1. “Old-school hydro”:
coastal charting with traditional tools
S. Max Edelson
Department of History
University of Virginia
A field study conducted from the NOAA Ship Thomas Jefferson
US Coast Guard Key West Station at Trumbo Point
July 21-23, 2010
2. A plane table needs:
a tripod
a level
a writing surface
a sight fixed to a straight
edge (alidade)
3. The table is moved
directly above a known
point, and from this
position, an unknown
point is sighted. A line is
drawn along the sighted
angle toward the distant
point on the paper
attached to the table.
When another line is
drawn to this distant
point from another
known point, the two
lines intersect, and its
position can be
established.
4.
5. Alidade
Real alidades have
telescopic sights
and can adjust
sights for altitude.
Lenses scored with
stadia marks allow
for estimating
distance when
stadia marks on a
distant rod are
sighted.
8. From A, a line parallel to
the edge is sighted, and
then we measured 100
feet along it to establish
point B.
A
B
This created the survey’s
only measured segment
of 1 inch, which gave our
map a scale of 1” = 100
ft.
9. B
C
From A and B,
we sighted
several points
and drew them
on the paper,
creating a
network of
angles
12. From the other side of the
harbor, we shot the angles the
other way to establish position
(although we might have used
our bearing)
Where A and B
intersected, we located
the lamppost point
lamppost
18. Sounding
To measure the depth of a
channel, we launched a small
boat and dropped a lead line
to the bottom. To locate the
position of the sounding, we
used sextants to measure the
angles to points we had
already fixed on our coastal
chart.
19.
20.
21.
22. With these angles measured,
we recreated the position
of the boat at two points
and used the station
pointer to locate these on
the chart.
More advanced versions mounted the table on a tripod—allowing it to be horizontal to the terrain. An alidade is fixed to a straight edge so that when the distant object is sighted, a corresponding line can be drawn on the paper. Training the alidade on a graduated stadia rod some distance away allowed the surveyor to estimate distance by counting the number of stadia hairs he could see through the sight with the gradations visible on the stick. This allowed for distances over inaccessible terrain to be measure using only a single known point. As the rod-holder walked along the high water mark on the shore, the surveyor marked a dot on the paper that recorded the observed distance from each measurement. After the survey, a line drawn through these points established the contour of the coastline. After moving the table to another location, this process began again; sightings of the rods made it possible to sketch the positions of visible structures and roads as well. These separate sheets, one for each position, were synthesized into an integrated image. (Monmonier, Coastlines, 51-53)
Beginning from two accessible point whose distance is known, the surveyor sights and draws the unknown (inaccessible) point from each corner, calculating the distance using the known side of the triangle and two of its angles. (Monmonier, Coastlines, 50-51)
