v20201130.1
If you are viewing Events on Notehub, you may notice a location
populated for an event, and when you investigate the location by
clicking it and viewing it on Google Maps ™ you may find the location to
be different than the true location, and perhaps even kilometers away.
There are a few nuances to location to consider: the type of location,
when and where the location was acquired, and location acquisition
failure.
GPS versus Tower Location:
The Notecard captures two distinct locations.
- GPS (global positioning system)
- Cellular Tower
GPS is the most accurate method of determining location and is defined
as the Notecard location or “where” field. This uses a satellite-based
system to locate the Notecard on the globe with a resolution of several
meters. However, GPS locations are much more power hungry than tower
locations, and they are only possible when the Notecard is outside with
a view of the sky. For these reasons, the Notecard also reports other
approximate locations.
The Tower location, in the tower lat and tower lon fields, is an
estimate of the latitude and longitude of the cellular tower that the
Notecard connected to during the most recent data sync. This lets you
know the general proximity of where the Notecard was when it synced
Notes with Notehub. Its effectiveness is limited by the accuracy of the
available tower location data, and the cellular operator using unique
location identifiers for all their towers. It is generally more accurate
in urban areas where cell towers tend to be more closely spaced, and
less accurate in rural areas. In some cases, if the notecard connected
to a cell tower across a large lake or ocean bay the tower location and
the true Notecard location can differ by many kilometers.
Location Change Since Last GPS Capture:
All data added to the Notecard will be location-stamped with the last
known GPS location, but the Notecard may have moved a significant
distance since the last location capture There are two reasons that this
can happen. First, the notecard will not update its GPS position more
frequently than the configuration of seconds in card.location.mode,
so if this is set to a large value, such as multiple hours, as would
often be the case for battery powered devices, the Notecard can move
hundreds of miles during this period. Second, if the Notecard has been
moved in a manner that has not allowed it to make a GPS position fix.
For example if the Notecard were placed in a shipping container and
shipped a long distance the GPS location would not change until it the
next time the Notecard was able to make a GPS position fix.
| Data Event | Timestamp | Temperature | GPS Location | Tower Location |
|---|---|---|---|---|
| E1 | T2 | 21 | A | C |
| E2 | T3 | 27 | A | C |
| E3 | T4 | 17 | A | C |
| E4 | T6 | 23 | B | C |
| E5 | T7 | 13 | B | C |
Each data recording event [E1 – E5] is provided a timestamp and location-stamp. The location- stamp is based on the last known GPS location.
In the scenario shown above, the Notecard moves from location A to location B. There are 3 temperature measurements [E1 - E3] that occur between GPS location updates. The temperature measurements are stamped with location A, including the two measurements when the Notecard has already moved to location B. Once a new GPS location has been acquired the location stamps will be updated as shown in E4.
Options to Improve Location Accuracy:
Increase the frequency of GPS location updates.
If GPS updates are configured to periodic mode, the Notecard will only
attempt to update GPS location if the Notecard has detected motion from the accelerometer. So, there is little energy cost to increasing GPS acquisition frequency (reducing “seconds”) when the device is not frequently in motion, but a significant energy cost if the device is frequently in motion.
Failed GPS Signal Acquisition
GPS location not updated when Notecard cannot acquire satellite signal
If a Notecard has detected movement and cannot acquire a GPS satellite
signal, it will not update its location data.
If the Notecard is moved some large distance and is now inside a
building and cannot acquire a GPS satellite signal, the current location listed by the Notecard is the last known GPS location. This could be any distance away from the actual current location of the Notecard.
| Data Event | Timestamp | Temperature | GPS Location | Tower Location |
|---|---|---|---|---|
| E1 | T2 | 21 | A | C |
| E2 | T3 | 27 | A | C |
| E3 | T4 | 17 | A | C |
| E4 | T6 | 23 | A | C |
| E5 | T7 | 13 | A | C |