JayBee
Well-Known Member
You came to it all quite recently, then?
All you Astro Navigators out there
- Thread starter Independence
- Start date
snowleopard
Well-Known Member
You came to it all quite recently, then?
Yup. for the first 15 years I sailed dinghies and navigation consisted of 'can you see the next mark?'
Then I decided to do the Ostar and thought it would be a good idea if I knew where I was going (unlike one frenchman who thought getting Air France pilots to give him a position as they went by would save him learning astro but had to withdraw when Air France went on strike). I was in the same race as Alain Colas with his 230ft monster and was amused to receive a notice before the start telling me I wasn't allowed to use satnav (which at that time cost many times more than my boat).
JayBee
Well-Known Member
Now that's something. Vaguely remember the Air France method!
Porthandbuoy
Well-Known Member
Now that's something. Vaguely remember the Air France method!
Anyone remember the chap who sailed from California to Hawaii without any navigational aids at all. He made it, and when asked how, replied. "I got out of my bunk every morning, had a look at the sky and simply followed the contrails."
True or not true? Feasible?
Skylark
Well-Known Member
Use a Portland Plotter to Create Long Scale
While it helps (me) to have a caculator on board to determine cosine of my DR/EP Lat, it is by no means essential and the point about it ending up in the bilge is perfectly valid.
Another, equally simple, way to construct a Long scale is to use your plotter.
If I knew how, I'd draw a sketch and attach, but I don't so I can't!.
Draw a right-angled traingle with angle at your DR Lat. Mark the hypotenuse with your chosen / convenient Lat scale (2mm increments for 12 cm = I degree, for example).
Each increment, draw vertically down. Increments on the base of the triangle are now Long scale.
It's easier to construct than to describe!
While it helps (me) to have a caculator on board to determine cosine of my DR/EP Lat, it is by no means essential and the point about it ending up in the bilge is perfectly valid.
Another, equally simple, way to construct a Long scale is to use your plotter.
If I knew how, I'd draw a sketch and attach, but I don't so I can't!.
Draw a right-angled traingle with angle at your DR Lat. Mark the hypotenuse with your chosen / convenient Lat scale (2mm increments for 12 cm = I degree, for example).
Each increment, draw vertically down. Increments on the base of the triangle are now Long scale.
It's easier to construct than to describe!
JayBee
Well-Known Member
Possibly a bit of a kid-on, but might be considered a modern day version of Polynesian navigation methods! As studied and written about by David Lewis, IIRC.
Link here:http://en.wikipedia.org/wiki/Polynesian_navigation
Last edited:
smartcom
Well-Known Member
Each sight you take gives a single line of position. Take multiple sights at twilight and they will give you a fix, or take sights at different times and you can then run them forwards (in the same way as with a running fix and hand bearing compass - anyone still navigate that way?).
Depending on what you want, you could try software such as WinAstro (www.winastro.co.uk) to avoid the arithmetic and save on the almanac. The web site also has reviews on astro books and sextants.
Tim
Depending on what you want, you could try software such as WinAstro (www.winastro.co.uk) to avoid the arithmetic and save on the almanac. The web site also has reviews on astro books and sextants.
Tim
maxi77
Well-Known Member
Emergency Navigation second edition by David Burch covers this and other techniques which can help when all goes tits up.
JayBee
Well-Known Member
Yes! Just discovered a copy on my bookshelf, which I had forgotten all about.
Must have another look at it.
Lady Campanula
N/A
While this 'construction of an approximate Mercator grid' is fairynuff - and I've done exactly that just prior to entering the London TMA Controlled Airspace at Flight Level 270 on a 'bad hair day' - it's rather easier, quicker, and a good deal more accurate simply to mark on the back of your plotting chart the intersections of the existing Lat/Long lines already printed on the front. Then 'join the dots' to make an accurate grid.
Anytime the daylight is strong enough to take a 'sun' astro sight is bright enough to see the markings through the chart.
If you're really smart, you can copy the correct enumerations ( 49 North, 50 North; 16 West, 17 West, etc ) - but mirrored ( west is on the RHS ) - so once you've plotted your 'assumed position' and intercept on the plotting sheet that is the back of your chart, you can prick this through to the proper side ( the coloured side ), turn over, and there's your Line of Position and/or Fix already neatly on your plotting chart.
5 minutes or less.....
Simples!
Jamesuk
Well-Known Member
I just had a master class in celestrial navigation at the MVS in Cowes in January so off to take some sights this Saturday at the beach.
Will pass on notes when I have scanned them in - sometime this year lol - where to find a scanner ?? (rhetorical question)
Will pass on notes when I have scanned them in - sometime this year lol - where to find a scanner ?? (rhetorical question)
JayBee
Well-Known Member
Lady C - gotta think about this for a bit, but my immediate problem is that I wouldn't be able to see through an Admiralty chart in normal daylight with any degree of confidence - and my eyesight is pretty good.
Don't know about "Simples!", exactly
Don't know about "Simples!", exactly
Lady Campanula
N/A
Lady C - gotta think about this for a bit, but my immediate problem is that I wouldn't be able to see through an Admiralty chart in normal daylight with any degree of confidence - and my eyesight is pretty good.
Don't know about "Simples!", exactly
Well, If you can't ( and I can ) then prick through with your dividers, from the front, the positions of the Lat/Long grid e,g, 48N/15W, 49N/15W, 50N/15W and so on.
Then turn the chart over and engage pencil and ruler....
Then you could do it in Braille!
AntarcticPilot
Well-Known Member
Well, If you can't ( and I can ) then prick through with your dividers, from the front, the positions of the Lat/Long grid e,g, 48N/15W, 49N/15W, 50N/15W and so on.
Then turn the chart over and engage pencil and ruler....
Then you could do it in Braille!
Just one word of caution, which might be a "gotcha" in some parts of the world! While the vast majority of charts are on the normal aspect Mercator projection, with rectilinear graticule lines so all the above suggestions work, a small proportion are on Transverse Mercator - where the lines of longitude are curves! This is true of some detailed charts of (for example) South Georgia. It is also the case that small scale charts of the Southern Ocean are on a completely different Map Projection (Polar Stereographic) - mainly because it is impossible to use Mercator projection to depict the Poles (unless you have an infinitely long piece of paper
)
maxi77
Well-Known Member
Just one word of caution, which might be a "gotcha" in some parts of the world! While the vast majority of charts are on the normal aspect Mercator projection, with rectilinear graticule lines so all the above suggestions work, a small proportion are on Transverse Mercator - where the lines of longitude are curves! This is true of some detailed charts of (for example) South Georgia. It is also the case that small scale charts of the Southern Ocean are on a completely different Map Projection (Polar Stereographic) - mainly because it is impossible to use Mercator projection to depict the Poles (unless you have an infinitely long piece of paper
Way back in the days when I did astro for real in one of her grey war canoes we always plotted on a plotting sheet which was rectilinear and could be scaled for any where, then the position was transfered to the chart you were actually using for navigation. The Jimmy used to make me do 8 stars which was pushing for a young midshipman it in the short time when both stars and horizon were visible.
AntarcticPilot
Well-Known Member
Sorry, but you CANNOT scale a Mercator plotting chart for use anywhere - they are specific to a particular latitude band, as the relative scaling between latitude and longitude varies in a non-linear manner with latitude. This is inherent in the maths of Mercator's projection, and I have had direct experience of trying to sort out data plotted on a chart where someone had done exactly what you suggest! That said, for large scales it is unlikely to be a significant error, but for oceanic scales it is most certainly NOT a valid procedure.
maxi77
Well-Known Member
The RN produced a standard plotting chart which you applied the scaling to. They came on a pad and were used once, very wasteful we were in those days.
Yes the longitude scale changes with latitude and on the basis you knew your aproximate latitude you could apply the correct logitude scale. I also remeber seeing instructions for creating such a plotting sheet from a blank sheet of A3
Try http://pdfdatabase.com/download/astro-plotting-sheet-pdf-12168399.html
AntarcticPilot
Well-Known Member
It is still mathematically incorrect, and liable to substantial errors when in high latitudes. The scale (Mercator is conformal, so the distance scale is the same in any direction at any point on the chart, but NOT from point to point within a chart) does not merely change with latitude, it changes non-linearly with latitude. You can't use a constant scale for any significant span of latitudes, and the rate of change of scale increases rapidly with latitude. For equatorial and temperate latitudes, this probably doesn't matter very much as the scale changes slowly in these latitudes, and will not change much across the extent of a plotting chart, but for latitudes higher than about 60 you will find significant errors. Remember that Mercator has the "interesting" property that the scale factor becomes infinite at the Poles!
I am not arguing about the practise of using such charts; in temperate or equatorial latitudes the errors will be less than those inherent in celestial navigation. But the approach will not work in high latitudes, and people using such plotting charts should be aware of their limitations.
As I mentioned, I had the interesting task once of capturing some data where the ship's officers had used exactly this approach on a small-scale plotting chart - they had relabelled the chart to span an interval half that the chart was designed for. I had to design a very complex procedure to get round the inherent systematic errors, which were of the order of kilometres.
If anyone wants a reference for the properties and mathematics of map projections, THIS is the Bible; heavy and advanced maths warning!
Porthandbuoy
Well-Known Member
Yeah Gods and little fishes!
I never realised there were quite so many map projections.
Downloaded for a bit of light reading
maxi77
Well-Known Member
I certainly do not dispute that such charts do ignore the fact that meridians are not pararrel, but for those working in the majority of the oceans other that the very high latitudes those errors are to small to worry about in the scale that these plotting sheets are used. In the prehistoric days before GPS it was accepted that astro was imprecise and the objective was to keep ones errors below 5nm, quite satisfarorary in mid atlantic, one was advised never to aim directly for the destination landfall, but to one side or the other, by more than your expected error so you knew which way to turn. I sometimes think we crave too much accuracy at times these days, but then I am an old fart who survived walking to school by myself at the age of 5, drank unpurified water as a child etc etc.
High latitude navigation and in particular charts and their projections is a special case, one I never had to deal with and I must admit I have no intyentions of starting now. Thus if I do find myself having to do a bit of astro then those hand made plotting sheets will suffice.