Figure 1: DSM Point cloud
Summary
Data Requirements
IMU Data: Angular Rate
Attitude Data: Yaw, Pitch, Roll
Navigation Data: Position, Velocity
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Georeferencing LiDAR data
Georeferencing is the process of applying a coordinate system to the point cloud so that it can be accurately located on a map. In order to georeference the point cloud the LiDAR sensor’s orientation and position, or Exterior Orientation Parameters
(EOP), need to be known in order to create a planimetrically correct scan that can be used for Geographic Information System (GIS) analysis
Unlike photogrammetry where Ground Control Points (GCPs) can be used to georeference mapped data using Aerial Triangulation. Ensuring that LiDAR data is accurately georeferenced requires the use of Direct Georeferencing, using an accurate GNSS receiver
and an inertial measurement unit (IMU) measure the pose (orientation and positing) of the LiDAR sensor.
To accurately georeference LiDAR data, operators need to take particular care when configuring the mapping system. Any misalignments or offsets incorrectly configured will directly impact the accuracy of the point cloud. There is little that post
processing can do to resolve errors and will most likely result in having to remap the environment/scene. When configuring the mapping solutions it is important to ensure that the following are correctly configured:
- Boresighting - this is the process of aligning the LiDAR sensor axes with the IMU axes.
- GNSS Antenna Offset - because the GNSS Receiver calculates the geographic coordinates at the antenna phase center we need to map this to the IMU origin. The GNSS lever arm needs to be configured in the IMU reference frame for x, y, z offset
- Reference Frame Rotation - The IMU coordinate frame may not align with the vehicle frame. It is therefore important that the IMU frame be mapped to the vehicle frame in order to ensure that the georeference data is correctly applied
Finally we need to implement synchronization to ensure that each range measurement is correlated with the correct pose data. LiDAR sensors typically have facility to accept a time synchronisation pulse, such as a Pulse Per Second (1PPS), from the
GNSS receiver and NMEA timing data.
Figure 2: MMS Reference Frames and Offset calibration
Sensor Selection for Direct Georeferencing LiDAR Data
Instead of using a stand alone GNSS and IMU, a more suitable solution for estimating EOPs of the LiDAR sensor is to use a GNSS/INS. The advantages of using a GNSS/INS include a more accurate attitude solution, ability to carry position and velocity
updates through short GNSS outages and several other advantages. To learn more about how a GNSS/INS operates please read Section 1.7: GNSS-AIDED INERTIAL NAVIGATION SYSTEM (GNSS/INS) in the VectorNav Library.
Using a GNSS/INS allows for all points in the point cloud to be geo-referenced to a fixed global reference frame. The following equation can be used to solve the absolute position of a point on the ground:
𝒑g = 𝒑a + [C]⍴
Where,
- 𝒑a is the estimated position vector of the LiDAR sensor,
- [C] is the estimated direction cosine matrix mapping from the LiDAR to the inertial coordinate frame and,
- ⍴ is the LiDAR point offset vector.
The estimated LiDAR position vector (𝒑a) comes directly from the GNSS/INS system and is subject to both GNSS position errors and timing errors between the systems. The coordinate frame transform between the LiDAR reference frame and an inertial
coordinate frame (eg. NED) comes from the attitude measured by the GNSS/INS. Errors in [C] come primarily from the sensor attitude misalignments, GNSS/INS attitude uncertainty, and timing errors between sensors.
Attitude Errors
Any angular uncertainty projects to the positional error based on the distance from the point to the LiDAR sensor. Therefore to achieve the required mapping accuracy, the attitude (Pitch/Roll) accuracy required is a function of the mapping height.
The higher the altitude the greater level of attitude accuracy required.
Angular Accuracy = Arctan (Accuracy/Range)
Figure 3: Attitude accuracy requirement vs range for 4cm point cloud accuracy
Positioning Errors
Positioning errors come from the uncertainty in position of the GNSS/INS sensor. This position uncertainty is represented in the North, East, Down (NED) coordinate frame The position uncertainty is mostly determined by the accuracy of the GNSS solution
alone.
In order to achieve survey-grade results it is necessary to use GNSS Receivers capable of either Real-Time Kinematic (RTK) or Post-Processed Kinematic (PPK) corrections to determine image coordinates to less than 1cm. As there is little need for Real-Time
mapping, most LiDAR mapping operators opt for PPK which has advantages of simplified operation and provides more accurate positioning data compared to RTK by utilizing precise satellite ephemeris data and forward/backward smoothing techniques.
Conclusion
As the design projects increase their reliance on modeling and verification, the demand for LiDAR scanning technology continues to grow. LiDAR mapping solutions rely on accurate range measurements coupled with accurate pose information. For LiDAR
mapping to produce accurate georeferenced point cloud data it is vitally important that the GNSS/INS solution not only have suitably accurate position and attitude data, but that the sensor be aligned and boresited correctly.
A suitably configured Mobile Mapping System is able to produce data that rivals traditional techniques with additional advantages of speed and safety.
