old school hydro

“Old-school hydro”: coastal charting with traditional tools

S. Max EdelsonDepartment of HistoryUniversity of Virginia

A field study conducted from the NOAA Ship Thomas JeffersonUS Coast Guard Key West Station at Trumbo Point

July 21-23, 2010

A plane table needs: a tripod

a level

a writing surface

a sight fixed to a straight edge (alidade)

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.

AlidadeReal 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.

Point A established at the position of

the plane table

The first point: A

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.

B

C

From A and B, we sighted several points and drew them on the paper, creating a network of angles

cornercleatlamppost C

BA

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

corner cleat

SoundingTo 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.

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.

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