LiDAR 101

In order to understand their function and importance, each component will be discussed in turn.
1. Laser Scanner
The laser scanner is the central unit of a full LiDAR system. Operating in the visible light band of the spectrum the laser scanner transmits a single laser pulse at a pre-determined pulse rate and measures the length of time it takes to transmit and return the pulse after it hits solid objects below the aircraft. Different manufacturers’ systems pulse at different rates which can impact the final achievable accuracy levels. Typically this pulse rate is controlled by the operator and can be altered depending on the desired results. Current systems offer pulse rates ranging from 10,000 Hz to 150,000 Hz.

Laser scanners usually transmit horizontally with the laser pulse striking a reflective mirror mounted at right angles to the path of the pulse, thereby transmitting the pulse to the ground. This reflective mirror is electronically driven to scan pulses perpendicular to the flight path in order to achieve wide ground coverage in a single flight mission. The width of this swath has significant operational and cost implications and is determined by the field of view angle that in most systems is varied by the operator. Again, different manufacturers’ systems scan across the flight path at different rates and use different techniques to achieve this scanning effect, both of which have significant implications on accuracy. Different systems currently offer scan rates ranging from 10 Hz to 65 Hz.

Mirrors generally operate in one of two modes; they traverse across the flight path, stop at the end of the scan and traverse back across the flight path or they rotate, allowing LiDAR pulses to reach the ground when the mirror is pointed down through the hatch in the aircraft. With rotating mirror systems, approximately 70% of the data is not useable since the mirror is pointed up and not to the ground for a majority of the time. Therefore, a stated pulse rate of 10,000 HZ may only have an effective rate of 3,000 Hz.

Technical parameters within the scanner’s electronics can impact accuracies. For example, most scanners today allow for multiple returns from a single pulse to be sensed, measured and recorded. This is an essential feature that allows for effective foliage penetration and foliage height determination. Systems vary in the returns that are timed and recorded from first and last return to up to 5 returns from a single pulse. However, it has been shown that in approximately 99.5% of the cases the third return from a single pulse is the last return, effectively negating the requirement to record more than 3 returns from a pulse.

2. Global Positioning System
GPS is an essential element of the entire system because it provides the geographic referencing of the LiDAR data set to the real world. Operating in a differential mode with two or more GPS receivers(s) operating at fixed locations on the ground, GPS provides a geographic coordinate for each LiDAR return (when combined with the Inertial Measuring Unit (IMU) data). GPS positions are typically recorded every second.

GPS also provides a very accurate 1 pulse per second timing pulse that is used to time tag and reference all data collected in the aircraft to an accurate timing reference frame and therefore to each other.

3. Inertial Measuring Unit (IMU)
The IMU is an accurate orientation device that is used to observe and record the precise pitch, roll and heading of the aircraft on a continuous basis throughout the LiDAR mission. IMU data is typically recorded at 200Hz to 400HZ and is used to correct the orientation of the mirror at the moment of each LiDAR pulse. These instantaneous orientation parameters are then used to correct the nominal mirror angle and allow for accurate transfer of the geographic coordinates from the aircraft to the ground. The IMU data is also used to bridge the position gap between the much less frequent GPS positions along the flight path and create a near continuous position of the aircraft and LiDAR sensor. It is important that the IMU data is optimized through post-processing algorithms. Some systems can only use the real time results from the IMU which severely limits the accuracy achievable in aircraft trajectory determination.

4. Data Collection Parameters
LiDAR manufacturers provide a number of variable operating parameters that the service company can alter to optimize performance in particular operating conditions and applications. These parameters ultimately affect accuracy and production:

• Pulse rate – Pulse rate defines the number of LiDAR pulses per second transmitted from the sensor to the earth’s surface where the time of transmission and time of return are accurately timed and used to determine the laser range. An important consideration in the published pulse rate is the effective number of pulses being directed toward the ground. For example, in a rotating mirror system, as much as 70% of the transmitted pulses may be unusable since it is not directed to the ground. A single pulse will yield multiple returns from different solid objects (ground, foliage, etc.) struck by the laser pulse.
• Flying Height – Different LiDAR systems allow for varying flying heights depending upon the power output of the LiDAR sensor itself. For a given field of view, the higher the flying height, the wider the area of coverage or swath width. Current systems vary in allowable flying heights from several hundred metres to several thousand metres.
• Output Power – Output power relates to the transmission power of the laser unit. Higher output power typically lends itself to higher flying heights, better foliage penetration and higher quality of return data. Power output can vary from 0.5 W to 5.0 W from manufacturer to manufacturer.
• Field of View – Field of view refers to the angular breadth of the scan angle of the mirror. Systems can vary from 10 to 70 degrees; however, the normal ranges are between 20 and 40 degrees. A larger field of view will provide a wider swath in a single flight line but will also result in a wider dispersion of the LiDAR returns since the data is being collected over a larger area for a given pulse rate.
• Returns Filter – Since multiple returns from a single pulse can be recorded, the operator may choose to specify the returns that will be recorded (eg. first, second, last or first, last, second last, etc.) As stated earlier, there is limited value in recording more than 3 returns from a single LiDAR pulse.

• Aircraft Position – The GPS data is differentially processed to obtain precise aircraft positions with respect to known ground control at each GPS position recorded in the aircraft. This interval is typically every second. Inertial data is then processed and combined with the GPS data to produce a final aircraft trajectory on a near continuous basis during the LiDAR data acquisition mission.
• Sensor Position and Orientation – With offsets in the aircraft, the aircraft trajectory data is reduced to produce a set of final sensor head positions (or trajectory) corresponding to the precise time of each LiDAR pulse. The orientation data derived from the IMU is also applied to the observed angular components of the mirror to provide a final corrected mirror position and orientation at each LiDAR pulse.
• LiDAR Range – Each observed LiDAR range is then converted to an x,y,z position using the final aircraft trajectory and the sensor orientation parameters. This step results in a final set of x,y,z coordinates for each LiDAR return, whether this return was at the top or middle of the foliage or on the ground. This resultant data set is known as a point cloud and represents all returns received during the survey mission.
• Data Filtering – There are a number of commercial and proprietary processing software packages available that will provide some level of automated data classification and filtering. The objective is to correctly classify a given data point with reference to its actual source (eg. ground point, middle of foliage, top of foliage, building, etc.) so that the data is represented correctly for GIS and other applications.
Filtering software packages allow operator intervention in terms of establishing filtering parameters. Tight filtering will tend to cause more subtle ground features to be removed. Loose filtering will allow more data which can result in incorrect ground classification and effectively create ground features which may not actually be present. Filtering parameters are influenced by the terrain and foliage cover and can vary from one area of a project to another. Experience in the use of filtering is critical to obtaining the desired results.
In spite of modern filtering software systems, the final processing of LiDAR data requires a significant amount of manual viewing and editing of the data. Experience in viewing and editing this data is also critical.

• Sensor Position Error – This error results from a combination of ground control errors and inherent GPS errors. Typically, with current equipment, these errors seldom exceed 10cm.
• IMU/Orientation Errors – These errors result in incorrect orientation of the mirror at the time of pulse transmission which in turn results in position errors in the LiDAR return data. These errors are typically in the 5-10cm range.
• Mirror Angle Errors – As the mirror is scanning across the flight path or rotating, its theoretical angular position is recorded for each LiDAR pulse. Errors in the sensing mechanism can contribute up to a few cm of position error.

• Final accuracy results are a function of a number of factors. High pulse rate is important but effective pulse rate is more important. Is the unit scanning or rotating? Does the system record true multiple returns? What is the output power of the laser? What is the resultant ground point density of the survey? Understanding the relationship between these parameters and accuracy is crucial.
• Solid operational survey design is critical. Ground control placement, the number and location of cross lines and bore sighting procedures are just a few factors to consider. It is impossible to produce and warrant high accuracy resultants without due consideration to these critical operational factors.