Old school hydro

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  • 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)
  • Old school hydro

    1. 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. 2. A plane table needs: a tripod a level a writing surface a sight fixed to a straight edge (alidade)
    3. 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. 4. 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.
    5. 5. Point A established at the position of the plane table The first point: A
    6. 6. 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.
    7. 7. B C From A and B, we sighted several points and drew them on the paper, creating a network of angles
    8. 8. corner cleatlamppost C B A
    9. 9. 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
    10. 10. corner cleat
    11. 11. 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.
    12. 12. 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.
    13. 13. 31.2 32.8 32.2 32.5 31.5 32.5 33.5 32.8 33.5 34.4

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