UAS Lidar Detects Terrain Change Under the Forest Canopy

Written by Eric Chabot

Editing and Data Viz by Lila Leatherman

UAS Lidar Detects Terrain Change Under the Forest Canopy

Imagine you need to evaluate erosional change across a small, forested field site. Traditional techniques like stream transects or erosion stakes offer only a narrow, localized snapshot, while crewed aerial lidar provides broader coverage but comes at a prohibitive cost. How would you address this challenge? 

One solution to this problem is lidar imagery collected from uncrewed aerial systems (UAS), also known as drones. Lidar, an acronym for Light Detection And Ranging, is used to measure three-dimensional forest canopy structure and surface characteristics. But, when lidar sensors are mounted on a plane or a helicopter, the data may be too low-resolution to capture detailed terrain changes– and to penetrate the forest canopy. Crewed-aviation lidar collection is also often expensive to collect; projects need a very large project area to make it economical to collect data, and may not be able to collect data as frequently. 

Interactive point-cloud viewer enabled by Potree. Point clouds displayed here were collected in summer 2025 to assess pre-conditions for a gully erosion prevention treatment in Kinkaid Lakes, IL. View the full site here, or check out the code.

Advantages of UAS-mounted lidar

UAS-mounted lidar represents a solution to these problems. First, because drones fly lower and slower than airplanes and helicopters, drone-mounted lidar sensors collect an almost overwhelming density of point returns– hundreds of pulses per square meter is the norm. This level of pulse density means it’s easy to generate super high-resolution bare earth digital terrain models: 10 cm resolution is easy to achieve and more than sufficient for most management questions. Second, that resolution is achievable without interpolation, even when the forest canopy is very thick. Finally, drone lidar can be easily deployed on a project-scale to measure before and after-impacts of a management action or significant change event. Aerial lidar collections from manned aircraft typically have much longer return intervals between data collections and require much larger project areas to be economically feasible.  

A Freefly Astro Max UAS equipped with a Phoenix Recon-XT lidar sensor being flown to monitor erosion on the Shawnee National Forest in southern Illinois, April 2026.

These factors make using UAS lidar ideal to evaluate erosional change at a site, as we have on several projects for the US Forest Service. Use cases include measuring sediment deposition and erosion following a high-severity wildfire in New Mexico, examining the soil impacts of tether-logging practices on steep slopes in the Pacific Northwest, and evaluating the effectiveness of slope stabilization efforts in the forested gullies near Kinkaid Lake Illinois. 

Case study: Gully Stabilization at Kinkaid Lake, Illinois

The key goal of this project was to stabilize forested gullies on the Shawnee National Forest near Kinkaid Lake, Illinois, south of St Louis. Because removal of trees can result in increased soil erosion, woody debris and stone berms were placed in the heads of steep gullies before timber harvest operations. Forest managers were interested in measuring the effectiveness of these mitigation structures in reducing the transport of sediment to protect water quality in the lake and improve fish habitat. 

The gully stabilization project takes advantage of each key aspect of UAS lidar. The site is heavily forested, making traditional photogrammetry approaches ineffective to create a bare-earth digital terrain model. Due to the scale of the project, traditional crewed-aerial lidar would not be cost effective to collect on a yearly basis. Finally, the affected areas and features of interest are relatively small, enhancing the value of a high resolution terrain model.

Project Timeline

March 2025	Site visit 1, pre-construction
Winter 2025/2026	Berm construction by FS
April 2026	Site visit 2, post-construction
March/April 2027 (planned)	Site visit 3, erosion control assessment

After an initial site visit in 2025 to collect pre-construction data, we re-flew the Kinkaid Lake sites in spring of 2026 to measure the changes after the USFS constructed berms to prevent erosion in the gullies. We were able to identify vertical terrain changes–that is, erosion or deposition–as small as ~10 cm with confidence. Use the explorer below to inspect the DTMs and difference raster for a small subset of our project area.

Interactive map viewer enabled by Map Libre. Digital terrain models were created from lidar point clouds and differenced to yield the outputs above. View the full site here, or check out the code.

Challenges to mitigate

Using UAS lidar data for change detection comes with challenges in both data collection and data processing. First, mounting lidar systems on UAS requires flight plan modifications to accommodate the sensors. Most Lidar sensors are heavier than traditional cameras, which reduces flight time.

When it’s time to fly, the UAS must always be piloted within the operator’s line of sight, making operation in a closed-canopy forest difficult. To properly co-register the thousands of point returns, the Phoenix Recon-XT’s lidar sensor’s location must be precisely known at all times throughout the flight, requiring a nearby GNSS base station (in this case, a Trimble R12i) collecting data on GPS signal variation used to post-process and correct the UAS’ trajectory. And, there is always the potential for issues with weather conditions or mechanical failures. 

After the data are collected, they need to be processed with proprietary software from the lidar sensor’s manufacturer, and algorithms used to classify points as ‘ground’ or ‘non-ground’ require tricky parameter tuning and manual cleanup. Finally, once bare-earth DTMs have been generated, they must be precisely aligned to reduce the noise that can occur when comparing this year’s DTM from last year’s. However, with careful consideration of all these factors– and with deep expertise in mitigating these concerns– the end result is an ultra-high resolution terrain map captured below the forest canopy capable of resolving vertical changes in soil height as small as 5 cm (2”). 

Lidar and RCR

RedCastle Resources has 10 years of experience in UAS-based data collection for environmental monitoring, and 5 years of experience in UAS-mounted lidar data collection. We are experts in mitigating data quality & accuracy issues, and operate National Defense Authorization Act (NDAA) Blue-certified UAS consisting of all American-made components when required by contract.

In addition to lidar-based terrain modeling, our UAS-lidar data collection can be used for modeling the three dimensional structure of the forest canopy, to measure forest fire fuel loads, or estimate the amount of harvestable timber at a site. Our expertise in UAS data collection also includes thermal mapping data for groundwater detection and stream restoration monitoring, multispectral imagery for forest health, and traditional photogrammetry to support a wide variety of management and monitoring needs.