"scan.time_increment" in laser scan message publishing

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The tutorial is correct. From the message definition:

float32 time_increment   # time between measurements [seconds]

That means that it is the time between each reading, not the time between each complete scan (that's the scan_time field of LaserScan). Because it's the time between each reading in the scan, it gets divided by the total number of measurements per scan. Note that this does assume that the scanner has 0 time between completing one scan and starting another, so obviously it may be different for different actual sensors instead of this example for publishing.