Anything without a global reference is dead reckoning. Optical flow sensors just give you a rotational velocity relative to ground. They don't globally locate you. If you combine it with an altitude sensor it gives the same as an odometer would in a car, or the number of steps you take while hiking.

Using an accelerometer wouldn’t be dead reckoning either. Dead reckoning in an aircraft is measuring airspeed and heading and correcting for expected wind. Using an accelerometer is intertidal navigation.

> measuring airspeed

> accelerometer

What exactly would imagine an accelerometer meters? Something other than speed?

Accelerometers measure acceleration. This can be used to calculate the groundspeed of the aircraft. Groundspeed is different than airspeed. For example, if a drone was hovering in a 5mph wind, its groundspeed is 0 and its airspeed is 5mph.

I was a flight instructor for a few years, and have taught many students how to do dead reckoning. That doesn’t mean I’m right, but hopefully gives me enough credibility that you’ll consider my argument. Each of the Wikipedia articles for these topics is robust if you’d like more detail.