I am a new user to the LapTimer App (last week) and have to say it has an amazing set of features packed into a small package. I was interested in 5 Hz acquisition, so I also picked up the Emprum device. Just having got an iPhone 4S, I hadn't really tested its GPS ability with any other application, so I didn't know what to expect. From the forum posts, the consensus seemed that at least for the iPhone 3 series, an external GPS was preferred. But I haven't seen a direct comparison posted for the iPhone 4S. So I conducted a very informal comparison using LapTimer as a readout and recording data to/from my daily commute to work, which is nearly all highway driving with unobstructed views of the sky. Just to note, the iPhone was in 3G only mode (WiFi disabled) using iOS 5.0.1 and for the Ultimate I am using firmware v1.0.6.
As I expected, the average acquisition rates of the iPhone GPS was around 1.0 Hz, while I achieved 4.6 - 4.8 Hz with the Emprum. Since the Emprum uses NMEA, I monitored the fixed satellites. In my travels, I normally had 9 to 11 sats in view, with fixes on 7 - 9 of them. Although their signal strength were all average, all signal strength bars gave a "green" indication for most of the open areas. On occasion, the signal did slip to yellow briefly. So far this is pretty much what I had expected and pretty good given I hadn't done any tweaking with in-car position. So although precision appears to be much better when using the Emprum, I was surprised to find out that iPhone 4S GPS was reading out 2-3X better accuracy than the Ultimate. According to the LapTimer in GPS view, the iPhone managed between 16 to 32 ft (~5 to 10m) positional accuracy, while the Ultimate was about 32 to 64 ft (~10 to 20 m). I've attached a screenshot comparison of GoogleEarth street view of my recording that illustrates these differences, showing the same road in both cases (just traveling in two different directions). I chose a section of my trip that wasn't on the highway, but around a couple of nice corners to illustrate the effect on accuracy. The differences of accuracy between these devices was still 16/32 ft vs 32/64ft on open highways.
I was wondering how others' experiences were with these devices. Honestly I'm shocked that the iPhone achieved 16ft accuracy, which was fairly consistent. How does it achieve better accuracy than an external device, perhaps I have not optimized the Emprum devices enough, though why would it be more particular about position than the iPhone, both of which were in the same location for the tests. Has anyone achieved 16 ft accuracy with their external devices? Overall, I'm not sure the increased acquisition rate outweighs the loss in precision? Any other insights into my informal Day 1 comparison are welcome!
PS As a side question, in the Satellite view, what is the difference between "3D" and "3D+" accuracy?
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