Hi all,
Just after some advice on the right settings to use on my Foretrex. Only just realised there was an option for setting up how often to log the trackpoints. 
Should I set the unit to log more than once every 2 seconds? what is the resolution on the device? I think the unit can hold 10,000 trackpoints, so setting it to 2 seconds would give me about 5.5 hrs of data?
Also, there seems to be a lot of discrepancies between the Max Speed on the GPS LCD screen and the top speed that GPSResults spits out. Does anyone know how often the "max speed" data is updated? Could it be more accurate than the speed thats being logged to the memory?
Thanks in advance,
Cheers
James
Hi James, the 2sec interval is correct. Don't go less unless you know your doing a very short session.
Also disable the option to wrap the data once memory is full.
Your max speed and previous track data should be cleared before each session. Also don't ever 'save' your sessions in the unit or you'll get a compressed form of data, when you download it will be active track that was there when you turned the unit off.
My understanding is also that the readout on screen for max speed is for any instant, the 2 sec peak from programs will be less than this as your averaging over a longer time, even tho its only 2 seconds. I'm happy to be corrected if someone knows better.
The 2 second setting is used in this device because it is more realistic and evens out the errors for the trackpoint data in the Garmin.
The vastly more accurate Doppler data from the GT-11/31 is logged at one second intervals.
The resolution of the Foretrex trackpoints in approx 1 .5m (do a search in GPS-SS for 'Grid Effect') which is why we use the 2 second setting.
Don't use the one second setting even if you are going for a short time. The correct setting for posting speeds is 2 second.
The display speed on the Foretrex (and just about every other GPS) is calculated from the speed over ground which is calculated from Doppler shift and is generally far more reliable and accurate than the trackpoint data saved to memory in the foretrex. (although you still will see errors from time to time). The advantage of the GT-11/31 GPS's is that they actually also save the 'Doppler' data to memory which is what allows much more accurate and reliable speed measurement from them.
Thanks for the replies all. I will set it to 2 sec logging for sailing this weekend.
Given that the display speed on the Foretrex screen is more accurate than that logged does that mean that is the speed we should be putting on GPSTC? I have always entered the max speed from the trackpoints, but maybe I've been selling myself short!
obviously if you're not using the downloaded track file you need to keep some kind of proof that the speed was real - maybe a photo of the screen?
cheers
james
Hi James,
If you're using the Garmin for the GTC, then post your 2 second max speed as calculated in software. Just make sure that you select "Trackpoint" as the calculation method, this lets us know that the numbers might be a bit less accurate than if you were using a NAVI.
Once you start getting speeds above 35 knots or so, it's usually a good idea to upgrade to a device that records doppler speeds. At the moment this is pretty much limited to a NAVI (GT-31 is the current model).
This means that we have a level playing field with the other competitors, and it also means that when you do crack 40 you know it's a genuine 40, not an artefact.
For example, (sorry to dob you in mate) Elmo was in exactly this situation about a month ago -- he had a 40 knot PB from years ago that was set using his garmin, but he didn't believe that it was a true 40. When he did crack 40 using his NAVI he was much more confident that he had in fact done that speed.
PB's of that size are incredibly valuable, I would bet that if you do manage to do that speed you would pay an arm and a leg to have the right equipment to record it.
However if you're just getting into this speed lark then we encourage you to post using whatever equipment you have, we believe that inclusiveness is more important than outright accuracy, at least at this level.
If I were after advice on Speedsailing and GPSs, Andrew would certainly be my first port of call, he's helped me out on numerous occasions, both with gps's and speedsailing.
James following on from sending you the cradle for the 201, I have a GT11 here gathering dust, all set up with an aquapac and 2GB card.
Easy to use, and simple to download your tracks to the pc. If you want it bud, its yours
cheers
H
