General aviation pilot reported they flew low over terrain to avoid entering overlying Class C airspace.

Narrative

I flew two identical VFR flights below the floor of ZZZ Class C outer ring within two hours. The weather was clear at daylight with unlimited visibility. These were sight seeing flights under VMC conditions. The floor of the southeast sector is 1700 ft so I kept flying around 1400-1500 ft MSL. stayed away from the inner ring. The southwest sector floor of the outer ring raises to 2300 ft as the terrain rises to around 1000-1100 ft elevation with an antenna farm to the west. Being local I am very familiar with all landmarks within my flight path. So I've not refreshed myself with the sectional chart recently. So I took all floor levels as 1700 ft; and kept flying within this southwest sector below 1700 as well. I flew over the peak of the hills at 1300-1400 ft (200-400 ft AGL) and safely returned to my base ZZZ1. Even though I believe I have not created any safety risk in such a clear flying day by flying within couple of hundreds of ft AGL just to stay below 1700 ft; over these hills; this aircraft may have appeared very close to the ground. Had I refreshed myself with the charts to see this floor raising to 2300 ft from 1700 ft; I would have attempted to fly over the hills at 2100-2200 ft. instead of 1300-1400 ft. My lesson learned is 'Always refresh yourself with charts and latest information no matter how familiar you are with the environment you are flying.'