Using a GPS Recording Device to Gather Data


 I found a deal on a BowFlex 905 model GH-625 GPS based running watch and took a chance getting it for my wife’s birthday present. It turned out to be a great device for her because it logs her runs showing the exact distance, while also posting her speed along the way. When I saw the site that described how to measure your coefficient of drag for your car, I thought the running watch might work.It actually works very well for measuring your Cd. This particular model records data at 1-second intervals, and has enough memory to record about 13 hours worth of data before you have to download it to your computer. It makes recording the rate your car slows down a breeze. You simply turn it on and drive. I would accelerate to about 60 MPH, and then push in the clutch and coast. When you get home, you can just find the point in the log where your velocity peaked, and assume that is where the coasting begins. The GPS is recording the speed at 1-second intervals, and it is also recording your altitude changes. That is a hint of what I’ll talk about in the article on how to use the GPS recorder to calculate your energy use on any particular route you drive..


Getting the data out of the GPS device

So, what I described above sounds very easy and straightforward. Unfortunately, in order to use the data recorded by the GPS you need to get it into a form that can be used by a program or an Excel spread sheet. For my GPS device, this wasn’t so easy. It comes with software that downloads your data and then displays in a graph. This is very nice to look at, but I wanted to get certain parts of the data, time, velocity and altitude out in tab delimited data so I could import it to Excel. The GPS program would export it’s data, but not in that form. The best format that I was able to get the data in was called an activity file. It turns out that this file is in what is called an XML data format. Many GPS units use a similar format, and I found a site that you can upload your data to, have it convert it to various forms, including tab delimited, and then download the result. This site was, and the Link to the conversion page is here. It turns out that browsers can interpret XML files as well, but I have only gotten the data into a table form so far, not a file I can use.

Your Cell Phone can do this!

Well, certain cell phones with GPS devices in them should be able to do this. I found a number of apps for cell phones that will do some form of running logging using the phones GPS unit. I haven’t had the time to investigate it further, but I wanted to mention it here. If you looking into such an application check to see the logging rate that the data is saved at. You’ll want to have something that can save data at a rate of 1 or 2 seconds to be accurate. You’ll also want to find out how to export the data in a format you can use to do the calculations needed measure your Cd and calculate your energy use.