Primary Offset ?

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capt_birdseye

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Working my way through the Passage Log of a recent trip, I notice that at certain points along the way there is a reference and values in the Primary Offset Column. How are these 'calculated', and is it possible to change the values ? Nice easy question for you to think about over the 10 o'clock coffee ! Cheers ! /forums/images/graemlins/grin.gif
 
The only context I've heard of a "primary offset" is for setting drills or probes. Sure you aren't sailing an oil rig, Cap'n? /forums/images/graemlins/wink.gif
 
Is THAT why it is a pig when the wind is on the nose ???

Primary Offset comes up in the Passage Log, and I suspect when there is a Chart change, have to check the log times and whereabouts of the vessel at that time.
It is definitely part of the Nav programs functions and has a column all for itself.
I will keep enquiring !
Cheers !
 
I am not sure on this but as no one else has come up with anything over the last 24 hours, here goes.

I think primary offset is normally the heading change away from the direct course needed to intercept the next waypoint.

Going further, I think that secondary offset is the offset in course to the next waypoint needed to miss the waypoint by a set amount ie perhaps as in an ECS where it drives the autopilot to an offset from the heading required to intercept the next waypoint so's that it intercepts tangentially to the vessel's turning radius around the waypoint.

As I say, not sure on this but may just prompt some answer (in which I would be interested).

John
 
I think that's something like it Cat. I have always thought of an "offset" in nav terms as the course difference required from that required to steer a straight line between 2 points with no wind and tide and that actually required to get there after wind and tide are taken into account. Thus if a chart plotter shows a "primary" course between 2 points and you have to steer off to one side to keep the boat on track this is a "primary offset" Anyone got any other ideas?
 
The only thing is that we were not using a chart plotter, or auto pilot, and the ref. to it by the actual Nav. program, was infrequent, and 'possibly' when we moved from one chart to another. I will have to delve further into the Log times, and Boat position before I can comment on that. I understand what you are saying, but I do not think it is relevant to this particular case. Thanks for your input ... Spread the word and ask the question ... There must be a 'logical' answer for it.
Cheers !
 
and the ref. to it by the actual Nav. program, was infrequent

Perhaps the infrequent reference to it was because you were not using an autopilot?

Not saying this is so but, for example, using your comment regarding it perhaps occuring on chart change, it may be related to some glitch in the software's management of the look ahead vector in the transition from one chart to another or some such creating a spurious report. As I say, am not claiming that exact case is so, but it would seem quite easy to imagine situations resulting in "compufusion".

While I would expect it to travel under another name maybe the reference is misnamed and actually refers to "Position Offset" (an offset introduced by the vessel operater to compensate for any georeferencing error in the chart in use). Again, it is possible to imagine a software glitch periodically throwing up a spurious output when transitioning between charts or such like (or maybe you unknowingly have an offset entered and the software properly reports it each time it transitions to a new chart?).

John
 
The Primary Offset is always given as a Lat. Long. figure and not Degrees with ref. to COG or CTS. The software is Tsunamis '99 and the charts are various for the South Coast UK. i.e. Ramsgate to Falmouth.
I will be looking at the Log later this w/e and hope to be able to post which Charts were used when the Primary Offset column was written to.
Interesting to find out why !
Cheers !
 
I have checked the Log and the Primary Offset is constant throught the voyage, despite several chart changes.
I also coincides with the positon shown on the chart ( Not always correct ! ), and the 'Actual' in the water position. So it looks like a deliberate 'error' in the software, and not the charts. Problem now is how to correct it, or remove altogether. Any Tsunamis '99 experts out there ? Cheers !
p.s. Charts A1827A, A2290_1 and A3315 were the main ones where the error shows up. I look forward to your answers !
 
A quick look at Tsunamis 99 site says that all charts are set to WGS datum 1984. This is referred to as an offset. I think it originated in the difference between the accuracy of GPS derived charts and the less accurate plotting using earlier methods.

This offset on their charts is not able to be altered, but the offset of the vessel on the chart can be changed, offshore only, if the need arises. I find it hard to think what that need would be.

http://www.navdynamic.com.au/p_tsunamis99-faq.html
 
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