Drift (Inaccurate Tracking) #
Drift is the GPS term for any skewing of the location by an outside factor. As you read in the “Accuracy” article, there are numerous factors in determining a device’s location. So, even a device location that is an inch off, is technically experiencing drift.
Below are the most common occurrences of Drift that customers see. Please note, how the device is worn or installed can impact accuracy and increase drift as well.
This is the most common form of drift. Interference is caused by buildings, large bodies of water, thick foliage – any “physical structure or material”. This interference impacts the signal before it reaches your device. As you read in other articles, essentially all GPS signals have some level of drift. This is most often seen when outdoors but near large buildings. The GPS signals are bouncing off the buildings around you before reaching the device and skewing the location by several meters.
To be considered a GPS device, the unit needs to always attempt GPS first before other location methods. Just because a device is indoors, does not mean it will always have to resort to WiFi. GPS signals are able to pass through windows.
So, this is most often seen when in areas of a house that are near windows, like bedrooms. The GPS signal bounces outside the house, enters the window and connects with your device. If the GPS coordinate is within 50 meters, it will publish. So, if you find anytime the device is in one particular room the location is off, that is most likely why. It is technically getting good enough GPS to publish and not resort to WiFi which might be more accurate at times.
(A) The first type of WiFi drift is also discussed in the FAQ. WiFi Touch reads a “tag” on the WiFi signal that has an assigned location and address. The device uses this location as its own. So, if in a store like Costco and on the perimeter, but the WiFi location is assigned as the middle of the store, the location will show in the middle, rather than where the device physically is.
(B) The location and address on the WiFi tag is assigned by the WiFi’s internet provider, generally in conjunction with Google Maps. Sometimes, these locations can be off:
- If you just moved to a new home/ apartment, the service provider/ Google Maps can be delayed in updating the “tag”. So, when locating via WiFi, it shows the old address.
- The lat/long associated with the address on the tag is not exactly where the home/ building is, but a few meters off
(C) This is an extension of the first WiFi drift and pertains to large warehouses. WiFi signals can only travel so far. So, there are “boosters” that you can set up around large warehouses/ buildings that boost the WiFi signal in areas the main hub can’t because it’s on the other side of the building.
These boosted signals though are an exact copy of the original WiFi signal. This means the tag has the same location and address as the WiFi source. So, if the WiFi tag is assigned to the far end of the building all WiFi locations in that building will show there, no matter where the device physically is.
This kind of drift is the rarest and also the most obvious form of drift. As the name suggests, this drift is when you see the device at home and then the next location it shows is in the middle of the Sahara Desert. A few minutes later it locates again and it is back at home.
This kind of extreme drift is actually caused by the GPS satellite itself. As the satellites orbit the Earth, they move towards the horizon. Once the satellite nears a 20 degree angle from the device, it stops the connection and hands the info off to a closer satellite so your device can continue getting accurate GPS data.
Sometimes, there is a hiccup with this handoff. That is when the location data becomes extremely skewed and drifts halfway around the world. The reason it is published is because the signal coming from the satellite “tricks” the system into believing it is accurate.
Satellite movement can cause smaller instances of drift to occur as well. Let’s say every day from 4-6, the person wearing the device does activities in the backyard. They are moving, but essentially staying in the same spot. So, looking in History, you see from 4-6 every day there is a cluster of locations there. But on Tuesdays, Thursdays, and Saturdays, you notice there is always a brief period where tracking “wanders”, giving the cluster a “tail”.
As the satellite was moving closer to the horizon, it was causing smaller drift to occur. Once it handed off the signal, no hiccup happened so the tracking went back to being accurate and not drifting/ wandering.