Geoscience Reference
In-Depth Information
Fig. 8
From the plain, surveyors sight on hill-top locations, establishing a trig point from
which they can sight back to further features in the landscape below. During the extended
period of observations from a mountain location, the surveyors can make astronomical obser-
vations, timing astronomical phenomena to establish the latitude and longitude of the trig
point, as well as its position relative to other places. Here, a surveyor consults a pendulum
clock mounted in a religious shrine, while his colleague sights on a monastic building below.
A third colleague on a hill-top across the valley sights on the same building to establish a link
between the two hill locations. César-François Cassini de Thury (1744)
structures that could contain signal fires), but also (in uninhabited areas) natural
features like large rocks and solitary trees. For key reference points to be used
repeatedly over time, which must be unambiguous and clear, surveyors erect artifi-
cial pillars as triangulation points (or “trig points” for short). These pillars remain
characteristic features of a hill-walk in a mountain landscape in France or Britain,
although they are all too often subjected to vandalism and in stressful times have
been used as target practice by invading forces of artillery.
In the valleys, surveyors sought tall, thin markers like the spires of churches,
weather vanes, and the corner turrets of towers. Church spires in particular are
likely to remain a permanent fixture. Moreover, they usually are at the centers of
communities where the maps were likely to be of the most practical use. In addi-
tion, they are usually a convenient tapering shape - if the church had a square-cut
tower, then a triangular tent-like shape could be placed on top of it at its mid-point.
To provide an intermediate baseline that helped establish a more important, more
permanent reference point, the surveyors used temporary structures such as a hole
in the roof of someone's house or a sentry box. (All the examples quoted in these
paragraphs are found as trigonometric points for the Paris Meridian surveys).
To survey the longer sides of the triangles, say over 100 km from one mountain
peak to another, the geodesists observed at night, using a lantern with an optical
