03 – Calibration

We have seen that calibrating (or fingerprinting) is the process of recording the WiFi / BLE signals that Situm needs to operate in a building. Situm Mapping Tool (download it here) is the application that allows any user to perform the calibration process: the user has to walk around the building, pinpointing where he is every few meters in the app screen.

In this Section, we explain step-by-step how to perform the calibration using our app Situm Mapping Tool. If you prefer to have a quick overview, you may view this video:

Calibrating a building #

Calibration configuration: WiFi, BLE, or both #

First of all, you should configure whether you want to record WiFi, BLE, or both signals when you calibrate. By default, both WiFi and BLE are enabled, but we encourage you to read this article to understand the recommended options for your case.  Please take into account:


The calibration screen #

First of all, you should go to the Calibration Screen. To do this, (1) open the application drawer by clicking on the hamburger menu. Then, (2) click on “Calibration” (center image).  At this point, you will be on the Calibration Screen. You will be able to (3):

Above all you can:

  • Visualize all your buildings on top of the floorplan and search for them using the Search Bar.
  • Visualize all the floorplans of each building.

You can too (3):

  1. Use the Calibration View Selector to visualize:
    • The paths that have been calibrated.
    • The WiFi signals that have been captured in previous calibrations.
    • The BLE signals that have been captured in previous calibrations.
  2. Floor.
  3. Floor selector.
  4. Positioning model overview. In this menu you can modify the Calibration Path width.

Starting the calibration #

In order to start a calibration, you should click over your current location on the floorplan and start walking straight at a steady pace From time to time, you should click again on the map. Typically:

  1. At every turn.
  2. At every recognizable point.
  3. At least, every few dozen meters 10-20 meters (although experimented “calibrators” usually perform calibrations with segments of up to 100 meters).  

As you can see in the following figure, Situm Mapping Tool connects all the calibration points with straight lines and interpolates all the measurements between them.  Please read the following guide on how to perform a good calibration.

  1. Sensors info:
    • Number of Wifi APs
    • BLE Beacons in last reading
    • Distance covered by current calibration
  2. Initial location
  3. Pinpoint your location at every turn and in identifiable spots
  4. Walk in straight line & at a steady pace between every 2 points marked
  5. Delete current calibration
  6. Delete last calibration point & Stop current calibration
  7. Save current calibration

You can see in the previous image that, at any time, you can: 

  • Save your calibration. This will stop the data recording process and upload all the data to Situm cloud. 
  • Delete the last calibration point. In case you misplaced your position in the map, you can undo it. This will also stop the data recording, and allow you to decide whether to save or erase the calibration. 
  • Delete the calibration. In case you are not happy with your calibration session, you can just delete it. This will not delete previously saved calibrations from the cloud. 

Saving the calibration #

When you (1) click on the “Save” (floppy disk) button, the calibration session will be stopped. Then, (2) a summary of the calibration session results will appear, indicating:

  1. The number of marked points: too few points marked will trigger a warning sign.
  2. The distance covered during the calibration session.
  3. The total number of WiFi APs and scans received.
  4. The total number of BLE beacons and BLE scans received.  

Based on this information, you can decide whether to save or discard this calibration. If you click on Save, the calibration will be saved on the device (3). The cloud icon (3) indicates that the calibration is being uploaded to the cloud; you may also click on this button to upload the calibration manually. When the upload finishes (4), a message will be shown accordingly and you will be able to proceed with the next calibration session.  


Visualizing the calibration data #

Once the calibration is saved, you will be able to see the results, just like in the Dashboard. On the Calibration View Selector, click on (1) the Paths selector to see the areas that have already been calibrated. You may also click on (2) the WiFi/BLE selector to see how many WiFi/BLE signal points have been captured: green stands for a high signal data density and red for a low one.  You should aim to cover most of your building “in green”. Please read the following article on how to maximize the quality of your calibrations. 


Maximizing your calibrations’ quality #

While you calibrate a building, you have to tap on the screen of Situm Mapping Tool from time to time, in order to indicate your location at that moment.

How to move and where to pinpoint your location? #

When you are performing a calibration with Situm Mapping Tool, we recommend that you indicate your location:

  1. At every recognizable point (e.g. doors, columns, entrance to elevators, etc.) or, as a rule of thumb, at least every few dozen meters 10-20 meters.
  2. At every turn you make  (e.g. in corners and intersections). Never change the walking direction between two consecutive points.
  3. Every time you stop. If you stop, tap your location immediately. If you start walking again, tap again in the same location. 

In between those points, you should always move straight and at a steady pace. Situm Mapping Tool will interpolate all the measurements in between every two calibration points assuming that you move this way.

In the following image, the fuchsia circles represent the best places to tap when calibrating, taking into consideration the building’s structure. Points 2 and 6 will have to be at the same point to close the calibration.

How long does each calibration have to be? #

There are no inferior or superior limits to the size of each calibration. Here are some recommendations:   

  1. Calibrations should not be longer than 200-500 meters, unless you are an experienced calibrator.  If you do something wrong in a certain calibration, you will risk losing a lot of work!
  2. Calibrations should not be shorter than a few dozen meters. Otherwise, you will probably advance too slowly.
  3. In general, a good idea is to try to cover small sections of your building at a time (e.g. a hallway, a room or a set of rooms, etc.).

How much should you calibrate? #

To know how much you should calibrate & when to stop, you may use the Calibration View Selector (or the  Situm Dashboard Configuration page as an alternative). 

The (1) Paths selector will give you an idea of the space that you have covered already: please aim to cover all the passable areas of the building, since Situm can only locate the users within the calibrated paths (green paths in the following figure). You may also modify the Path Width to cover more areas with less calibration effort.

  1. Situm will localize the user within the green paths.
  2. You may perform several calibration passes in each area.
  3. Aim to cover all passable areas.

You may also click on the WiFi or Bluetooth calibration view, in order to visualize the exact points where  WiFi/BLE readings where gathered (each reading may contain the signal of several WiFi APs / BLE beacons gathered at that point). As shown in the following image, you should aim at painting all your building areas in “green” or “orange” at least.

  1. “Not enought” readings. Please calibrate again these areas.
  2. “Just enought readings”. You may calibrate again or leave it like this.
  3. “No readings”. Please walk slowly!
  4. “More than enough readings” captured. Further calibrations in this area will not improve the quality.

How to maximize the data captured? #

You should always aim to capture as much WiFi/BLE signals as possible, within a reasonable calibration cost. There are 2 main strategies to do this: 1) walk slowly, and 2) perform at least calibration passes per area. You should aim at implementing both.

Strategy #1: Walk slowly #

Obviously, the slower you walk, the higher the density of signals you will capture. We recommend a walking speed of 0,50 meters/second – 1 meter / second as a reference. The following figure illustrates the effects of walking slowly.

  1. At 2 meters/second, you will not capture enough data to have a good accuracy.
  2. At 1 meters/second you may capture just enough data in a single pass.
  3. Don´t run! or else you will leave uncalibrated spots!
  4. At 0.5 meters/second you may capture all the data you need in a single pass.

Strategy #2:  Perform at least 2 calibration passes per area #

A complementary strategy is to calibrate each area 2 or more times. For example, in corridors this is usually accomplished by calibrating in the two opposite walking directions. 

How to make Situm work with most smartphone models? #

If you calibrate with just 1 smartphone model, Situm may “overfit” to that particular model: you will get good accuracy with that specific smartphone, but not so much with others. To avoid this, we recommend that you calibrate each area at least with 2 different smartphone models. This way, you will capture enough signal variability to make Situm robust enough to work with a wide range of smartphone models. 

Path width #

When you calibrate a building, you draw a virtual path that connects all the calibration points. This path contains the points where Situm Mapping Tool has captured the WiFi/BLE readings.  For example, the following image represents as blue dots the points where Situm Mapping Tool has recorded a WiFi reading.

Situm Mapping Tool allows you to modify the Path Width, which represents the width of the virtual paths that you draw when you calibrate. This width has a direct effect on the space that you will cover with each calibration: the wider the path, the farther each calibration point will have influence. The difference is clear in the following image. When we set a Path Width of (1) 3 meters, the influence of each point is limited. However, with a Path Width of (2) 12 meters,  we cover more areas with less calibration effort. Be wary, because this usually comes at the expense of a inferior positioning accuracy.  

Adjusting the Path Width to the right number #

Finding the right Path Width may be tricky. As a rule of thumb, the Path Width should be approximately equal to the width, in meters, of the building’s narrowest hallways.  Examples:

  • In a supermarket, the path width should usually be low (e.g. 1-2 meters). Otherwise, signal points from different aisles could be mixed, reducing the chance of positioning the user in the correct aisle.  
  • In a shopping mall, the path width can usually be larger (e.g. 3-10 meters), since the corridors are wide and widely separated.  

As shown in the following image, this allows you to calibrate always in the middle of the corridor and increase the path width until it covers the whole corridor.  

You may increase the path width a bit more to reduce the calibration effort…
… Until the calibration paths cover perfectly the narrower corridors of your building…
…But be careful! If you over increase the path width, you will end up covering non-passable areas.

Another option, useful if the corridors of your building are too wide (e.g. more than 10 meters), is setting a lower Path Width and calibrating the sides of the corridors as shown in the following figure (1). This way, (2) you will cover the whole corridor with a lower Path Width, which will increase the accuracy.


Tricky Situation #1: Obstacles #

Sometimes, you will calibrate areas (e.g. open spaces) that contain elements that the users can not traverse (e.g. a bench, an escalator, etc.). You should avoid calibrating over those elements because otherwise, Situm will provide locations on top of them (which is wrong).

In addition, when adjusting the Path Width, you should take care not to cover the space covered by those elements: otherwise, Situm may provide locations on top of them, which should not be possible.

If the paths are too thin, you will need to perform a lot of calibration passes.
You may extend the path width to cover wider areas and reduce the number of passes required.
Do not over-reach! You may end up covering non-passable areas by accident.

Tricky Situation #2: Rooms #

In case you are calibrating rooms or areas separated by walls, you may want to distinguish the side/room where each user is. A small path width will allow you to do this (1), although you may need to perform more calibration passes to cover the whole space. On the contrary, (2) if you increase the path width too much, you may end up connecting both sides of the wall (both rooms) together, and Situm will treat that area as an open space.


Calibration Manager #

As in Situm dashboard, the calibration manager in Situm Mapping Tool allows you to select, delete, enable and disable the calibrations that you have done for your building. To activate the calibration manager you must go to Calibrations and click gear button.

Click on the menu button in order to visualize all the calibrations of your building, you will be able to see the calibrations of each floor using the floor selector.

  • Calibrations are selected by touching them.
  • Selected calibrations are shown in gray.
  • Multiple calibrations can be selected

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