Lidar CANEX 2025: Keynote: Some Lidar Observations from the Past and Some of the Latest Developments

This keynote presentation will revisit some of the fundamentals behind lidar that influence the precision and detail the systems produce. The laser ranging systems that are used strongly influence how the lidar system works and factors including: wavelength, beam divergence, pulse width and pulse repetition frequency will be reviewed and discussed. Most airborne topographic lidars use a 1064 nm Near-Infrared laser, while topo-bathymetric lidar systems deploy both a NIR laser and a green laser at 515 or 532 nm. Terrestrial and mobile mapping lidars often utilize lasers at 1550 nm wavelength. All lidars have a dead-zone which is a result of their range resolution, the minimum range between two targets that allows them to be resolved, that is controlled by the laser pulse width. Older lidars in the 2000’s had a dead-zone on the order of 2 m whereas today the shorter pulse widths result in a dead-zone of ca. 30 cm. The beam divergence is measured in mrad and determines the laser footprint diameter which increases with flying height. For example, a typical NIR laser beam divergence of 0.5 mrad at 400 m altitude equals a laser footprint diameter of 20 cm. The pulse repetition frequency continues to increase, now routinely at 1 MHz resulting in higher point densities. For topo-bathymetric lidars, they’re typically equipped with a NIR and green laser to map the topography and water surface with NIR and to penetrate the water and map the sea-river-lakebed with the green laser. Light slows by 25% when traveling in water compared to air (vacuum), causing the beam to slightly change direction in the form of a refraction angle at the air-water interface. This speed change and refraction angle must be compensated for to correctly position the laser returns on the sea-river-lakebed. The green laser pulses both spread when it passes through water and the energy exponentially decays with depth. As a result, the lidar intensity of the green laser returns should be compensated for this energy loss to allow proper interpretation of features on the sea-river-lakebed. Since the laser footprint increases with water depth, objects will appear larger with depth, for example a 1 m cube will appear larger with depth and can also be influenced by the reflectivity of the cube. A new generation of bathymetric green only laser equipped drones are on the market but require large drones because of the increased weight compared to topo-lidar which is steadily advancing.
Speaker: Dr. Tim Webster, Research Scientist, Applied Geomatics Research Group NSCC
This presentation comes from the Lidar CANEX livestream on January 29, 2025.
Thank you to our Lidar CANEX Sponsors:
GOLD: NV5 www.nv5.com/
SILVER: XEOS xeosimaging.co...
Presented by
GoGeomatics: GoGeomatics.ca