Measurement can be direct measurement, like measuring length using a measuring tape; or indirect measurement, which is measuring something by measuring another.
Flat-Earthers love to discredit results of measurement which they deem ‘impossible’ or not supporting their beliefs; for example the distance to the Sun, the distance to stars, Sun’s temperature, the orbital and rotational velocity of the Earth, etc. Their usual excuse is that such measurements were not done directly, but indirectly. In reality, most measurement instruments we use every day do their measurements indirectly.
For example, to determine the speed of Earth’s surface at the equator due to Earth’s rotation, we can first determine the circumference of the equator, and the period of Earth’s rotation. Earth’s radius at the equator is 6378137m according to WGS84; and the Earth’s sidereal rotational period is 23 hours, 56 minutes, 4 seconds. From the two data, 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.
In many cases, flat-Earthers would reject such measurement, because it was not done directly, but derived indirectly from two different variables. They would say that no instrument can detect the speed of Earth’s rotation.
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, the distance to the Sun and stars, etc.
Just because a measurement result was determined indirectly, it does not mean it is ‘wrong,’ as frequently claimed by flat-Earthers.