Harry's GPS Suite Forum

Hi,

First, while I have autocrossed my WRX, I'm writing here because I need a high frequency GPS acquisition app for my iPhone for the purpose of accurately mapping trails through the woods. I'm a mountain biker and trail builder/maintainer and I'm stumped: I can't believe that the GPS apps I'm using so far can be so bad about creating accurate tracks. I recently bought the Magellan ToughCase which adds a SiRFstar 3 GPS chipset and good antenna, among other things, to my iPhone 3GS because I truly thought that was the limiting factor to getting accurate tracks. But I'm not seeing much improvement at all, which means it's either A) iOS's fault; or B) the app's fault.

I'm here because LapTimer seems able to actually obtain data at a frequency >= 1Hz and in cases 5Hz or even 10Hz. So, it seems that iOS can't be to blame, unless LapTimer is using a different API beyond Core Location (such as direct NMEA access?).

I've contacted support for both MotionX GPS and Cyclemeter, two of my favorite apps for the purpose of general GPS and/or ride recording. So far, both have said they record all the unique points that iOS's CoreLocation provides to them. How can this be true? The chipset in my new case can do 1Hz and I have also installed the Core Location Info (CLInfo) app and see it updating the data at least at 1Hz, so something isn't adding up because my tracks through the woods are sampling at an obviously slower rate and producing jagged lines.

I'm considering returning the ToughCase and going with one of the other solutions I'd considered (Emprum UltiMate or Dual XGPS). I am curious if LapTimer can be used for the purpose I'm interested in: creating highly accurate GPX or KML plots for individual trails.

Harry, I consider you an expert in this area and I'm hoping you can shed some light on this question. I just don't understand the difference here and I'd be happy to buy your app if you think it'd help.

Thanks,
Brett

Hi,

First of all check if the GPS of the ToughCase really works and is used. I have no experience with this device, so I cannot help further here. The webpage is not very informative on technical topics, but maybe you can contact their support. It should clearly increase accuracy as a) the iPhone3GS's GPS is weak and b) the Sirfstar III is an absolutely solid and sensible receiver. In case a Sirfstar III does not work, all other solutions will not work either... Another issue might be the shielding from the trees. Bad reception leads to decreased update rates as some of the fixes delivered are not useable any more. LapTimer shows the effective update rate, as soon as reception is poor, it decreases below 1 Hz.

Have you ever tested the update rates outside the forest, on a free field? Is there is difference?

LapTimer's integration technology does not use iOS's location service at all, it directly accesses the NMEA data delivered by the external chip. The GPS connected needs to be compatible with LapTimer to allow this, so there is a limitation to the ULtimate, the XGS150, TomTom car kit, and the Posimotion G-Fi (out of production). However, as stated above, as long as a Sirfstar doesn't work, LapTimer's direct integration will not cure the problem.

- Harry

Hi,

Thanks for your reply. The ToughCase GPS is definitely used as the iOS reported accuracy immediate jumps seconds after I turn on the external GPS, and drops again when I disable it. I've learned a lot in the last day, as I received answers to several requests I'd made to specific app's support departments. The short version is, the apps I've been preferring so far are among those that filter the data returned by iOS Core Location, so no matter the update rate from iOS, which is stated to max out at a sample every 700ms (~1.4Hz), these apps are making their own determination whether to keep the data points or not. This explains the behavior I was seeing even in open fields where my serpentine path was straightened out.

I do have a support request out to Magellan as well, but I have a feeling I know some of the answers already and won't get too far there. For one, I don't think the ToughCase is NMEA compatible, since neither you nor they listed it as such.

I do wonder if:
-there are other apps available that leverage NMEA and have a "trail" focus. I'll look as well but maybe you know of other similar but non-competitive app efforts here?
-LapTimer can be used for trail mapping purposes and how stressful this would be for the user.
-I should just be happy with the granularity I have since for either of the above I'd need to buy a NMEA compatible GPS device.

Thanks again Harry,
Brett

Hi again,

There are no apps I'm aware of featuring direct access to NMEA input - except the utility apps supplied by the device manufacturers themselves.
LapTimer has a compression mechanism for stored fixes too. However, it can be turned off, and it does not remove any fix that adds information (so it is mainly about removing fixes on straight lines).
As I do not have a ToughCase available, I cannot say if Magellan allows direct access using the so called EAAccessory interface.

- Harry