How much difference (in dB) is approximately between a broadband impulse noise which is mistakenly calculated back to a near field distance and a measurement, which considers the near field? For example: A noise is measured in 2m distance with 80dB and then it is calculated back to 118dB at 0.025m. Or: The same noise, but the near field characteristics are included in the measurement. What is the approximately decibel difference between those measurements?
This question has no definite answer. The beauty of the far field lies within the fact, that you don't need a very rich knowledge about the geometry of the object you're surveying.
In the picture you provided the sound source seems to be some kind of engine. Parts of the engine are especially loud, some parts do not contribute to the overall noise level. Different parts may also have different resonance/vibration patterns, and therefore sound differently.
Your near field measurement has a very high dependency, of where exactly the microphone is placed. Along the exhaust pipes I expect higher values, but the away-facing side of the engines flywheel may even be a bit quieter than expected.
So there is no "approximate difference" between far field extrapolation and near field measurements, as it's not even clear, whether this would result in a positive or negative difference.