I noticed that you can change your 1 hour result by deleting slow points in realspeed.
Mine is set to .85 NM in Realspeed which seems to be the default.
My GT31 is set to not record under 3Kn.
Questions: Should everyone have the same setting on their GPS & the same default setting in Realspeed so as we are comparing apples with apples or am I just totally confused.
If you set the gps to not record under say 20 kn you would then get a really good 1 hour wouldn't you?
Probably no alphas though
Any thoughts / explaination welcome but try & keep it simple if possible
Paul
Its one hour continuous on the clock. Continuous being the key word.
Therefore if you didn't record anything under 20 kts all you would do is murder your average speed for any one hour period.
ie....Half an hour at 20 and half an hour under 20 and not recorded gives you an average speed of 10 for the hour.
I think most sailors set their GPS not to record under 2 or 3 kts (so as not to use up memory) and this would have no effect on the one hour average since you are never going this slow.
I think you are right Paul. Sometimes I've definately sailed for less than an hour, come in and left the GPS on. As a result I get a one hour result. Its something dismal like 2.9 knots but its my average best speed over an hour.
Programs such as Realspeed are smart enough not to count invalid points... you don't need to delete any points.
I just look at the one hour figure and put that on the site.
As far as I can tell it would put your average up if you sail for more than 1 hour in a session and realspeed doesn't insert zeros for when your GPS is not logging. Some of us don't sail for an hour without stopping 
..... so it could make a big difference for some or if a large min speed is set.
Also the GT31 has two min speed settings, one for the card and one for the data logger. On my GPS the card is set to 2 kts but the data logger min speed is off and my SBP files have all data recorded, regardless of speed. Appears the card minspeed doesn't change the data, what is the card min speed then?
I think I found the answers to my questions, though please correct if wrong:
Memory Card (SBN Files)
If log switch is set to ON, not ON-FIX, then the min speed setting is ignored and all data irrespective of speed is recorded. Set to ON-FIX to use min speed.
The SBN file is created as soon as you turn your GPS on and it automatically records until you turn your GPS off. When you turn your GPS off you will have an SBN file automatically on your card with the data from your session. No need to save data, but the file will be time stamped to when you turned your GPS on.
Data Logger
This is always saving to the GPS internal memory and you need to clear the memory of your GPS when you turn it on or else it will still have the data from your last session. The internal memory has room for about 5 hours and 40 minutes of data if you record at 1 second intervals and have min speed turned off. I assume if you don’t remove the data from your last session it will start to overwrite that data when it runs out of space? You have to save the data logger to your card at the end of a session and it will be time stamped to when you saved to the card.
So back to the hour question, next session I’ll set the memory card to 5 knots min speed and then compare to the data logger which I’ll leave minspeed off.
Some other questions:
Is there any pros or cons to using the SBN or SBP files for the GPS Challenge?
Is saving continuously to the memory card with a SBN file a good way to make your GPS lockup? Maybe it will freeze if the card is not “seated” correctly? What happens if your card gets filled up during a session – what happens to the open SBN file?
It can be confusing when you first start using these things, but once you get it figured out they become a very useful tool.
Hmmmm well there ya go! Learn something new every day.
Just did a check, and my data logger had around 7.5 hours in its memory... I thought it would be a lot more than that.
One thing related is that if you are going for a big distance you may run out of memory on the log memory. I certainly did last week. But the SBN file will cover any amount of time of course.
I always use "ON" and don't notice many invalid points or spikes. Perhaps I mostly use it in a pretty good reception environment. I think the GT-31 should not have much of an issue but the GT-11 would have more reception issues so there may be a good argument for favouring "ON-FIX" for the GT-11.
Ben, why did you say the SBN file is easier to work with? I prefer to use the SBP file when I need to send them as they are smaller to email and they also open more quickly.
Of course, I always download and save both.
I thought I could shed some light on the minimum log speed issue by comparing some files. I am pretty sure one of my GT-31's is set with a min speed and another is not. On good days I usually run them side by side in my helmet so I should be able to compare distance and 1 hr results. I don't have them with me so I will have to wait 'till the weekend to know which is which. :-(
One thing I have discovered though is that GPS-Results has a minimum speed filter. Mine was set at 8kts by default. When I returned it to zero I got a slightly different 1 hr result (23.869 v's 23.884) Given that I only briefly stopped once to back the weed off my fin it made very little difference.
It did make a difference to the total distance by almost 5km which brought the result up to the same as RealSpeed.
You can quickly see the difference by just unticking the 'Filter' box.
I would set the GPS-Results min. speed filter to zero. I can't see any good reason not to for our use.
I try to make a habit of clearing everything before a new session. In the GT-31 there is a great menu which clears the Genie, Max speeds etc and the logger all in one click. Look in MISC / DELETE / DELETE ALL.
I did my experimenting today and got some interesting results. Whilst I'd like to say I purposely stuffed all my gybes and stopped for many rests to get lots of data below five knots, its not quite the case......
I set the SBN files with a minspeed of 5 knots and the SBP files with no minspeed. Also minspeed on the SBN files definitely only works when ON-FIX is selected. I got the following out of realspeed:
SBN File (5 kt minspeed)
2 sec = 28.45, 5x10 sec = 27.36, Alpha / Waterstart = 11.29, NM = 13.76, 1HR = 8.3, Dist = 25.91
SBP File (minspeed off)
2 sec = 28.45, 5x10 sec = 27.36, Alpha / Waterstart = 10.14, NM = 11.96, 1HR = 6.41, Dist = 20.62
All data is doppler.
The Alpha, NM and 1Hr make sense, but I'm confused about the distance.
It appears if you are a good sailer and spend most all of your time above your minspeed, then the above changes won't be so dramatic, if at all. But then again we now all have large memory cards, do we really need to use minspeed?
Must admit I'm very confused about the distance. I thought that was one of the reasons for min speed to be at about walking pace, so that walking around on the beach wasn't recorded as part of the sailing distance.
So are you reading distance from, "total doppler distance" from the data columns?????
Yep, just checked. Interestingly my GPS reckons it moved 400m sitting next to my car while I derigged! Shame we can't use the DGPS function in WA....
How does realspeed calc distance? Does it use position or speed multiplied by time? If it used speed multiplied by time then it makes sense. Ie if at 9:15 you drop below 5 knots and at 9:20 you get back above 5 knots, realspeed might assume you have been doing 5 knots for the last 5 minutes when really you have been standing still.
.
Hi Te Hau,
Did you have the memory card log switch set to ON-FIX?
How much time did you spend at below 3 knots in your hour? Can you see any data below 3 knots in your SBN file? If you spent some time below 3 kts the SBN file should have less total points in it than the SBP file, so it doesn't make sense it gives exactly the same results. Even if its only a small change, the results have to be different because the raw data from the two files will be different. Question is whether the difference is worth worrying about?
I'm happy to send my SBN or SBP files to someone to check and I will get some more data on the weekend.
Ahhh, good thinking Decrepit.
SBN Track Dist = 18.86 & Doppler Dist = 25.91
SBP Track Dist = 20.52 & Doppler Dist = 20.62
Makes sense now Maybe minspeed is why people are getting such big differences between track and doppler distance?
You're certainly making a good point for setting min speed lower, or off.
For some strange reason, I have min speed set to 2kts on memory card and 3kts data logger.
Guess I'm more concerned about data logger memory.
I remember a while ago somebody said we shouldn't be using doppler distance because it ignored the min speed setting, and still recorded all the low speed distance, but it looks worse than that.
I can't quite get my head around what you're saying.
Do you think Real speed assumes that the last doppler speed reading before min speed cuts in, is maintained until min speed cuts out? Then calculates distance (speed X time) on that basis????
Maybe Mal could change it to assume 0 speed while min speed is active.
I've emailed him, so there should be an explanation forthcoming.
I try to make a habit of clearing everything before a new session. In the GT-31 there is a great menu which clears the Genie, Max speeds etc and the logger all in one click. Look in MISC / DELETE / DELETE ALL.
Paul, I think you need the latest firmware on a GT31 for this to work, must admit I haven't tried it on mine yet.
Paul mine doesn't delete the trip meter either, even though TRP is highlighted for delete all, but it does delete the data logger and speed genie in one go. I've got the latest firmware and I'm fairly sure Paul does too!
Misc/delete/"trip meter" seems to work. Maybe the delete all needs an update for the next firmware or perhaps we have a setting wrong somewhere?
Also when you hit "delete all" I think it just happens that the delete progress bar is just below "save to card". If you look carefully the delete option will still be highlighted whilst its deleting.
Hi Te Hau,
The software seems to be where we are going different. As far as I can tell realspeed doesn't insert zeros when data is not recorded. So say in your first hour where you have 11 minutes under 3 knots that gets changed to zero, realspeed ignores this zero data and takes the next 11 minutes of data above 3 knots from the next hour. So you can see why minspeed and realspeed seem to be causing a problem. Perhaps there is a setting I need to turn on, shall wait to hear from Mal.
I'm still a little surprised you are getting exactly the same results with minspeed for below 3 knots. When your SBN data in GPSAR goes to zero, the data in the SBP file might still be 2 knots instead of zero..... Though this does mean the SBN file results would be conservative in this case, rather than inflating the results.
Am I understanding your post correctly - so for the 11 minutes under 3 knots, GPSAR gave an average speed of 0.64 knots for the SBN file and 1.12 knots for the SBP file? If so I wonder how it calculates a speed when it has no data! Either way it does sound like your results are slightly different, but more accurate than what I'm getting out of realspeed!
I just ran the same files through GPSResults and got
SBN File (5 kt minspeed)
2 sec = 28.49, 5x10 sec = 27.37, Alpha / Waterstart = ?, NM = ?, 1HR = 5.912, Dist = ?
SBP File (minspeed off)
2 sec = 28.49, 5x10 sec = 27.37, Alpha / Waterstart = ?, NM = ?, 1HR = 5.907, Dist = ?
GPSResults inserted gaps for data below 5 knots and gives almost the same 1HR speed, though realspeed gave 6.4 for the SBP file.... It didn't like my waterstart alphas for either file and something strange was happening with the NM. Couldn't figure how to get distance out of the program - will look harder later. Shall also try KA72 tonight.
I'm not sure if this is so simple anymore Paul!!
I've run the files through Realspeed, KA72 & GPSAR. Te Hau this is the first time I've used GPSAR, so I hope haven't made a mistake - cool program!
Its seems all 3 programs don't like minspeed being set on your GPS. GPSAR works best, but there is still some error. The less time you spend stationary the smaller this error, but given the way I sail, having minspeed off will be more accurate! Even whilst talking BS in the carpark with my GPS on!
Scotta, if you set GPSar to doppler & trapezoidal, it should give results closer to the others.
Here's another tricky one, as doppler measurement is virtually instantaneous, to get a 2 sec avearage result you need 3 measurements not 2, for 10sec you need 11 not 10.
Realspeed and gpsresults do this by default but with gpsar you have to set trapezoidal first.
Gpsar seems to have a min distance for alphas, it doesn't recognise some short alphas that realspeed does.
And did you manage to get doppler distance from gpsresults and gpsar??? I've never been able to figure out how to do that.
Very interesting that they all show similar errors!!!!
