Measurement can be direct, like measuring length directly with tape or a ruler. But it can also be indirect, like measuring distance by emitting a sound, listening for it, and measuring the delay the sound is received after being bounced back by an object.
Flat-Earthers like to discredit measurement results —like the distance to the Sun— by mentioning these are just results of calculation, not measured directly. In reality, many everyday instruments do measure indirectly. They measure a different value, then use calculations to get the desired calculation.
For example, to determine the speed of Earth’s surface at the Equator due to Earth’s rotation, we can first determine the Equator’s circumference and the period of Earth’s rotation. Earth’s radius at the Equator is 6378137m, and the Earth’s sidereal rotational period is 23 hours, 56 minutes, 4 seconds. From these values, we can calculate the speed of Earth’s surface at the Equator due to Earth’s rotation is 2 × π × 6378137m / (23 hour + 56 minute + 4 second) = 1674.4 km/hour.
Flat-Earthers like to reject such measurement, because it was not done directly, but derived indirectly from two different variables. They claim no instrument can measure Earth’s rotational speed.
In reality, many results of measurement are determined indirectly. An electronic speedometer in a car measures speed by measuring the rotational frequency of the car’s driveshaft. The speed of an airplane is determined by the height difference of the fluid in the different column as a result of the difference in pressure inside a pitot tube. An anemometer indirectly measures wind speed by measuring the rotational speed of the instrument.
Most measuring instruments we use every day are showing the end result in the unit we want to see. However, in most cases, they come up with the results indirectly, the same way we determine the speed of Earth’s rotation as above. Just because a measurement is indirect, it does not mean it is ‘wrong,’ as frequently claimed by flat-Earthers.