Math question RE: GPS track

[Benh]

I want to calculate rate of change of track acorss ground (in degrees). You could call it rate of turn.

OK, it's simple....all you do is subtract the incoming reading from the privious one. Well not quite.......all is dandy until you sweep over the 360 degrees line. If you were turning clockwise you could get 359 - 1 = 358, where what you really want is '2'.

Any ideas how you do this would be appriciated.

Many thanks, Ben

[derez]

The problem happens only for one reading, until you reach the next time around.
You can ignore this specific reading by a simple test:

if abs(new_track - old_track) < x then
.....
end if

where X must be larger than the maximum expected turn per second.

for continuity of display use for this specific "non reading" the previous value.

Edit:
If you do want that reading, since you know by using the test above that you have the difference of 360 degrees to the result, add or subtract according to the sign of the difference, the value 360.

[Benh]

Thanks derez

Sounds like a good idea. I'll give it a try.

Ben

[klaus]

If you take the difference of the incoming reading - previous reading you get a value.
Clockwise turning is positive and counterclockwise turning is negative.
In your example:
incoming reading 1°
previous reading 359°
so 1 - 359 = -358°
This means a counterclockwise turning of 358°
But this also means a clockwise turning of 2°
The following code will give you the turn in +/- 180°

DeltaDir = Direction(i) - Direction(i-1)
If Abs(DeltaDir) > 180 Then
 If DeltaDir < 0 Then
  DeltaDir = DeltaDir + 360
 Else
  DeltaDir = DeltaDir - 360
 End If
End If

Best regards.

[Benh]

Thanks Klaus

That's exactly what I ended up with!

Cheers, Ben

[nl1007]

you don't need the Abs() line, and extra 'If' level; just use:

If DeltaDir < 180 Then
DeltaDir = DeltaDir + 360
ElseIf DeltaDir > 180 Then
DeltaDir = DeltaDir - 360
End If

[klaus]

Are you really shure ?

In the case when
incoming reading 150°
previous reading 120°
DeltaDir=150-120=30

in your example DeltaDir<180 Then
DeltaDir = DeltaDir + 360 = 390
Do you want that ?

That's not what I want, I want results between -180 to + 180

In my code the result remains 30 because Abs(DeltaDir) is less than 180

Best regards.

[nl1007]

sorry, should have been < -180