Track on Track: Reflections on GPS Accuracy on a Running Track
Recently, while at the Applied Geography Conference in Atlanta, I decided to test the spatial accuracy of my smartphone’s GPS in a challenging environment–a rooftop running track. Although located on a roof, the track was surrounded by buildings far taller, and in downtown Atlanta, a location with many other buildings impeding signals from GPS, wi-fi hotspots, and cell phone towers. A further challenge to the GPS positional accuracy was that each lap on the track was only 0.10 miles (0.16 km), and therefore, I would not travel very far across the Earth’s surface.
After an hour of walking, and collecting the track on my smartphone with a fitness app (Runkeeper), I uploaded my track as a GPX file and created a web map of it in ArcGIS Online. As I expected, the track’s position was compromised by the tall buildings–I only had a view of about half the sky during my time on the roof. As you can measure for yourself on the map linked above, the track lines formed a band about 15 meters wide, but interestingly, were more spatially precise along the eastern side of the track, where the signal was better, as you can see in my video that I recorded at the same time.
Also, as I have encountered numerous times in the past, a line about 100 meters long stretches to the north. Rest assured that I did not leap off the building, but rather, the first point that the GPS app laid down as I opened the doors to walk outside was about a block away. Then, as I remained outside, the points became more accurate. When you collect data, the more time you spend on the point you are collecting, typically the more accurate that point is spatially.
Another interesting aspect of this study is that if the basemap is changed to satellite imagery, it appears that the track overlaps the tall building to the west. Try it, using the map link above. However, a closer investigation reveals that this is a result of the orthocorrection that was performed on the imagery; the buildings do not appear from “straight overhead”, but rather, they “fall away” to the east. Turn this into another teachable moment: Images, like maps, are not perfect, but they are very useful. We can learn to manage error and imperfection through critical thinking and through the use of geotechnologies. This is a central topic of our book and of this blog.
To dig deeper into issues of GPS track accuracy, see my related post on errors and teachable moments in collecting data, and on comparing the accuracy of GPS receivers and smartphones and mapping field collected data in ArcGIS Online here and here.
Despite these challenges, overall, I was quite pleased with my track’s spatial accuracy, even more so considering that I had the phone in my pocket most of the time I was walking.