The distance I'm putting on the speedsailing site is the total Real Speed corrected distance (after deleting invalids and slow points) - I presume this is in km? Also what is the Tot Dop Dist - this seems to be much more?

are these KM's or NM's, and which should we use - total dist or total Dop distance?

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TonyC said...

The distance I'm putting on the speedsailing site is the total Real Speed corrected distance (after deleting invalids and slow points) - I presume this is in km? Also what is the Tot Dop Dist - this seems to be much more?

on my realspeed, total distance is in metres.
my tot doppler distance is only a small percentage more than trackpoint distance, certainly wouldn't say it was a lot more. What sort of difference are you getting???
As far as I know doppler distance is just that, distance calculated by using doppler. I assume it's doppler speed X time.
And I assume trackpoint distance is the sum of the distances between each trackpoint, these are a series of straight lines, so a bit of the circumference of any curve in your path is left off.
I asked a question about this yesterday here.
www.seabreeze.com.au/forums/topic.asp?TOPIC_ID=32739

Thanks Decrepit. My Tot Dop Distance is about 15% more - maybe I sail all over the place more than normal - or do I have a problem with my setup? I have been using the smaller distance (trackpoint?).

Thanks

Tony

15% sounds a lot, I can't access realspeed at the moment it won't work in this operating system, but I'll check later and see what my variations are, I was doing 700m runs yesterday and covered 100kms so there should be as much variation as I'm going to get, but from memory the difference was less than 1%.

In my ramble yesterday,
www.seabreeze.com.au/forums/topic.asp?TOPIC_ID=32739
I proposed that if you've set up realspeed to use doppler results for speed, then also use doppler for distance, but if realspeed is using trackpoints for speed calculations then you should also use trackpoints for distance.
So far nobody has commented on this proposal, so I guess it's do what ever you feel like.

I always use the trackpoint distance because at the moment the Doppler total distance can be wrong if you use a speed threshold for data recording.

I always use the 'file' 'properties' menu to see the total distance in KM's. The distance that is printed out on the results sheet is in NM if that is your setting. I have asked Mal to change this to KM.

If you have made your mouse 'active', when you hold it over the very end of the speed graph you will see the trackpoint total distance in meters.

Thanks Decrepit and Sailquick - explains a lot - I am set up for doppler on each of the divisions and I have a high minimum speed - so I presume that explains the difference. Will continue to post the trackpoint distance. Good tip on using the "Files" "Properties" to check. Mine also shows a -9m min altitude - explains a lot, I always thought I spent too much time under water

I've always used the bottom right hand data window in realspeed for my distance, also hadn't used file properties, that's a good tip.

I've just checked a file with very few invalids, tot distance 36426.88 tot dop dist 36594.16.
That's about half a percent difference, if you're getting 15% perhaps you're deleting too much????

Sailquick, I don't understand your point about minimum speed settings, surely that affects trackpoints just as much as doppler? Neither of them will produce a result under the minimum threshold.

I've checked a few files, ones without bad invalids doppler is more, files with bad invalids doppler is less. makes me think doppler is just more accurate, recording what's actually happening.

Hi Decrepit. The GPS stops recording points when below the min speed set but the trackpoints still add up to the correct distance. The distance spans the gap. Doppler distance is worked out on speed. If there is a gap in data points, the software assumes the last speed recorded continues for the time of the gap. But it may not of course. I think Mal is working on a way to minimise this difference.

Yes, it seems that the Doppler speeds are more reliable and more consistent. As the whether they are more accurate.... not totally sure about that one. The big question mark for me is why the Doppler speeds come out so much lower than the trackpoint speeds. There is a small grid effect in the trackpoints but this should mostly equalise over 10 sec or more. There can be some accumulation of error but this again should be quite small when sat signal and number is good.
The trackpoint positions vary because of changing atmospheric conditions. These changes tend to happen quite slowly causing a drift in location. This should have little effect over short times so relative speeds in the time periods we are mostly interested in should not be affected very much at all. Most of the errors we see from trackpoints are because of loss of satellites or sudden switching from on sat to another.
On the other hand, Doppler should not be affected by the same atmospheric condition changes so the speeds should be as accurate as the Doppler calcualations all the time, and indeed this seems to be reflected in the more consistent results from Doppler.
So why do we consistently see up to half a knot difference?
For comparison in a competition, Doppler is clearly more consistent and therefore fairer. But as to what the actual speed is.............who knows?

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sailquik said...

Hi Decrepit. The GPS stops recording points when below the min speed set but the trackpoints still add up to the correct distance.

Thanks sailquik, guess that should have been obvious.

I was assuming that over longer times trackpoint speeds would be a little less than doppler, as trackpoint calculations don't take the full circumference of any curves in the track into account. But of course I haven't actually checked that yet!!!