LiDAR Survey Through a Forest Corridor: A Transmission Line Case

Day 1 on site and the client's biggest worry was simple: how do you survey ground you cannot see. The proposed 40km transmission line corridor cut straight through reserve forest for nearly a third of its length, and photogrammetry alone was not going to get us a usable terrain model under that canopy. This is the field report from a drone LiDAR survey that had to succeed where camera-based methods would not.

Why Photogrammetry Was Never Going to Work Here

Drone photogrammetry builds a 3D model from overlapping photos, and it only sees what the camera sees, which under a closed canopy is the top of the trees, not the ground beneath. For a transmission line survey, the actual ground profile determines tower height, sag clearance, and foundation design. We needed a sensor that could see through gaps in the leaves, not around them.

Drone-mounted LiDAR flying the forest section of the corridor at 80m AGL.

How the Survey Actually Went

The Numbers That Made the Client Comfortable

LiDAR pulses travel at the speed of light and the sensor records the time delay for each return. A single laser pulse can produce multiple returns as it passes through gaps in leaves before finally hitting the ground, and it is these last returns that get filtered out and classified as bare earth.

Classified point cloud: green returns are vegetation, brown returns are bare earth used for the DTM.

Why This Approach Holds Up

The ground does not change just because you cannot see it. LiDAR just gives you the honesty photogrammetry cannot.

This corridor is now fully surveyed, geotagged, and cleared for detailed design, with a defensible dataset if the forest department ever questions the tree count or the ROW documentation. If your transmission line, pipeline, or road project runs through forest or dense vegetation, this is the method that actually delivers usable ground data, not an approximation of it.