Part 107 UAS pilot reported finding out post-flight the UAS exceeded 400 feet AGL due uneven terrain elevations in the mission area.

Narrative

Who:I am a Part 107 remote pilot in command; conducting contracted methane detection operations for a pipeline operator. I was operating a DJI Matrice 350 RTK equipped with a Purway Gen III TDLAS CH4 sensor. The flight was flown using the Purway Captain flight planning/flight application. All missions were flown manually by me as PIC.What:During post-flight data review; I identified an altitude deviation in one or more flight logs; with a maximum reported altitude approximately 185.1 m AGL; exceeding my intended operating altitude of 70 m AGL. This deviation was not recognized during flight; and no loss of control; loss of VLOS; or airspace conflicts were observed at the time. Personally; I am distressed and disappointed in myself for this and am asking for suggestions; help; and support to prevent this from ever happening during my sUAS operations moving forward. I am new to the methane inspection space and am trying to learn and develop good operational procedures to ensure safety.When:The operations occurred Day 0-Day 2; during a multi-day methane inspection project.Where:The inspection took place along a natural gas pipeline right-of-way (ROW) south of Location X; within rural lowland terrain near a reservoir. All flight operations were conducted from ground-based access points along or near the ROW.Why (What I believe led to the deviation):Prior to flight; I was informed by the pipeline company representative on site that the terrain was relatively flat lowland. Visual observation from the ground confirmed a generally flat landscape; and from each takeoff point I maintained VLOS with the aircraft above the tree line. I routinely used the top of my truck's camper shell to increase my vantage point and ensure VLOS.During operations I did not detect any significant terrain drop-offs visually; via the aircraft's FPV camera; or via sensor payload imagery. However; I recognized the deviation on Day 8 while reviewing the data and based on the post-flight altitude logs; it appears that there were sections of the ROW with unexpected elevation changes or depressions not visible from my operating position. These terrain features likely caused the aircraft's actual height above ground level to increase without my recognizing it in real time.The flight was conducted manually; and additional factors such as long linear ROW geometry; higher-than-expected vegetation height; and potential GPS-referenced altitude drift may also have contributed.Contributing Factors:Unrecognized terrain undulations or depressions along the ROW.Ground vantage points that did not reveal actual elevation changes beyond the visible tree line.Manual flight profile maintained at a constant indicated altitude above launch point rather than confirmed AGL.Lack of pre-flight high-resolution terrain elevation modeling for the area.Possible sensor or GNSS altitude variation; typical in long; linear flights.Corrective Actions / Planned Improvements:Incorporate detailed terrain analysis prior to future missions (digital elevation models; USGS; county LIDAR data; or project-specific GIS).Implement AGL-based altitude monitoring tools; such as mapping software with terrain-following capability or mission-planning apps that use DEM data when available.Adjust operational procedures to account for terrain variability; including lower baseline altitudes when uncertainty exists.Improve pre-flight briefing questions with client representatives to verify terrain beyond what is visually apparent.Increase frequency of cross-checks of the aircraft's relative altitude warnings and elevation indicators during manual operations.Continue to self-report and evaluate unexpected altitude deviations or any other issues to improve future flight safety.Summary:While conducting methane inspection operations; I unintentionally exceeded my intended AGL altitude due to unrecognized terrain variation along a pipeline ROW. The deviation was not observed in real timebut was discovered in the flight log review. I am submitting this ASRS report to document the incident; acknowledge contributing factors; and outline steps I will take to prevent recurrence. I welcome any guidance you may offer.