I have been using an iPad for forestry field work for several years – iPad, iPad 3 and an iPad Air 2. For most of that time I have relied on the GPS unit built into the iPad for navigation and for the capture of spatial data. Since the beginning of 2015 I have been using a Gamin Glo paired with my iPad via Bluetooth. Recently I had the opportunity to compare the:
- Garmin Glo
- Bad Elf Surveyor
- iPad Air2
Where I have a good view of the sky I would normally expect that an iPad will give me a positional error less than 8 meters 95% of the time – most of the time it is much better than this. In forested conditions the error is less than 12 m 95% of the time. The problem is that you don’t know when the accuracy is worse than this! When the iPad is paired with a Garmin Glo the error is reduced by about a third – less than 8 meters error 95% of the time in forested conditions.
When I rely on the GPS Receiver built into the iPad there is always a delay in determining my location as the iPad does not get a good view of the sky when I carry it against my body. So when I pull the iPad out of my chest pack (or vest) it takes 15 to 60 seconds to get a good position fix when there are trees overhead. What I really like about pairing my iPad with a Bluetooth GPS is that there is no delay in determining the position as the GPS receiver can be carried in a location where it has a good view of the sky all of the time. Ideally above your head but I have found that it works well when carried in the top of a backpack.
Recently I had the opportunity to borrow a Bad Elf Surveyor Bluetooth GPS receiver. This GPS Unit has a lot of potential as it promises better accuracy than the Garmin Glo at a modest price – $120 for the Garmin Glo and $600 for the Bad Elf. Also, when Bad Elf delivers on their promise to provide the ability to post process the GPS data we should be able to improve the data.
What I really wanted to know was how the Garmin, Bad Elf and the GPS receiver built into an iPad Air2 compared when used in a dense forest. As I don’t have access to a surveyed GPS test range I had to come up with an alternative for test purposes. Fortunately not too far from home there is a forestry test site on which the trees are planted in a very uniform grid. This makes it easy to walk in straight lines under a dense over story. By looking at how much each device’s track deviated from a straight line and the variability of the waypoint locations for a few fixed points we should get a good idea how reliable each is.
Please note that the Bad Elf Surveyor is “optimized for slower or stationary outdoor environments”. For this test I spent only about 20 to 30 seconds stationary when capturing waypoints, which may not be enough time to get the high accuracies that Bad Elf Surveyor is capable of. However, this test does represent how I would normally capture GPS data.
For this test:
- Bad Elf was paired to an iPad 3
- Garmin Glo was paired with an iPhone 4S
- iPad Air2 was on its own
I carried the iPad Air2 in my hand facing up so it would have a decent view of the sky. The Garmin Glo and Bad Elf were carried on top of my hardhat.
When out in the open all devices performed very well. When I plotted each device’s track on a map the lines were often overlapping or parallel with only 2 meters between them. The Bad Elf and Garmin Glo were very similar in performance. The iPad Air was noticeably less consistent – this is illustrated in the plots below.
Open Space Testing
All performed well when recording waypoints in the open:
- iPad Air – scattered within a 4 m diameter circle
- Garmin Glo – scattered within a 2 m diameter circle
- Bad Elf Surveyor – scattered within a 2 m diameter circle
Overhead Tree Testing
Things got a little more interesting with trees overhead. Again the iPad Air2 was the poorest performer. The Garmin Glo and Bad Elf Surveyor performance was very similar. The iPad Air2 waypoints were scattered within about a 12 to 14 m diameter circle. Both the Bluetooth devices waypoints were scattered within about a 10 m circle. I may have been able to improve this by spending more time at each fixed location before logging the waypoint.
The tracks for both Bluetooth GPS receivers were reasonably parallel lines spread over about a 6 m wide area – the Garmin was actually a little more consistent than the Bad Elf. The iPad Air 2 was just a little more variable with the tracks spread over an 8 m wide area. This is illustrated below.
When an iPad is paired with a Bluetooth GPS receiver the quality of data that is captured is significantly improved. For most of my work the accuracy of the iPad’s GPS receiver is good enough. However, I much prefer to use a Bluetooth GPS receiver I don’t have to wait for the iPad to acquire an accurate position “fix” as the Bluetooth GPS always has a better view of the sky than the iPad.
I was surprised how well the Garmin Glo performed when compared to the Bad Elf Surveyor. With price considered it would appear that the Garmin Glo is a better value. However, the Bad Elf has several features that make it worth considering:
- A great app for the iPad that provides information regarding the position accuracy, number of satellites, and signal strength. Paying attention to this will give you some idea as to the accuracy of the data being captured (this is not available for the Garmin Glo)
- The ability to store data on the GPS Receiver rather than on an iPad. This can greatly extend the battery life of the iPad
- Post processing of the GPS data. This functionality is coming soon via third party software providers.
For the work I perform I will stick with the Garmin Glo. It provides the accuracy I require at a very reasonable price.
However, for many applications the Bad Elf Surveyor will be a tremendously valuable device, especially when the capability to post process the data is made available. For many applications the ability to post process the data will make the Bad Elf Surveyor a clear winner