NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Accuracy of sextant observations at sea
From: Hewitt Schlereth
Date: 2010 Nov 30, 22:10 -0400
From: Hewitt Schlereth
Date: 2010 Nov 30, 22:10 -0400
Alan - I know what you mean by just diving in and thrashing away. That's my preferred technique, too. Do try averaging a run of five sun shots and compare the intercept to the GPS. People I've taught on the beach here in St John (USVI) using a Davis Mark 15 pretty routinely get .8' to .6' intercepts. Hewitt On 11/30/10, Alanwrote: > Hewitt: > > Iam still "here", likely more than somewhat overwhelmed by the input from > helpful professionals such as Frank Reed, Garry, Anabasis and others.. > > Re sights, I haven't tried averaging, something I likely should as when at > the beach, spring and fall, weather allowing, I usually do a run of shots, 5 > upper limb of sun, 5 lower limb of sun. I try o allow 5 or 6 hours between > shot runs, to get a better spread between LOP's. > > I also, when both are available, shoot sun and moon, usually a string of 5 > shots each upper and lower limbs, when possible, so I would have sufficient > data to average, I believe. As to your observation about "From your posts I > have the impression you've started right in on multi-body fixes, and > difficult ones at that." I've tended to doing things the hard way for years > and years. Fools rush in and so forth. At home, in Pittsburgh, it's a case > of shooting across the Monongahela River down town, the distance is 500 or > so yards, requiring the dip short correction, and then there are boats going > by, or standing in the parking lot at our apartment complex, using an > artificial horizon, shooting the sun in AM and again in the PM 5 hours or so > later. Using the AH, once I opted to actually read the instructions, my > calculated positions are usually quite close to GPS coordinates, within a > couple of miles, sometimes less. Shooting sun, as above described, at the > beach, I usually end up within a couple of miles (2 to 3) of GPS > coordinates, and have come as close as 1 NM. Of course, there were some > shots about which the less said, the better. The three body fixes I've done, > at the beach were shot at evening civil twilight or thereabouts. > > Greg: > > On your comments regarding the three body fix I posted, Altair, Fomalhaut > and Jupiter, 23 October, the data I posted are, as I recall, what Frank had > requested. As for Height of eye, and IC that you questioned, the H/E was 6', > IC was 0.1' Off Arc, or so it appeared. My a distances were all Toward, and > my "fix" was inside the "cocked hat" or triangle. I determined it's location > via an exercise in geometry, didn't think I remembered any, bisecting each > side of the triangle, erecting a perpendicular, and extending this line for > each side. Where the lines crossed, is the location of my fix, or so I > assumed. The time between shot of Fomalhaut and the others, being longish, > could have thrown the whole thing off, but I ended up, measuring with > dividers, with my fix being less than 2 NM fro GPS coordinates, as mentioned > previously. Maybe just dumb luck, I cannot say. > > Even with weather permitting, here in Pittsburgh, doing star shots "downtown > by the river" is, due to light pollution, an exercise in futility, and > beside that, it's often cloudy, not to mention cold this time of the year. > I'm limited to using the AH in the parking lot during the daylight hours, > weather permitting or delving through a couple of years worth of data > derived from standing on the beach in North Carolina. > > As I mentioned to Anabasis, being at sea, I never have been so, where one > has a 360 degree horizon must be and entirely different situation, for > shooting in North Carolina, from my location there, I'm limited to shooting > with an arc running from roughly 110 Degrees to 250 Degrees. I plot stars > with the 2102-d, but realize that I'm missing one hell of a lot of sky. > Shooting to the North is out, as I'd be over the land. Problems, > everywhere.. > > All: > > Lest I forget, thanks for your patience, time, attention and efforts. > > > > ---------------------------------------------------------------- > NavList message boards and member settings: www.fer3.com/NavList > Members may optionally receive posts by email. > To cancel email delivery, send a message to NoMail[at]fer3.com > ----------------------------------------------------------------