I have had many clients and colleagues ask what is the accuracy and reliability of the GPS receiver that is built into a tablet computer and what can be done to improve on this? Recently I did some digging to find out what options are available.
Ideally I would have collected all of these devices and run them through a surveyed GPS test range to get data for comparison. However, this was not possible for me to do, so my comments are based on my own experience and interviews with field users of these devices. I have included some other commonly used GPS receivers for reference. The “Accuracy” numbers in the table indicate the error will be less than the stated amount 90% of the time – at least this is my best estimate anyways. Which means that 10% of the time you can expect the error to be greater than this – the challenge with many of these devices is that you don’t know when this occurs.
|Device||Description||Accuracy Forested (m)||Accuracy Open (m)||Cost Estimate|
|Apple iPad||To get a built in GPS/GLONASS receiver you must purchase the version that is capable of connecting to the cell network. The WiFi only model does not have a built in GPS receiver.||10 to 15||4 to 8||$700|
|Garmin Glo||Bluetooth GPS receiver that is GPS and GLONASS capable and works with Apple devices.||8 to 12||3 to 6||$100|
|Trimble GEO XH (with External Antenna) Trimble GEO XH (with External Antenna)||These devices are an all in one unit (no tablet required). I included it in this table, as these devices really are the gold standard for GPS/GLONASS receivers.||2 to 4 (<1m when post processed with sufficient data points)||<2 (within centimeters when post processed)||$10,000|
|SX Blue II||To achieve good position accuracy the GPS receiver must connect to an SBAS satellite at least once every 40 minutes. This should be possible in most forested environments.||2 to 4 (Provided SBAS signal)||<2 (Provided SBAS signal)||$3,000|
|Bad Elf GNSS Surveyor||A GPS/GLONASS Receiver that works with an iPad or iPhone.||?||Advertised <1m when stationary & <2.5m moving||$600|
|Garmin GPSMAP 64||High-sensitivity GPS and GLONASS receiver with quad helix antenna.||5 to 8 m||2 to 4||$300|
I included an iPad in the table to represent all tablet computers as I have found that with Android and Apple tablets I get about the same accuracy with the GPS Receiver built into the device. As a Forester I have found that my position is usually better than I can achieve with a map and compass unless I am willing to tie into a known location every hour or so.
I believe that one of the reasons that a Garmin Glo achieves better accuracy than the GPS built into an iPad is because it can be positioned in a location where it gets a better view of the sky. Your body easily blocks the signals from the GPS satellites. This means that even if it is on your shoulder you will still be blocking some satellites with your head. The Garmin Glo represents the “consumer” class of Bluetooth GPS receivers. Others include those made by Bad Elf and Dual. One benefit of using an external GPS is you will increase the battery life of the tablet computer.
Trimble GEO XH with an External Antenna
This device sets the standard by which other GPS units are measured. They are amazingly durable and very reliable. Their real time accuracy is great and by post processing the accuracy is improved significantly. Even without the external antenna they are very accurate and reliable. The position accuracy is diminished slightly without the external antenna. However, the cost is prohibitive for many businesses – about $9000 without the external antenna. For most applications in the Forest Industry these units provide better accuracy than is really necessary.
SX Blue ll
I had the opportunity to try one of these earlier this year. The position accuracy is not as good as a Trimble – but not a lot different. The position is corrected in real time by using a correction signal (SBAS) broadcast by a satellite. The SBAS signal needs to be received at least every 40 min. In most forested environments this should not be a problem. On the SX Blue website there is a good explanation of how real time corrections are performed. These GPS receivers are a great fit for most applications in the forest industry as they are provide a good balance between cost and position accuracy. EOS Positioning Systems will be releasing a similar Bluetooth GPS receiver shortly.
Bad Elf GNSS Surveyor
This is a really interesting low cost GPS/GLONASS receiver. It has been specifically designed to work with Apple (iOS) devices. It was released in September this year. Consequently I have not been able to find anyone to talk to that has used it in a Forestry application. What makes it really interesting is that it will be possible to post process the data. This means that you will be able to determine the accuracy of the data collected and improve it by removing the suspect data. As of yet there is a lack of applications written to utilize this data but you can bet they will be available soon. You can learn more on the Bad Elf website.
Garmin GPS Map 64
This is the lowest cost receiver in the 64 series. These “recreational grade” GPS receivers work amazingly well. My experience has been with the older 60 series, which were GPS only. The 64 series work with both GPS and GLONASS which means twice as many satellites to connect to. This may give accuracies better than I have estimated. These GPS units are head and shoulders above most recreational grade receivers largely because of the “Quad Helix Antenna”. Most consumer units use a patch antenna, which does not work nearly as well in forested environments.
This is by no means intended to be a complete review of the options available but my hope it is a good starting point for field users of tablet computers that are looking to improve the GPS data they are collecting – or at least make an informed choice regarding the options available.