe positions of the earth banks and cuttings then
to set up wooden profiles to define the slopes. If that had
been done before work commenced, the limits of the construction
could have been fenced in and the land-take valued and agreed
with land owners. Outram, in common with other engineers of
the period, preferred not to do this and left such matters
until work was finished. Not surprisingly, this practice led
to the disputes and claims for trespass and damages to property
already mentioned.
A difficult matter was the control of line and level in
the tunnel at Standedge. The principal surveying tasks comprising
of establishment of accurate surface alignment between the
tunnel mouthings at Diggle and Marsden and the location of
the shafts; the transfer of bench marks across the mountain
and also to the top of each shaft to ensure a common basis
for levelling; and the transfer of line and level down the
shafts and into the workings.
Surviving records fail to describe how any of these problems
were resolved, although survey methods and instruments used
were of the simplest kinds. Surface alignment was probably
obtained by an interactive process of traversing using a magnetic
compass and chain, or even by means of a series of long poles,
each of which was moved laterally by trial and error until
a satisfactory alignment was observed.
Accuracy was poor and the surface positions of existing shafts
were only between 3 feet and 7 feet laterally of the correct
positions. One of the two shafts at Red Brook was offset from
the centre line of the tunnel at the bottom by about 26 feet,
but no doubt this was deliberate so as to ensure that the
deep sump and pipe work at canal level would not impede movement
along the tunnel.
Transfer of surface alignment to the base of the shaft would
be made by means of a pair of plummets hung at pit bottom
on cords suspended from points a short distance apart on the
survey line at the surface. This parallel line could be projected
into the workings for the guidance of the miners. the correct
level of the tunnel was generally obtained by calculating
the depth of the shaft and measuring down from a bench mark
by chain, as sinking proceeded.
Thomas Telford, after visiting the canal during the winter
of 1806, wrote that 'the season was totally unfit for proving
the general line of direction over the mountain and dialling
and levelling below, but each end appears very direct in itself'.
He was confident that all was satisfactory because he understood
that 'they have been tried and found accurate by several properly
qualified persons'.
In fact, some serious surveying errors had been made prior
to Telford's visit. David Whitehead, a surveyor associated
with Outram on the Peak Forest canal, checked the tunnel in
1802 and found several mistakes which were described by Rooth
in later years. At the Marsden end the tunnel invert was found
to be 2 feet deeper than the sill at the first lock. Moreover,
the workings then dipped 3 feet deeper over a shorter distance
to the rock face.
Of greater concern however, was the difference between levels
at the tunnel ends. The invert at Diggle was much higher than
that at Marsden. Rooth did not record this difference in measurement,
but on excavating at Diggle to equalise the levels, the foundations
to the side walls were undermined and these, as well as the
completed masonry arching, had to be dismantled and rebuilt.
Beyond this length was a long, partially-finished section
of tunnel which also had to be cut down to a correct level.
Although these levels had been corrected before Telford's
visit, a major error of alignment occurred in the months that
followed. The final section of the tunnel from Red Brook to
Brunn Clough was driven with a maximum deviation of 26 feet
to the north-west of the true centre line.
Not surprisingly, this was never mentioned in the company
minutes, but that was not all: there were errors of 120 feet
and 42 feet respectively in measuring between the longitudinal
positions of three of the shafts. These were not discovered
until the mountain was finally pierced in 1809 and direct
measurement was possible. The corrected length of Standedge
Tunnel was then given as 5,477 yards.
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