Headcount snapshots are a count of the number of people in the target detection zone at a *moment* in time. Snapshot counts work best when the number of people in the detection zone can be counted in just a few seconds…a time short enough that the number of people does not change during the count. Ideally, several snapshots should be recorded at low counts (e.g. 1), at the maximum count for that area (e.g. 25), and at a midway point (e.g. 12). Several counts that deliver the same number are much less effective.
Threshold counts measure the number of people entering and leaving the target detection zone during a *period* of time. Threshold records work well when the number of people in the detection zone is too many to be recorded in a headcount snapshot. Ground truth collectors, generally one at each entrance/exit (“threshold”), record the entry and exit of all persons during the collection period, typically at least 15 minutes.
Threshold transitions must be recorded simultaneously for each entrance/exit and only data recorded when all entrances/exits are being measured is useful for calibration. It is possible for one person to treat multiple entrances/exits as a single threshold if traffic is low or the area is small enough, but all of these entrances/exits must be visible to the ground truth collector at all times. Such a collection is more difficult than having multiple ground truth collectors, one at each entrance/exit.
| Type of ground truth → | Headcount Snapshots | Threshold Counts |
| What is counted? | Persons in detection zone at a moment in time | Persons crossing the threshold of the sensor’s “inner perimeter”, both into and out of the detection zone |
| How many people can do the ground truth collection? | One | One or several, with each person counting threshold transitions for one subset of the perimeter |
| Time needed to collect ground truth? | Instantaneous (i.e. just a few seconds for a dozen people; more time if larger number) | At least 15 minutes per collection (i.e. because the timestamps of each transition must be recorded) |
| When to collect ground truth data? | On multiple occasions, covering cases when small, moderate, and large numbers of people (i.e. occupancy) are in the detection zone | On multiple occasions, covering cases when small and large numbers of people are entering and leaving the detection zone (i.e. low and high traffic) |
| How many times should data be collected? | At least four times | At least two times |
| Relevant to measurements of | Visitors (aka occupancy) | Visits (aka impressions), Dwell time |
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