Situm is able to provide the location of the user with a certain degree of accuracy. This position is always provided cartesian coordinates (XY) with respect to the bottomleft corner of the floorplan (see next figure). Situm also provides this location to latitude-longitude (WSG 84). In this section, we will explain the location estimation features in detail.
Convergence or Time To First Fix
When the Situm positioning system is initialized, it starts from zero. During the first seconds, Situm has little information so the first locations may be slightly inaccurate/erratic. Little by little, the system receives sensor readings (Wifi, BLE, gyroscope, etc.) and the location estimate will converge to an stable and accurate position. The following figure shows a representation of how the system error is initially high, but decreases as it approaches the convergence.
In Situm, the convergence time, usually called the Time To First Fix (TTFF), is usually 4-15 seconds. After this convergence, Situm accuracy is usually within 1-5 meters on average.
Both, the convergence time and the accuracy, depends on:
- Movement of the user. If the user moves, Situm needs less time to yield a first accurate position. Further positions will also be more accurate.
- Magnetometer performance. If the smartphone has a magnetometer and it is well calibrated, the convergence time will be greatly reduced and overall accuracy increased.
- Other factors, such as sensor quality, calibration quality, etc.
Accuracy
Once Situm has converged, we may meassure the accuracy of the location estimate (distance from the real location). In general, Situm yields an average accuracy of 1-5 meters, although this may depend on a number of factors. The following table can be used as a reference for different environments and situations.
| Environment | Example | Typical accuracy on hand | Typical accuracy in the pocket |
|---|---|---|---|
| Wide corridors | 5-15 meter wide corridors present in airports or shopping malls | 1-5 meters on average | The accuracy when the smartphone is in the pocket / backpack is reduced a 20%-30% on average. |
| Narrow corridors | 3-10 wide corridors present in hospitals or offices | 1-3 meters on average | |
| Open areas | Wide diaphanous areas that do not have (many) delimitations. E.g. central plaza of a shopping mall, modern airport terminal, etc. | 1-7 meters on average | |
| Mixed areas | Mix of open areas and corridors. These are diaphanous areas with some delimitations such as shelves. E.g. supermarket, warehouse... | 1-5 meters on average. Detects correct shelve 70%-90% of time. |
In addition to the geolocation, Situm provides an estimate of the accuracy as the "area surrounding the geolocation where the user most likely is". You can see this in the following figure: the user is within the circle centered at the arrow with a certain degree of confidence. When the confidence is high (see next figure), the circle is small and the real location should be close to the one returned by Situm. On the contrary, when the confidence is low, the circle is wide and the real location might be distant from the one returned.
