Sampling is exactly right! The higher the sample frequency the more accurate the average by calculation.
Jim...
PS: The other comments about GPS precision can introduce error in a sample.
So, which one of these is more accurate?
- Thread starter Tally Ho
- Start date
Jim...
PS: The other comments about GPS precision can introduce error in a sample.
Yeah....have noticed that too. Luckily, I am usually in visual contact with the shore, so I am not getting too lost!
Greg
The GPS in my Chartplotter is built in to the unit, so I am not getting the big "swings" with an antenna at the top of the mast. The iPad was in the cabin, so again, no big swings there.
Greg
Does the speed really matter when your sailing? Chances are good you will be late for almost any scheduled event unless you leave 3 days early and get lucky with the winds or the motor runs long enough to get you there 5 minutes after the event starts.
Sam
Sam
Exactly correct. The almanac data describes the current and future (24 hours) bird locations, and the chipset uses its rough (within 100 miles) earth location to calculate the Doppler effect on the expected p-code frequency.
Turning on a GPS with valid almanac data allows a 'warm start' and normally the GPS locks in 20-60 seconds, assuming an un-occluded view of the sky.
Without this, the cold-start must 'sweep' through all possible frequency shifts, looking for the p-code pattern. That can take awhile!
Newer chips do this somewhat faster, and of course A-GPS provides the most current almanac along with a very close initial extimate of position so the TTFF (time to first fix) is now very quick for moble devices.
Last edited:
This is also why a warm start can fail in two ways;
Either the almanac data expires, meaning the device has been off for too long, or
It MOVES too far while off
That explains why your Garmin/TomTom/etc carnav locked fast on your way to the airport in New York, but too forever to lock in Los Angles later that same day. The alminac was current, but the unit was no near where it thought it was (last use) and so now the calculations it did are off. The warm-start will fail, and the unit will being a cold-start sweep.
Either the almanac data expires, meaning the device has been off for too long, or
It MOVES too far while off
That explains why your Garmin/TomTom/etc carnav locked fast on your way to the airport in New York, but too forever to lock in Los Angles later that same day. The alminac was current, but the unit was no near where it thought it was (last use) and so now the calculations it did are off. The warm-start will fail, and the unit will being a cold-start sweep.
Greg, it appears to me that you are comparing two sources of GPS for just Max speed and Avg speed. Depending upon decimal point, it appears that the Avg speed is identical. I never even bother to consider what the Max speed says, I know it is probably wrong in both cases, but your chartplotter appears more reasonable. Does the 7.7 knots coincide with your observations? Also, your screen shots don't appear to be recording what you observed underway (except for the graph, which isn't helping for comparison). Were the two sources concurrently showing different speeds while moving, or did they correlate?
As far as the Datamarine with paddle wheel, I calibrate mine with GPS based on speed under engine power on a very calm day. It rarely correlates when sailing. Like you, I find it is more accurate on one tack vs the other, probably due to the offset from center and the angle of the hull where it is mounted. There isn't much you can do about that, I think.
As far as the Datamarine with paddle wheel, I calibrate mine with GPS based on speed under engine power on a very calm day. It rarely correlates when sailing. Like you, I find it is more accurate on one tack vs the other, probably due to the offset from center and the angle of the hull where it is mounted. There isn't much you can do about that, I think.
I get no speed readout with paddle wheel due to sea grass. Even if it worked I would not trust it due to error possibility. GPS readings are not perfect but close enough for me. Chief
GPS is always more accurate than a knotmeter! Many factors prevent a totally perfect speed reading. Capt Bill on STARGAZER
Tally Ho, You are comparing apples and oranges.
The GPS units as previously discussed, are measuring speed over ground SOG. Simple time traveled from point to point. Variations in units comes from different algorithms used by the developer and which map datum the GPS is set up for. The knot log measures speed in the water. That includes wave action and current, which can be reduced to Drift in dead reckoning navigation to determine actual speed or "SOG".
The GPS units as previously discussed, are measuring speed over ground SOG. Simple time traveled from point to point. Variations in units comes from different algorithms used by the developer and which map datum the GPS is set up for. The knot log measures speed in the water. That includes wave action and current, which can be reduced to Drift in dead reckoning navigation to determine actual speed or "SOG".
I have to ask the one question that seems to be over looked... why are you so worried about speed...on a sailboat?
Are planning on running a race? When I go out I trim the sails so they aren't flapping and enjoy the day. I can kinda see paying attention to the speed to "check" your sail trim, but other than that - who really cares...
You have to trust one device over the other and I suspect the Garmin is more accurate than an iPad which is trying to run background apps without an Internet connection.
And the accuracy depends on the number of satellites it has at any particular time... and that comes from placement of antenna and that includes how your iPad is being held - consider that it is an internal antenna and may be blocked at some points... blah blah blah
Raise the sails and enjoy the day... leave the speed to the racers.
Are planning on running a race? When I go out I trim the sails so they aren't flapping and enjoy the day. I can kinda see paying attention to the speed to "check" your sail trim, but other than that - who really cares...
You have to trust one device over the other and I suspect the Garmin is more accurate than an iPad which is trying to run background apps without an Internet connection.
And the accuracy depends on the number of satellites it has at any particular time... and that comes from placement of antenna and that includes how your iPad is being held - consider that it is an internal antenna and may be blocked at some points... blah blah blah
Raise the sails and enjoy the day... leave the speed to the racers.
my gps came from the Garmin factory set on miles per hour while the signet knotmeter was set to knots....it does make a difference... setting the garmin to knots helped a lot...still not exactly the same but very close. good luck
NotCook
.
- Dec 29, 2008
- 806
"Like"! There is also the potential of induced friction in the paddlewheel from various forms of growth, causing it to read consistently (or inconsistently) low. And, obviously, GPS antennae placement must be a factor in reported speed - especially if it is whipping around on top of a mast, unless the unit has a way to dampen out those highs and lows.
Likewise, I take instantaneous wind speed and direction with a grain of salt when the masthead transducer is whipping about from side to side or fore and aft!
I am always amazed at these threads. I always learn something new. I now know the difference between a warm and cold boot which explained why the newer units are so quicker to lock on. I did have to stop and think, just how long have I been using GPS now? 25 years to be exact. It started with a Sony Pixis unit. What a hoot that was. Released before the GPS constellation was even really ready to go. I can remember lock on times taking from forever to almost never then loosing signal shortly after it just locked on. When it worked it did work, but with none of the frills of today's units. Now our Lowrance Elite 4 locks on to the satellites under our tent top which total surprised me. I was just turning it on to check the depth and it locked on to enough satellites to acquire our position.
While the live speed read out is nice, it really isn't necessary, much like the chart plotter feature on a 20 foot boat on a 15,000 acre lake. But it is fun to see your path at the end of a sail and then realize it just took 4 hours to go nowhere! Given the fact that it was not much more than just a depth sounder it seemed like a good deal to us.
Sam
While the live speed read out is nice, it really isn't necessary, much like the chart plotter feature on a 20 foot boat on a 15,000 acre lake. But it is fun to see your path at the end of a sail and then realize it just took 4 hours to go nowhere! Given the fact that it was not much more than just a depth sounder it seemed like a good deal to us.
Sam
I don't disagree with you. I do try to enjoy the journey, but I also try to squeeze out as much speed as I can from the given wind and conditions.
I was just really curious about different GPS devices showed a fairly significant different top speed. Not really all that important, but more about my curiosity.
I know why a knot log and GPS are going to vary, but was not sure why the GPS units would vary and why 2 apps on the same iPad show different results...
Thanks all.
Greg
Amid all the technical wrangling it's important to remember the original poster posed a simple question: Which is more accurate?
On my boat, that question is easy to answer - whatever source shows the faster speed has to be the most accurate!
On my boat, that question is easy to answer - whatever source shows the faster speed has to be the most accurate!
In lieu of a knot meter (mine is always clogged with marine growth) I use the stern squat method. On a day when there is no current I use the GPS to determine my speed through the water and note how "squatted the stern is". By squatted I mean how high the water is on the stern. I know that at 5.6 knots on my 40.5 the stern to topside joint just starts to take on water. At hull speed (on a calm day....lots of variables here) (7.2ish) the joint is completely submerged by about 2" of water.
I find this to be a very accurate gauge of my speed through the water in all cases.
I find this to be a very accurate gauge of my speed through the water in all cases.
There are lots of ways to estimate boat speed. Bill has identified one method. The more you sail the better your estimate of boat speed will become. Probably not to the same level of precision that a clean knot meter or GPS will provide, but close enough for most purposes.
When all else fails toss a floatable object off the bow and measure the time it takes to reach the stern and do the math.