Chartplotters & Speed Filters

[macd]

Are there any chartplotters out there with a speed filter facility? There ought to be.

Let me explain: my plotter (Lowrance) displays speed (and thus also heading) with a sample rate of less than a second. So the speed fluctuates all over the place and, since small boats yaw, so does the heading. In other words, in these respects it doesn't tell me anything very useful.

How much should I tweak the windvane steering to hold the right course? No idea, the damn thing doesn't tell me.

What's my CPA to that merchant ship five miles away? Well, anything between 100 metres (scarey) and a mile (no worries). And one of these days I'll kill it for emitting all those spurious alarms.

This is not useful. In fact it's hard to escape the impression it's downright frivolous, mainly intended for impressing your mates with the 11 knots you did today, for all of half a second.

My old Garmin 128 GPS does have an adjustable speed filter. It can be set to 'auto' or to display a mean speed/heading over anything from 1 to 250 seconds. The speed and heading it shows are settled and real. So that's what I tend to use on passage. But why, oh why, if a decade-old instrument can do this, can a jazzy new one not? Preumably it's simply a software issue. Not hard for the maker to fix.

So, does anyone have a plotter which embodies such a function? And does anyone feel like me that the lack of one is bonkers?

 

[pvb]

My Raymarine C-series has a 3-level COG/SOG filter.

Many Lowrance plotters have a "Track Smoothing" option, which will help stabilise COG. But you really need a high-speed compass input to get anything sensible out of CPA calculations.

 

[Ex-SolentBoy]

You are right. It is bonkers not to have one.

All modern Garmin plotters have several options.

"Off - This setting will turn the Speed Filter setting off so the unit will not average the speed reading

Auto - This setting will make the unit automatically control the speed filtering

On - This setting will allow the unit to manually control the speed filter by entering a value in seconds. As an example, if this value was set at 5 seconds, the GPS unit will record your speed readings and average them over those 5 seconds to provide a gradual change in the speed reading"

Auto works fine for me.

 

[john_morris_uk]

The Raymarine series have an option of an electronic gyro which stabilises the whole thing fairly well. Is there such an option available to fit to your plotter or interface with it. Try asking the manufacturers?

 

[Robin]

I've never had one that didn't have the averaging feature somewhere in the setup menu. That said with the current GPS accuracy with SA turned off (ie it's very good) any small jumping around of the ships position should be undetectable at pretty well all speeds above stationary and SOG, COG, DTW, BTW, XTE VMG to WPT etc very stable. Is there perhaps another reason for what is seen, is the GPS fix accuracy being affected by a bad signal, poor aerial position etc?

 

[macd]

Thanks, all, for your suggestions.

It's heartening to know that at least some plotters have a solution, and equally that I'm not being neurotic.

I doubt that the lack of gyro or high-speed compass is at the root of it (except perhaps as far as AIS is concerned). The old Garmin 128 has neither, and works just fine with the speed filter turned up. Solentboy's description of his Garmin plotter's options exactly matches my GPS.

I've searched endlessly and can find no 'averaging' or 'track smoothing' setting in any menu. If anyone knows better, I'd be delighted to hear.

The GPS aerial (built-in to the display unit) is fine. Signal is good.

I queried the UK Lowrance importer about the issue. Someone who sounded like a Home office bureaucrat replied that no-one else complained, best-selling plotter on market, etc, etc and if I thought it was faulty I should send it back. He sounded as though he last went to sea in a pea-green boat and seemed to think I was being picky. No suggestions for a remedy offered.

Incidentally I've just Googled 'Lowrance plotter' and various permutations on 'smoothing' and 'speed filter' and found...you guessed it...this thread. I've since e-mailed Lowrance (HQ) asking if there's a solution. If there is one, I'll post it here.

 

[pvb]

What software version is your HDS7 running? Lowrance issued updates in summer 2010 and summer 2011. The 2010 version 3 included faster GPS sampling for improved accuracy.

 

[Robin]

I just looked ast the Navman 5600 plotter manual I have in pdf format and it is in the menu section for GPS. I just copied and pasted this which has the factory settings (time in seconds) in brackets.

Internal GPS (On)
Tank full DGPS Source (None)
Set remaining Restart GPS
Clear Used Static Navigation (Off)
Fuel Tank size (0)
Position Filter (Off)
Num Engines (0)
Speed Filter: (5)
Fuel cal
Course Filter: (4)
Flow filter (5 seconds)
Track Record

However I still don't understand how a modern GPS fix, usually accurate to +/- five metres can generate a discernible error in the form of a rapid variation in the SOG or COG, assuming that the boat is moving, unless it really is yawing about or crashing into headseas. I can't remember what settings I used at the time but it was in seconds not minutes and might even have been zero.

 

[richardbrennan]

I think it is software problem so you have a chance that Lowrance can put it right in a subsequent update. I had a Garmin 2010 and orginally had no problem with the SOG read out as it was pretty stable. However, I subsequently upgraded the software and got the exactly the symptoms you describe. Sanity was restored by manually setting the speed filter which had been happily working on auto with the previous software version.

I have subsequently changed to a Garmin 5012 and this seems OK regarding speed filtering; what I find annoying is the large number of information points that do not disappear swiftly enough when you reduce the scale and virtually block out the important chart features if you are looking at somewhere like Portsmouth Harbour.

I compained about this a few boat shows back and was told that the point was valid and would be remedied, but several software upgrades later it is still the same. If others feel the same, please add your voice to my protest!

 

[macd]

However I still don't understand how a modern GPS fix, usually accurate to +/- five metres can generate a discernible error in the form of a rapid variation in the SOG or COG, assuming that the boat is moving, unless it really is yawing about or crashing into headseas.

Ta for the info, Robin.

I'm not suggesting there's anything fundamentally wrong with the plotter: the variations in speed are real, mainly the effect of seas. As it is, functions such as ETA are a joke.

The larger practical issue is in variations in displayed heading, which are also real, caused by yawing. It doesn't need much of a yaw to trigger an AIS alarm for a vessel which is actually 'Safe'. Equally, the displayed CPA of the vessel will always jump about, which is precisely what I don't need.

pvb: software is latest version, thanks. As I wrote in my original post, I believe the issue is entirely software-governed, and therefore remediable by Lowrance were they so inclined.

richard: couldn't agree more. At least Garmin seemed to think you had a valid point, unlike the pillock I dealt with at Lowrance UK.