gps sog vs log

cagey

cagey

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Hi
often noticed 1+ kn difference between gps sog and log, always put it down to usual things, current, crud on impeller etc until today. Hove to to make tea and whilst sitting in cockpit log showed 0 took transits and i was stationary but gps showed 1.1 kn played around reduced genny etc to ensure i was really stopped (Carrick Roads huge no of transits) time 1630 low water so wasnt too hard.
So what is the explanation, from experience I think the log (albeit the slower reading) appears the more accurate

thanks

keith
 
1 - GPS SOG will always wander around 0 due to nature of calculating a position.

2 - SOG and log show two totally different things. You can be dead in the water and still doing 5 kts over the ground and visa versa. If you were making 5 kts upstream in a river running downstream at 7kts your SOG would be 2kts, not -2 kts, GPS does not display negative speed!.
 
[ QUOTE ]
GPS SOG will always wander around 0 due to nature of calculating a position

[/ QUOTE ]This was more pronounced on older GPS models. Modern sets average multiple positions records so the long/lat does not jump around when stationary or at very low speed. Hence modern GPS models have more accurate COG and SOG, especially at low speeds.
 
Your log works by measuring water flow across your impellor, so is affected by tide flow etc.

GPS SOG, is calculated using actual movement across the earths surface & has no tide/current influence.
 
If the boat was rolling even a little, a sensitive GPS can record the motion as speed particularly if the aerial is mounted high. My handheld fell across the cockpit once, and was found later to have logged a 'maximum speed' of 11kts! Even waving it around when holding it can log suprisingly high speeds.
 
I suspect that you were actually doing 1kt. If you were hove-to you will have been making leeway and the amount can be surprising. In most hove-to situations your leeway is sideways so it is hard to see and the log won't show it.

I often have the GPS on in harbour actually tied up to the dockside and I seldom see more than 0.2kt and then only for a very short spell.
 
I do find that my log is not accurate at low speeds. Anything under 2 Kn and I take with a pinch of salt. Can be anchored where you know you have over 1Kn of tide, and it often barely registers.

I even find that the log reads differently depending on whether the boat bottom is foul (and I am talking about accuracy of reading not boat speed). I take the impellor out when not using the boat so this is always clean.

Agree with GPS comments - they are so accurate and can be sensitive (depending on what level of damping you are using - on mine you can change this and no doubt you can on most) that it will measure your boat yawing in a big sea.

Log - you can check by trailing log or dropping something over the bow. GPS - try a bit of traditional navigation to check.
 
Just in case the GPS is recording random movement while hove to or movement of the antenna due to the boat rolling have a look in the manual and see if there's a setup for averaging the SOG. Some receivers allow you to specify the amount of averaging done. If you increase this it may reduce the problem.
 
Hi. This thread reminds me of times when others, on this forum state that their 40ft ish yachts make speeds in excess of 13 kts .Out in the Med the same type and style of boat make 8kts tidal flow perhaps.cheers bobt
 
a sensitive GPS can record the motion as speed particularly if the aerial is mounted high.
__________________________________________________

Why would you mount the aerial high? when all the recommendations are to mount as low as possible, to eliminate spurious 'bounce' signals off the surface of the water.

Only VHF needs a high point.
 
Modern GPS unit software filters out spurious movements (ie rapid out of character long/lat changes), and reports a constant long/lat based on average multiple rapid long/lat fixes. It is this average that is used to calculate COG and SOG (not the last two fixes).

On older GPS units you could see the 'ship' icon jumping across the display, especially when SA was active. This was due to the older crude method of attempting to plot every literal fix. Newer systems dampen the error effect. Next generation systems will make more use of ground augmented (corrected) signals
 
I know /forums/images/graemlins/smirk.gif But Cagey makes no mention of the age or configuration of his GPS set up. People do put aerials up the mast in spite of the recommendations. He wanted to know why his GPS was logging movement when he clearly reckons his boat was not moving - measurement of spurious local movements is one entirely possible cause.

It was quite a game for youngsters on my boat with the original Garmin 12 handheld - which I still use as back up - to see how high a speed it could be made to register swinging it around at arms length /forums/images/graemlins/cool.gif
 
Old vs New GPS sets ...

Newer sets are not necessarily less susceptible to wander of position ... that wandering around rate is due to filtering and even old sets you could often alter the sampling rate - creating an old GPS set apparently being better than a modern set ...

Of course parellel ch's increase on modern sets ... 12 / 20 instead of 8 as before and of course garmin in old days didn't do parellel - it was sequential..... ie GPS50 ....

I have both an old 8 ch Magellan Meridien and the Lowrance 3500C on board ... I can reduce wander on the MM and alter LR 3500 opposite ways to make MM far better .... if I was silly enough !!
 
Re: Old vs New GPS sets ...

I know that your special interest is GPS....looking at Cagey's original post, he told us that he had hove-to, adjusting his genoa to reduce way, and confirmed that he was stationary in the Carrick Roads by transits. Now, have you ever, when tied-up (i.e. without doubt not moving), seen a prolonged display on a GPS of the order of 1kt ? I certainly haven't. Sure, you see odd numbers flashing up but never a consistent speed of that magnitude. And have you ever managed to heave-to without making significant leeway? My suspicion is that somehow Cagey was deceived and the he was, in reality, moving.
 
Re: Old vs New GPS sets ...

Leeway is inevitable with hoving to ....

The problem here is the intial comparison of SOG and SOW ... (GPS speed vs Log speed) - they are completely different and cannot be compared without other info. - but I think you already know that.

Have I seen speed indicated for more than small intervals etc. - yes I have but only because GPS doesn't indicate -ve speed. If you are stationary and your GPS is on fast sampling ... you can have position calcs jumping all-over the place....... so speed is cal'd when it jumps away from true position ... AND speed is calc'd when it jumps back or further other way ...
Also when you reduce sampling rate and calcs ... that also lengthens the time that a speed is displayed ......

So how to answer ?
 
Re: Old vs New GPS sets ...

[ QUOTE ]
Leeway is inevitable with hoving to ....

[/ QUOTE ]I rest my case. We are trying to explain something that possibly did not happen.
 
[ QUOTE ]
Your log works by measuring water flow across your impellor, so is affected by tide flow etc.

GPS SOG, is calculated using actual movement across the earths surface & has no tide/current influence.

[/ QUOTE ]

Surely this is the other way around...

Your log works by measuring water flow across your impellor. This is true BUT because it measures JUST your speed through the water and nothing else it means that it DOES NOT take into account tides/current/wind. Not only that but it only measures your speed in the direction that the boat is heading. It won't take into account tides/current/wind.

GPS SOG, is calculated using actual movement across the earths surface & so it DOES take into account tide/current/wind influence.
 
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