Monday, April 22, 2013

Speedometer v1.5.1 – KML export and looking at GPS altitude data

One of the new features of many in version 1.5.1 is ability to export track/trip data in proprietary Speedometer’s format or KML/KMZ (Google Earth)

To keep the information more intense let us combine this feature showcase with analysis of how good altitude data built-in GPS receiver in iPhones and cellular iPads gives you.

Here is part of a track exported to Google Earth from Speedometer:


Google’s Earth is configured to show the absolute elevation and show the bars from the altitude reported by iPhone’s GPS receiver to what Google believes is the “ground”, Speedo’s exported track contains altitude data.

Ideally, given Google is right, we would see a path everywhere with no bars to the ground. As we can see from this example (quite typical, compared to the rest of the route) there are bars and path is disappearing from time to time when altitude reported by iPhone’s GPS  is less than the Google’s ground level is.

So, should we tell GPS receiver in iPhones (and cellular iPads) is really bad? Here is the truth, all GPS receivers are generally bad at telling you the altitude. Here is a link to a short and GREAT article on this subject. Let me cite from it:

“New GPS buyers are frequently concerned about the accuracy (or lack of it) of the altitude readout on their newly purchased GPS. Many suspect their equipment may even be defective when they see the altitude readout at a fixed point vary by many hundreds of feet. This is NORMAL.”
”Generally,  Altitude error is specified to be 1.5 x Horizontal error specification.  This means that the user of standard consumer GPS receivers should consider +/-23meters (75ft) with a DOP of 1 for 95% confidence.  Altitude error is always considerably worse than the horizontal (position error). Much of this is a matter of geometry.”

”What this means is that if you are walking on the seashore,  and see your altitude as -15 meters,  you should not be concerned.  First,  the geodetic model of the earth can have much more than this amount of error at any specific point and Second,  you have the GPS error itself to add in.  As a result of this combined error,  I am not surprised to be at the seashore and see -40 meter errors in some spots.”

Having iPhones and cellular iPads GPS honor restored, lets continue with what speedo in its 1.5.1 version will be capable of in area of KML export:

- Ability to export multiple tracks under the same trip as Google’s Earth folder with all info nicely organized track by track with A and B point with their corresponding start/end times
- Export in KMZ zipped format to save on data transfer

As version 1.5.1 was released to AppStore on June, 9th - you can read the complete GPS tracking manual here:


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