>>18153552land surveyor here.
the earth is not a sphere and neither is it a geoid. It's like a potato. But if you take into account that the curvature of the earth is approximately 6200 km, you can estimate that the maximum value of error due to curvature occurs around 20 km, it is the line that separates topography from geodesy. If you have the data from the place where you work, you can make this calculation more precise.
