Launches and returns are events
A launch records a boat going onto the water through the monitored area. A return records a boat coming back through it. Count each event once using a consistent rule. The same boat can make more than one trip in a day, so launch events are not necessarily unique boats.
Boats still out is a running estimate
Estimated boats still out = the starting number out + launches − returns. The starting number and the observation period matter. Resetting to zero at midnight is only appropriate if there are actually no boats remaining out at that time.
For example, if the known starting number is 4, and the monitored ramp records 30 launches and 22 returns, the estimate is 12 boats still out. These are illustrative numbers, not BoatSight customer results.
A ramp is not a closed lake
A boat can launch at one ramp and return at another. Private docks and unmonitored access points can also change the number of boats on the water. A single-ramp estimate must be interpreted in that context.
For a network-wide estimate, add movements across the relevant boundary and use an appropriate starting count. Avoid counting the same movement at both a ramp and a nearby waterway camera as two separate departures. If coverage is incomplete, describe the result as activity at monitored locations rather than total lake occupancy.
Handle missing and uncertain events
Glare, overlapping boats, missed returns, and camera outages can change the estimate. Do not force a negative balance to zero without investigating the cause; it may indicate a wrong starting count, a missed launch, or a return from another access point.
When coverage is interrupted, distinguish the last known estimate from a current measurement. Restore confidence through footage review or a new known starting point where possible. An estimate of boats still out does not identify the boats, their passengers, or their locations.
Choose the number that answers your question
Use launch and return events to understand ramp traffic and peak periods. Use a clearly qualified still-out estimate to describe the balance of monitored departures and returns. Use dedicated observation or a wider monitoring design when the question is how many vessels are on an entire lake.