Harry's GPS Suite Forum

Hello, I recently purchased a TomTom car kit for use with LapTimer and an iphone4. When reviewing the GPS page (while connected to tomtom) I noticed my reported Accuracy is usually around 32ft and my GPS Rate is around 0.8 - 1.0hz. When using the built-in iphone4 GPS (not connected to tomtom) my Accuracy is always 16ft and my GPS Rate is always 1.0hz. I am curious if these are typical numbers for the tomtom kit. It seems to me a lower Accuracy number is better and a higher GPS Rate number is better? Thanks!

-Mathew

Hi,

Both accuracy and update rate are not directly comparable. On the update rate side LapTimer has a very picky calculation of effectively new fixes coming in. 0.8 to 1 Hz is exactly what you get with a 1 Hz device like the TomTom. On the accuracy side again, calculation is different again. While LT takes what Apple delivers as accuracy for the internal GPS, it calculates this accuracy from the so called HDOP field delivered from the NMEA stream. This calculation is, well - conservative. It dates back to 2006 when receivers were much weaker. I'm currently working on this - and make it specific to the supported receivers and the chips used. The 32ft for the TomTom should actually be something around 15ft or so - today it displays roughly double the distance.

- Harry

Harry,

Thanks for the explaination! But now I question why use the tomtom car kit? It appears I get the same GPS Rate and Accuracy with the tomtom as I do without. I assume there is another advantage to the tomtom other than GPS Rate and Accuracy?

-Mathew

Hi,

Yes, certainly. Please check http://www.gps-laptimer.de/GPS_and_Accuracy.html for the factors influencing accuracy.
In addition you receive a mount and power supply.

- Harry