I'm trying to figure out the 2 sec calc because today I got 7 readings in a row:
29.28
30.25
30.67
30.27
30.22
30.52
29.59
but my RealSpeed and KA72 2 sec peak is (http://www.ka72.com/TrackRecalc.aspx?fid=32417)
29.89

I found a post from TimeMachine (RealSpeed author): www.seabreeze.com.au/forums/Windsurfing/Gps/Bug-in-Real-Speed

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40, 42, 43 (wishful thinking in these windless days I know...)

The two second result should be calculated as follows:
((40+42)/2 + (42+43)/2) / 2 = (41+42.5)/2 = 41.75

So mine would be
((30.25+30.67)/2+(30.67+30.27)/2)/2=(30.46+30.47)/2=30.465

Using GT31, latest firmware.

What am I missing? Something to do with Doppler?

Just created a 2 sec division based on trackpoints and got 30.47 so I guess doppler is the reason. But still none the wiser really.

Patrick,
Doesn't look right.

Check your Realspeed settings, see my post re: Realspeed Settings.

I would be interested to see your settings to compare.

Send me your track & I will check it.

Yes paddymac, you are looking at trackpoint speed.
I thought that it was actually your doppler distance (in m) (30.75m from screenshot) between three points divided by 2 for m/s *3600 / 1852 = 29.89
Sorry

Thanks Mark

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paddymac said...

Just created a 2 sec division based on trackpoints and got 30.47 so I guess doppler is the reason. But still none the wiser really.

Well, I never realised that before
When looking at the 3 x 1 seconds that make up the 2 sec max speed they are actually Trackpoint & not Doppler even when you select Doppler

I also thought it was an average of the 3 x 1 second, so I have learnt something else as well

In real speed the speed graph at the bottom can show both trackpoint and doppler lines in different colours.
(I think you right click in the bottom pane to select what's in there).
If you expand the 2s result out you can see the doppler data that's being used.
It's a pity the chart only shows trackpoint speed.

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decrepit said...

In real speed the speed graph at the bottom can show both trackpoint and Doppler lines in different colours.
(I think you right click in the bottom pane to select what's in there).
If you expand the 2s result out you can see the Doppler data that's being used.
It's a pity the chart only shows trackpoint speed.

I am not sure what you mean by the chart only showing trackpoint speed. If you make a Doppler speed 2 sec division the speed shown is Doppler speed.

I'm fairly sure Paddymac meant when he made a 2 sec division based on trackpoints he got the same average from when he calculated by hand using the trackpoint data.

2 sec should definitely be doppler or select doppler if available.



Paddymac - right click over the track data, select grid fields, deselect speed and tick doppler. Do the same for distance and total doppler distance. In the graph window you can also right click for fields and turn on doppler.

Paul - Realspeed calculates the 2sec speed centred on the one second peak. So in the trackpoint sample Paddymac posted its saying he did 30.67 knots for 1 second, then it assumes 30.25 for half a second prior and then 30.27 for half a second after, so 2 seconds total. If you average just 2 numbers the average won't be centred on the peak, it will be off to one side of the peak.

Thanks Froude, I've got the Doppler speed displaying now in the track grid.

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sailquik said...

>>>

I am not sure what you mean by the chart only showing trackpoint speed. If you make a Doppler speed 2 sec division the speed shown is Doppler speed.

I'm not talking about the division results, and I'm not in windows so can't open real speed, only going by my failing memory here.
By "chart" I'm referring to the pane that shows, date, lat and long, total distance, total doppler distance, etc, and "speed" I haven't seen "doppler speed" there. If it had been paddymac wouldn't have been puzzled by unexpected results.

EDIT. just read Froude's message above, and it seems I haven't tried changing speed to doppler, next time I have realspeed open I'll have a look at that.