A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: David Pike
Date: 2017 Nov 5, 17:10 -0800
Bob Goethe you wrote:
From a sheltered location on the east side of Vancouver Island (e.g. 49° 02' 16" N, 123° 44' 47" W), the water is frequenly as placid as a lake. Standing on the shore to take a sight, it is easy to estimate your height-of-eye, and hence dip.
However, on the west side of Vancouver Island (e.g. 49° 03' 47" N. 125° 44' 35" W), you can be standing on a beach with rollers coming in.
How best does one estimate his height-of-eye on a beach with rollers coming in? Do you assume that the soles of your shoes, if just barely wetted by the last wave to come in, are at the same height as the tops of the waves out to sea?
I don’t think I’d stand there for long risking getting my feet wet, especially if there was a nice shelter to offer support on the promenade. My reasoning is as follows. Although height is probably the least accurate function of GNSS, differential height at the same location is probably quite reasonable. So if my height of eye above my feet is 1.5m, lets suppose my GNSS, held at eye level, reads height 10m at the waters edge and reads 16m in my shelter. Then in my shelter my height of eye above the water’s edge is 16m – 10m + 1.5m = 7.5m. Now if the sea horizon was the same elevation as the water’s edge, dip of the sea horizon from the water’s edge would be -2.2’. However, if the sea horizon was 1m higher, dip would be -1.2’. That would build in a 1nm error in my position line, which would be disappointing.
If I observed from my nice warm, dry, comfortable shelter instead, dip of the sea horizon would be -4.8’, and if the elevation of the sea horizon was 1m higher, the true dip would be -4.5’, so the error in my position line would be 0.3nm, which is more acceptable. This is because dip increases with the square root of the height of eye, so assuming you can still measure your height accurately, any error in dip due to wave height will be less the higher your eye is above the sea.
I tried this at Littlehampton, West Sussex in early September this year with my 1943 small mirrored Hughes Mate’s Three Circle sextant, ‘Navigator’ on my net-book doing the calcs, and Mrs P noting the time. With practise we got down to about 1.5 nm error overall. The hardest part was judging where the sea horizon was for the observations and to check index error. The sky was grey, the sea was grey, and there was a patch of reflected sunlight just below the horizon. (DaveP)