International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
he was not allowed to erect permanent signal structures,
because those were often removed on purpose. Thus, in order
not to attract attention, he marked the places using small posts
on the ground level or a piece of iron in the rocks. During the
measurements his five or six helpers set a purpose-built vertical
signal pole on the assigned location and guarded it until the
operation - as he called surveying - was finished. For
communication with his assistants Pfyffer used a big flag in
black and white. From time to time he was also forced to use
natural points such as single tall firs as signals; in this case he
considered the displacement of the station during the
subsequent angle measurements at the concerned position. We
can conclude that in the time when surveying was not (yet) a
state- or military-supported enterprise but rather a cause for
suspicion, Pfyffer solved the problems of missing immovable
signal structures by means of good preparation and organization
of his work.
Pfyffer ordered his surveying equipment and accessories at
various places in Europe. To prove the procedures and the
accuracy of his instruments to measure angles, he measured at
each station with several different pieces of equipment. As a
practitioner he was at most fond of a plane table and a
graphometer, both instruments which could deliver the
graphical results immediately in the field. Pfyffer constructed
some surveying instruments himself, too, such as the alidade
with a telescope and two diopters in combination with a plane
table (Fig. 8 right). The most precise instrument of Pfyffer was
a simple English theodolite which he called /e chassis, the
frame (Fig. 8 left). It was a stable, 2 feet high device with a
horizontal and a vertical circle, both divided in degrees and
minutes. Instead of a telescope the instrument was provided
with two peep sight diopters. The major difference of this
"frame" in comparison with his other instruments was that it did
not allow the underlying with a piece of paper for graphical
interpretation. It belonged to a new generation of instruments
coming from England, which supplied the surveyor only with
the numerical values whereas the abandonment of drawing
accessories made them more compact and more accurate.
Fig. 8: Two instruments reconstructed according to Pfyffer's
description: left an ancestor of a present theodolite, right the
alidade with a telescope in combination with a planar table.
The results of numerical analysis using statistical and image
processing procedures allowed for coming to a new interesting
conclusion concerning Pfyffer's triangulation. As shown in
Table 1, the deviation of the relief and the analysed maps from
the northern direction amounts to peculiar numbers (around 30
and 15.5 degrees), while the orientation of Pfyffer's manuscript
map of Central Switzerland at a scale of about 1:247'000 is
almost perfectly northern. What motivated Pfyffer to orient his
major topographic works in such an unusual manner? At the
end of the 18th century the northern orientation of small-scaled
maps became widely accepted and on the other hand, large
scale maps still kept an arbitrary orientation according to the
position and form of the territory to be mapped. However, the
latter could not be the reason for Pfyffer who himself depicted
the same area in a north-oriented manuscript drawing (Fig. 4).
The answer to this question can be found when looking at the
secular variations of the geomagnetic field of the Earth for over
400 years. On the basis of the old measurements we can
recognize a steady progressive change in magnetic declination,
or angle between magnetic north and geographic north. The
publication of (Fischer, 1994) shows that between 1740 and
1790 the westerly declination in Central Switzerland varied
from 15° to 19° with a value of about 15.5° for 1750. Thus,
Pfyffer oriented his large-scaled topographic works - including
the inner constructional parts of the relief - simply according to
his compass (Fig. 9). Since Pfyffer's measurements took place
within a long time span and over a large area, changes of
magnetic declination certainly had a negative influence on the
accuracy of his results. To eliminate these errors, repeated and
precise astronomic measurements would have to be undertaken,
and this was by Pfyffer probably not the case.
15.299
(b) Georeferenced
map of Clausner
(a) Georeferenced Pfyffer's
map of the Rengg area
^
30.30?
15,5?
(d) Georeferenced
Pfyffer's Relief
(c) Georeferenced inner
parts of Pfyffer's Relief
Fig. 9: Orientation of Pfyffer's works, corresponding with the
westerly declination of magnetic north (a-c) The original
declination of the relief was 15.5? (c). Later on, Pfyffer must
have decided - probably for political reasons - to add new
constructional parts under a different angle so that the whole
relief deviates about 30? from geographic north (c, d).
Pfyffer's surveying was performed before the theory of errors
and least squares adjustment had been elaborated. He strived
for accuracy in terms of possibly small deviation of the angle
sum in a triangle from 180?. His observations were always
excessive and thus they enabled the elimination of insecure
directions and also a simple averaging of error-afflicted
measurements. When content with the triangulation, Pfyffer
turned to the detailed measurements. He mapped all the
significant terrain objects with his plane table: ways and paths,
rivers and small streams, houses, meadows, hedges, ravines and
even big stones. He also differentiated oak, beech and fir
forests. Under bad weather conditions he transferred the
recorded sheets to a large-scaled map and hereafter he began to
construct the relief. ——
Inter
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