NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Range from calculated dip of the shore horizon
From: Don Seltzer
Date: 2018 Oct 5, 15:07 -0400
From: Don Seltzer
Date: 2018 Oct 5, 15:07 -0400
The reticle binocular is used for precisely this range estimating technique. The military has used this in the past for shore observation stations. Marine biologists use them to estimate how far marine mammals are from the research ship.
The internal reticle measures the angle between the horizon and the ‘hull up’ object. A simple table lookup that is precalculated for the viewers height is used to determine range.
Don Seltzer
On Thu, Oct 4, 2018 at 6:19 PM David Pike <NoReply_DavidPike@fer3.com> wrote:
Robert VanderPol II you wrote:
Have you ever used the table for distance off based on vertical angle between waterline and horizon beyond? You don't need to know the height of the object, just your own height of eye. Only works well for fairly close objects and ones where the horizon beyond can be seen.
That took a bit of figuring out. Are you saying that you add the angle between the object’s waterline and the sea horizon behind it to the dip of the sea horizon? That would give you the dip of the waterline of the object. Then go into the “DIP OF THE SHORE HORIZON or Dip at Different Ranges” table (P519 in my 1991 copy of Nories). Go vertically down from height of eye. Then when you get to the calculated dip of the waterline of the object, go left to get range. As you say, for a height of eye of 5ft, as might be encountered in a cruising yacht, it doesn’t look particularly useful. However, for a pole sticking out of the sea, there is the advantage that the range calculated doesn’t seem to be affected by the height of the tide. DaveP
