Modern LiDAR-based aerial surveys

CK Aerial Surveys' Adriaan Combrink explains the techniques processes of Modern LiDAR-based aerial surveys.

“Typical modern LBAS data sets contain five to 10 points surveyed per square metre, to accuracies of five to eight cm”
-Adriaan Combrink

A modern LiDAR-Based Aerial Survey (LBAS) offers a surveying and planning solution to the mining, energy and civil engineering markets, which older surveying techniques could not. Light Detection and Ranging (LiDAR) is an airborne surveying technique that combines the trajectory of the aircraft (from differential GPS), orientation of the aircraft (from an inertial measurement unit) and laser ranging to survey the earth's surface accurately in high resolution.

Aerial surveys have always been able to cover larger areas in a shorter period, compared to conventional ground surveying, while also offering imagery to visualise the survey sites. Furthermore, very little ground access was required, which is a positive from a security point-of-view.

However, conventional aerial surveys - for example stereo photogrammetry - had some major shortcomings. Firstly, digital terrain models (DTMs) provided could not be accurate in vegetated areas, since the stereo-derived DTMs would only give the heights of the top of the vegetation. Secondly, some ground access was required to do so-called pre-marking, whereby a white cross is built on the ground to serve as a known point. And thirdly, turnaround on the delivery of survey products was slow and expensive because of all the manual labour involved.

The advent of LBAS solved many of these problems. Firstly, the high-power short-pulse laser used will penetrate vegetation and also ensure returns off dark surfaces such as tarred roads or coal stockpiles. Furthermore, multiple returns are recorded for each pulse, so that the coordinates of the ground and vegetation are recorded, thus providing a very accurate DTM. Secondly, the ortho-rectified aerial photos from LBAS are produced through direct geo-referencing whereby pixels are projected onto the DTM, while the indirect methods of photogrammetry required pre-marked survey beacons. Using LBAS, site access is only required for quality assurance purposes, but can strictly be conducted without any site access. Thirdly, with the automated systems used turnaround time has greatly been reduced and this allows for rapid calculation of volumes, contours, vectorisation of buildings, et cetera.

Modern LBAS systems improved significantly on older LiDAR systems. The modern surveying systems, such as those used by CK Aerial Surveys, has laser pulses in the air 51 percent of the time, while the other 49 percent is spent turning the mirror to direct the laser pulses. The three engineering limitations that prevent these systems from surveying any quicker are the speed of light, the speed at which the mirror can be turned side-to-side without introducing any significant deformation, and the recording rate of data. Currently, systems can transmit up to 200,000 pulses and record up to 800,000 returns per second.

One of the major benefits of the improved ranging rate is lower cost to the client. Various steps can now be taken to reduce the cost of conducting the surveys, for example flying lower and using a wider field-of-view, thus covering a larger area to better resolution and accuracy than possible before and doing so in a shorter period. CK Aerial Surveys exploits this property by operating mostly from a helicopter platform and thereby further negating the need to operate from airfields and reducing aircraft turn-around time at the end of flight lines by 75 percent.

Earth's atmosphere causes dispersion of laser pulses as they travel to the ground, hence another advantage of modern helicopter-mounted systems is the smaller laser footprint on the ground, typically eight to 12 cm diameter, which offers better resolution and accuracy of the ground points than the 20 cm diameter from systems based in fixed-wing aircraft. With the smaller footprint, one is able to also survey features such as earth wires on power transmission lines.

Typical modern LBAS data sets contain five to 10 points surveyed per square metre, to accuracies of five to eight cm. The high-resolution photography combined herewith, yields ortho-rectified aerial photos with seven to 10 cm pixel sizes. These data sets have a myriad of uses in mining, energy and civil engineering, including volume calculations of stockpiles and pits, DTMs, contouring, audits, hydrology, flood predictions, detection of sinkholes, three-dimensional mapping of power lines relative to the ground and other features, and environmental impact assessments.

Adriaan Combrink holds a PhD in Geomatics from the University of Cape Town and an MSc in Physics from Potchefstroom University. He spent 6 years as a full-time researcher at the HartRAO Space Geodesy Programme and has been heading up the Geospatial Operations at CK Aerial Surveys since 2008.