Starting grid versus bearing
Think of the inputs as a start location plus a direction. The aircraft begins its photo run around the requested grid and continues along the bearing, so choose a heading that sweeps across the area you need to inspect.
Use recon after battlefield state changes
Recon photos are especially valuable after new enemies appear or when your existing map information is stale. The official game description explicitly treats aerial photos as a way to reveal the outcome of your actions and update battlefield understanding.