Plotting Questions; Radar ranges; magnetic compass bearings; positions on Open CPN...

[jjjacq]

Originally Posted by El Pinguino

Like this? You can also lay off range and bearing.

Most of us would be working with magnetic bearings.

Your chart is not up to date, the light shown on Seal Rocks is now on Grant Hill/ Phillip Island, so your fix is unlikely to be very good

[jjjacq]

On the other hand, I'd like to see a circle drawing tool added to openCPN. Pick two objects on shore and nominate the angle as seen from the boat between them (measured to high accuracy with a sextant) and the tool would plot the circle passing through the boat and the two oblects. Do that twice (with a 3rd land object) and you get two circles that intersect at a point giving a high precision intercept, likely to be better than the GPS position. This would be very handy.

[El Pinguino]

Originally Posted by jjjacq

On the other hand, I'd like to see a circle drawing tool added to openCPN. Pick two objects on shore and nominate the angle as seen from the boat between them (measured to high accuracy with a sextant) and the tool would plot the circle passing through the boat and the two oblects. Do that twice (with a 3rd land object) and you get two circles that intersect at a point giving a high precision intercept, likely to be better than the GPS position. This would be very handy.

Ah ha... the 'station pointer' plug in!

Re my chart... if I zoom in it shows the light to be on Round Island which is where I recall it being. Mind you its a few weeks shy of 20 years since I sailed in those waters.

[jjjacq]

Originally Posted by El Pinguino

Ah ha... the 'station pointer' plug in!

Re my chart... if I zoom in it shows the light to be on Round Island which is where I recall it being. Mind you its a few weeks shy of 20 years since I sailed in those waters.

Notices to mariners shifted it to Grant Hill less yhan a year ago. The o-charts were updated 2 weeks later. If you're interested, I also edited my old kap files so AUS150 and AUS151 are about right. (I got the pdf from the hydro department and pasted it onto the old tif file until it was about right (scaling and so forth) and then turned it into a new kap file. If you're interested I can put the relevant kap files on my server for you to grab.

[El Pinguino]

Thankee, I don't see myself sailing those waters again but I'm sure others will appreciate it if you put them on your server.

[jjjacq]

The notice that shifted the light is: https://www.hydro.gov.au/n2m/2022/bl...1_268_2022.pdf

The kap files edited by me are:
https://madinstro.net/charts/AUS/met...150/A00150.kap
https://madinstro.net/charts/AUS/met...151/A00151.kap

But the original tifs are year 2000 vintage, the o-charts have better enc type charts (except for Deal Island where the old AUS 148 is still better).

[Jim Cate]

likely to be better than the GPS position.

Ho, ho, ho! If true for you, perhaps you should consider a new GPS.

Jim

[El Pinguino]

Originally Posted by Jim Cate

Ho, ho, ho! If true for you, perhaps you should consider a new GPS.

Jim

Not so.
What do we want to know when we are sailing coastal?
Where we are in relation to the land.
Horizontal sextant angles and a station pointer are simply the most accurate way to ascertain that.
Ever wondered what those triangulation beacons are all about?

[jjjacq]

Originally Posted by Jim Cate

Ho, ho, ho! If true for you, perhaps you should consider a new GPS.

Jim

My GPS has a typical error of the order of 10 metres, with angles measured with 0.1' accuracy which is what my sextant provides, I can plot to within 1 metre. This is very accurate.

[Jim Cate]

Originally Posted by jjjacq

My GPS has a typical error of the order of 10 metres, with angles measured with 0.1' accuracy which is what my sextant provides, I can plot to within 1 metre. This is very accurate.

But are the plotted positions of the beacons or geographical features laid out that accurately on the chart? And can you resolve the featured point on the navobj to that accuracy? and from a moving vessel, can you advance the measured bearings that accurately? And really, modern GPS rx using all available satellites and systems can do a bit better than 10 meters, can't they?

I'm a believer in traditional pilotage, but I don't imagine that I could come close to the accuracy you propose... still a useful skill to have.

And for that matter, I've never been in a navigational situation where 1 meter accuracy would have any practical advantage over 10 meter. Land survey? Sure, such things matter, but IMO not in even inshore pilotage.

Jim

[jjjacq]

The accuracy is not a problem, no matter the boat's movement, or the observer's shaky hands, when the two lighthouses are superposed, they stay superposed.

Do I need it? Probably not, yet I can use it at times, say my anchor is stuck and I have to leave (maybe the weather forces me to) then I can quickly measure 2 or 3 angles accurately, write them down and what the shore objects were, then let go of the anchor chain. When I return later, I can find the very same spot and go about retrieving the anchor.

It's the same accuracy as two leading lights for a narrow channel. You know you're on the right track when the leading lights are on top of one another or do you use your GPS then? Well here, two marks and one angle act like the two leading lights except the track is a circle, not a straight line, but the accuracy is the same. Pick a circle that passes between two submerged rocks and you've made your own leading marks.

[El Pinguino]

Originally Posted by Jim Cate

But are the plotted positions of the beacons or geographical features laid out that accurately on the chart? And can you resolve the featured point on the navobj to that accuracy? and from a moving vessel, can you advance the measured bearings that accurately? And really, modern GPS rx using all available satellites and systems can do a bit better than 10 meters, can't they?

I'm a believer in traditional pilotage, but I don't imagine that I could come close to the accuracy you propose... still a useful skill to have.

And for that matter, I've never been in a navigational situation where 1 meter accuracy would have any practical advantage over 10 meter. Land survey? Sure, such things matter, but IMO not in even inshore pilotage.

Jim

A lot - all? - of the original - and still in use - coastal survey work was done with trig beacons set up along the shore.

And who you going to believe - your GPS or your eyes?

[El Pinguino]

A good read - if Santa is still taking orders - is 'The Great Arc' https://www.goodreads.com/book/show/....The_Great_Arc

In the meantime you can go here https://archive.org/details/amiralty...phic-surveying and download the full text of the Admiralty Manual of Hydrographic Surveying.
Therein - in the preamble - you can find this

'Triangulation.—The surveyor’s first step in the field is to fix the number of
marks, known as trigonometrical stations, which will provide control points for
the survey. These marks must be plotted on the chart in the same relative position
that they occupy on the earth’s surface thus providing a fixed framework by
which every detail in the survey may be accurately located. The survey will
be plotted on some predetermined scale and it is therefore necessary that the
distances between the various trigonometrical stations should be calculated.
If, for instance, the survey was to be carried out on a scale of 1/25,000, two
stations 1,000 feet apart would be plotted exactly l/26th of a foot, or
0-48 inches, from each other on the sheet. In general, the precise measurement
of the distance between points on the earth’s surface is a matter of some
difficulty and it would be impossible to make direct measurements between all
the various points of the framework. If, however, the distance between any
two of the stations can be found and if horizontal angular measurements are
made from these to other stations in their vicinity and thence to the remainder.






Chap. I 5

thereby connecting all the points of the framework by a system of triangles
it is possible to calculate the distances between all other points in the suiwey!
In the absence, therefore, of any known trigonometrical data it will be necessary
to measure a " base,” the distance between two selected stations or points
which are marked in a convenient position where the measurement can be
carried out over ground as level and free from obstruction as possible. The
ends of the base are then connected to other selected stations by observing
horizontal angles to and from them, and these in their turn are similarly con¬
nected to the remainder, by observing angles between all that are interAusible.
The result will be to provide a system of geometrical figures, known as the
tnangulation, which embraces the area to be surveyed. From these main
stations of the triangulation, horizontal angles are observed to secondary
stations and points, natural and artificial, of lesser importance which can
thereby be fixed and will provide additional marks for locating all the detail
of the survey. For the purpose of fixing the soundings it is usually necessary
for the surveyor to see at least three fixed points in the survey from every
part of the water area, and this is an important consideration in selecting the
sites for the main and secondary stations and soimding marks. '

[ChrisJHC]

Please allow me to put my pedant hat on for a moment

A GPS able to record a position down to the metre is precise but not necessarily accurate.

Exit pedant mode.

I’m sure many people have noticed their boats being shown as being on land when they’re actually at anchor or on a mooring.

I use GPS all the time, however when it comes to situations where metres matter, nothing beats looking outside and using the physical references that are available.

I well remember coming into Eden (NSW, Australia) one night in the middle of a major bushfire. The smoke was so bad you could only see about 10 metres past the bow.
I was navigating via GPS only and my instructions to the skipper were along the lines of “I think there’s a concrete wharf about 50m dead ahead”.
Neither of us could relax until we finally saw the wharf loom out of the smoke and could confirm what the GPS was telling us.

[shipahoy]

When the GPS-position and the map don't agree, it is usually the map which is inaccurate. Many of the surveys are much older than the GPS-system.

It is however becoming more and more frequent disturbances on the GPS-signal, especially in the north. Northern part of Norway had 81 losses of GPS since the war in Ukraine started. 10 last month.


For me a simple way of plotting distance and bearing from a point would be useful.