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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
32 Height measurements
The height measurements represent an area where the
pioneering spirit of Pfyffer can be shown in its strongest point.
The European maps produced until the first quarter of the 19th
century contain no - or only isolated - height information.
However, to form the landscape of his relief, Pfyffer needed the
area-wide height coverage, which he obtained by performing
systematic height measurements as one of the first. The result
of his work is not only the relief, but also the map of J. J.
Clausner that contains numerous height values (Fig. 6).
Pfyffers principal instrument to measure heights was the
barometer. During his walking-tours he annotated the terrain
elevation in regular intervals, obviously based on reading off
the barometer. The barometric observation must have been self-
evident for Pfyffer, because, as opposed to angle or distance
measurements, he never mentions his instrument or the process
of his work. The presumption of barometric measurements
relies on contemporary travel reports as well as on the results of
the accuracy analysis. However, still many questions remain
open, particularly Pfyffer's way of derivation of the height
values from the quicksilver stand of the barometer, which was a
great scientific challenge in the 18th century. The interpretation
of Pfyffer's letters and topographic works allows to conclude,
that except of barometric observations he applied trigonometric
measurements as well. First, he possessed instruments which
enabled a straightforward determination of vertical angles (Fig.
8) in his scale-defined trigonometric network. The second clue
for trigonometric measurements of Pfyffer are the heights of
alpine peaks in the map of Clausner, which were not conquered
within the lifetime of Pfyffer (e.g. Jungfrau and Finsteraarhorn,
firstly climbed in 1811 and 1812) and thus their heights must
have beed determined indirectly. As he was aware of the results
of the famous arc measurements in Lapland and Peru, he
supposably took the earth curvature into consideration,
however, with an unknown amount of flattening, and he
probably disregarded the influence of refraction.
Pfyffer had chosen Lake Lucerne as the zero horizon for his
height measurements and he estimated its height as 220 toises
(428 m) above sea level. A comparison with the lake height in
the modern map (434 m) shows an excellent result of Pfyffer's
measurements. The small difference of 6 m is evidence of long-
term thorough barometric observations as well as of a
progressive procedure for the derivation of the heights.
The numerous height values published in Clausners map
constitute the best basis for the evaluation of Pfyffer's
measurements. After the conversion from toises above Lake
Lucerne into meters above sea level, the heights of 41 well
identified localities and summits could be compared with their
present values. The average difference (to-be minus is)
amounted to -35 m; the negative sign is in accordance with the
contemporary phenomena of unreachable mountains considered
to be much higher than in reality. In average of absolute values,
the 41 tested heights deviate from the present map by 57 m. A
clear dependency on terrain elevation can be observed: the
average absolute difference in the flat northern area is only 23
m, in variable middle land 61 m, whereas in the mountainous
southern part it reaches 82 m. The detailed accuracy analysis of
Pfyffer's Relief confirms that due to the same surveying basis
the height accuracy of the relief roughly corresponds with the
one of Clausner's map. The slight accuracy decrease of the
relief in comparison with Clausners map (the 208 tested
identical points of the relief deviate from the current map of 76
m in the height in average of absolute values; Niederóst, 2003)
can be explained by the procedure of relief construction. When
considering a great number of height measurements undertaken
in a large area with the elevation range from 500 m until almost
4'300 m, Pfyffer's achievement is very respectable.
3.3 Relief construction
Pfyffer formed the landscape of his relief on 136 mostly
rectangular, transportable and clearly numbered wood plates
(Fig. 9c). He marked the position and height of peaks and major
localities with vertical wooden sticks and afterwards he
modelled the terrain using ordinary materials found in his
household: wood, brick shards, carbon rests and cardboard
pieces. Then he took a mixture of gypsum and sand for surface
formation and to close the gaps between the single
constructional parts and he covered it with a layer of beeswax
for detailed modelling. Finally, he worked out the surface
objects. The forests are made out of green dark- and light-
coloured drapery pieces, the water streams of chenille and the
ways and small paths of white or red string strained between the
nails. The houses appear as red finger-like metal and wax
pieces, the churches stick out as small nails plugged upside
down into the surface. A surprising result of the accuracy
analysis is the unity of relief scales in three coordinate
directions: contrary to previous assumptions it can be
concluded that Pfyffer did not magnify the heights of the relief
on purpose.
i S avs = emm
Fig. 10: (a) View of Mt. Rigi, one of 94 existing sketches of
Pfyffer, (b) The same view in the present 3D model (DHM25 of
swisstopo and a Landsat image of NPOC, www.npoc.ch)
showing the distortion of the left-side hill and a perfectly
detailed depiction of the background mountain forms.
Except of surveying data, Pfyffer also needed additional
information to model mountain forms and the landscape cover
of the relief. For this purpose, later relief constructors usually
acquired single oblique photographs or terrestrial stereopairs.
Pfyffer's way of documenting and indicating the objects to be
depicted in the relief was the drawing of coloured landscape
