Expedition Expedition Navigation Software

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9/23/2018 4:56 am  #1

Set & Drift - B&G versus Expedition

When I compare the set & drift displayed by B&G (previously H3000 Hercules and now H5000 Hercules) it was often different to that calculated by Expedition. A lot of effort has gone into calibrating the log over a known distance by 3 laps to minimise current effect, so it is believed this is reasonably accurate now - at least within the tolerance of the device..
If I adjust the speed calibration factor on the B&G the results for each can become reasonably close. To get sensible output I have found that damping needs to be quite high - about 9 for B&G and between 60 to 120 secs for Expedition.
Is the data used for this calculation different between each system? On the face of it the data is the same in each system - boat speed, heading, position, COG & SOG as there is only one source for these. Why would adjusting the calibration factor by only a small amount bring Expedition and B&G into reasonably close alignment?

Thanks Frank

If you rest you rust

10/03/2018 1:32 am  #2

Re: Set & Drift - B&G versus Expedition

These are very valid questions. I am sailing on one boat having a H3000 system and the current set and drift calculated is questionable. ...so it would be great to use Expedition numbers if they were better.

However, the fact that the damping of current set and drift is set independently worries me as it indicates that they are damped as two separate numbers. This will exaggarate the current drift. The proper procedure should be using vector averaging (https://en.wikipedia.org/wiki/Mean_of_circular_quantities). Using vector averaging would at least give reasonable numbers even with persistent offsets if using big damping values...

Maybe Nick could clarify how these numbers are handled and computed...


10/03/2018 1:46 am  #3

Re: Set & Drift - B&G versus Expedition

The set & drift is essentially just the difference between bsp and course (heading + leeway) and the GPS COG & SOG

I always presumed the instruments should do better at this as they may have more precise data. How are you getting data from the H5?

In Exp, the damping of some vars is handled as vectors (awa & aws, twa, tws & twd, set &drift, cog & sog). You can set the damping for each differently though.

I've always liked quite a high mean. There is quite a lot of noise in the system and small errors will have a large influence. Leeway is mostly just a  guess too.

Interestingly, I had an email a while back from someone using the Exp currents as he found them better than H5000 which I found surprising.


10/03/2018 2:11 am  #4

Re: Set & Drift - B&G versus Expedition

Thanks Nick & Barne
I take the point about damping values being different for different elements of the vector calculation.
The data comes into Expedition over the websocket from H5000 so it is the same raw data as for the H5000. The data displayed in the websocket application for set and drift appears to be undamped as it bounces around a lot.
I have consistently found with the H3000 and now the H5000 to have quite different values from Expedition. I rely on Expedition but the guys in the cockpit generally only have access to B & G numbers as the system is only Hercules and there are no Garmin GNX/GNI displays for custom output.
Doing this by terrestial navigation with pencil and charts in the navy it was always at least 30 minutes between observations which is effectively 30 minutes damping.

If you rest you rust
     Thread Starter

10/03/2018 11:35 am  #5

Re: Set & Drift - B&G versus Expedition

Which is essentially how B&G does it for currents from Dfw to WTP. I understand Dfw uses positions a similar time apart.

Exp uses the raw cog, sog, bsp & course (heading + leeway) to calculate set & drift. None of those are damped, which is why you need to damp the output.

It may be that H5000 uses damped values as inputs, which would probably reduce the damping needed on the output.


10/05/2018 1:38 am  #6

Re: Set & Drift - B&G versus Expedition

I shouldn´t have expected anything else that Exp handing vector channels correctly! :-)

That makes Exp calculations for set and drift a good option to me, if instrument system numbers looks questionable.


10/05/2018 4:27 am  #7

Re: Set & Drift - B&G versus Expedition

Perhaps obvious but worth checking - Are you doing any calibration or calculation within Expedition? Recalculating Leeway, for example, would have a big impact if the numbers were even slightly different.


10/05/2018 7:05 am  #8

Re: Set & Drift - B&G versus Expedition

Good point benhowett. I don't know about pivata but I don't.


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