Harry's GPS Suite Forum

Hello Harry,
I would like to know if you could add GPS based acceleration.
As a motorcycle rider, I keep my iphone in my leather suit and therefore can't use the embedded accelerometer.

Would it be possible to add lateral/longitudinal acceleration based on the GPS signal?

Longitudinal acceleration can easily be processed as it is the derivative of the speed.

Lateral acceleration is a bit more complex, but can also be done:
It is expressed as Alat = v^2 / R ,with v = speed and R = radius of the corner
R can be found knowing 3 GPS points (latitude, longitude) :
R = a*b*c/(4*K)
with a, b, c are the length of the 3 sides from the triangle
and K = sqrt(s * (s-a) * (s-b) * (s-c))
with s = 0.5 * (a + b + c)

All in all, we finally have :
R = a*b*c / (4 * sqrt(0.5*(a+b+c) * (0.5*(a+b+c) - a) * (0.5*(a+b+c) - b) * (0.5*(a+b+c) - c))

To convert latitude/longitude into X,Y cartesian, here is a link : http://stackoverflow.com/questions/5983 ... coordinate

Let me know if you think it can be included in a future version, or if you need other details.

Well prepared, but already available

Just check Lap List -> Lap -> Recalculate -> Acceleration, select the level of smoothing you want to apply (that's actually the best about he feature - and absolutely necessary to generate reasonable results), and press Done.

Please take care about your GPS accuracy. Putting the iPhone / Android phone into your suit will often degrade GPS accuracy to a degree it cannot be used any more. An external BT GPS is a good idea here.

- Harry

Neoraptor,

The higher data rate of an external BT GPS improves precision and accuracy of calculated acceleration as well. I think it's easier to calculate lateral acceleration as the product of yaw rate in radians/sec and velocity in m/sec. Then divide by 9.81 to get units of g. But in either case, a simple three point fit to calculate lateral acceleration from fitting a circle to three position points or yaw rate from the derivative of heading is incredibly noisy, especially at a data rate on the order of 1/sec. Harry has implemented a method that works much better.

Perfect Harry
I didn't see that option !!

I already use a XGPS150 that is fixed in the tail of my bike (so in direct line to the sky).
It gives me a refresh rate of 4Hz.

@gplracerx: thanks for the details

I was able to press recalculate in my ipad w retina to 10 hz, but not able topress smooth data calculation, I figured that would make the lean angle smoother..
On my iphone 4 tried several way and at best it got half way to process ONE lap and then the program closed.
there was plenty of space even with raw video, over 5 GB
The one that was done in the ipad will be uploaded to you tube shortly.

Qomomoko wrote:I was able to press recalculate in my ipad w retina to 10 hz, but not press smooth data.
On my iphone 4 tried several way and at best it got half way to process ONE lap and then the program closed.
there was plenty of space even with raw video, over 5 GB
The one that was done in the ipad will be uploaded to you tube shortly.

What do you man by "but not press smooth data"?
For the iPhone4 you are on the same topic (smooth data)? As this process is usually not memory constraint, there is probably some data not handled correctly... You may send the lap your iPhone4 crashes with to me: please use .hlptrl as export format and add a comment / link to this thread when you send the mail generated.

- Harry