Recreational / Hobbyist UAS pilot reported flying in controlled airspace without authorization. Pilot exited airspace after realizing an incursion had taken place.

Narrative

As the Remote Pilot in Command (RPIC); I was operating a small UAS quadcopter for recreational purposes in an open field. It was daytime with clear skies. My drone was flying under 400 feet above ground level. During pre-flight I went to an area I thought was outside the area of controlled airspace based on GPS on my map. The closest airport was a regional airport; with class D airspace; around 5 miles away (which I thought was a little farther away). However; before takeoff I forgot to doublecheck my exact location. I took off and flew a short distance (around a few hundred feet) before remembering to double check my exact location. At that time; I realized I had been mistaken about where I was starting from on the map; and I was actually at the far outer edge within the class D airspace; so I promptly returned the drone and landed. If I had remembered to double check my exact location before takeoff; I would have either moved farther away to outside of the class D area before takeoff; or if staying in the area I would have requested LAANC authorization before takeoff/flight. I have highlighted the step of double checking my location using an approved app in my pre-flight checklist to ensure this does not happen again. I regret making this mistake and will make sure to be extra careful about this and all rules going forward.