LIDAR provides the data for highly accurate 3D point clouds and has a much lower noise ratio than those based on photogrammetry. It sounds perfect, so why add a camera?
• LIDAR does not provide color information other than near infrared (NIR), because the laser wavelength only operates within the NIR spectrum.
• Objects can be difficult to interpret if the point density on the ground is insufficient.
• Details can be difficult to interpret, such as the differences between a traffic light, a power utility pole or similar object.
To augment the usefulness of LIDARs, image sensors are now commonly combined with LIDARs to offer new possibilities in deliverables for clients. The matched images from the sensor make the recognition of objects on the ground easier, and the fast production of orthophoto and true orthophoto images possible. Many LIDAR systems are purchased with additional image sensors, either digital, thermal or multi- and hyper-spectral cameras. The combination of LIDAR data (3D point cloud) and RGB image data, produces a new product that is more useful than each element by itself.
Medium format cameras offer several benefits over large format when combined with LIDAR systems:
• The small-sized medium format cameras can use the same belly hole as the LIDAR, which represents savings in the use of an existing aircraft, without the need for expensive alterations.
• When combined with a LIDAR system, today’s medium format cameras offer large savings over digital large format cameras. When purchasing a LIDAR system, the medium format camera is a small additional expense, compared to the LIDAR, but is capable of contributing significantly to the end product.
• Phase One aerial cameras are well suited to LIDAR systems, because the opening angles are very close to the maximum LIDAR opening angle.
Adding another sensor, such as a digital camera, to a LIDAR system, means that it works in tandem with the LIDAR sensor, and shares data from the IMU/GNSS system. Despite the fact that people talk about the integration of
a camera with a LIDAR, the most important integration is done directly with the IMU/GNSS system.
To enable the camera to make use of the IMU/GNSS system, which is most often located inside the LIDAR housing, close to the laser diode and avalanche sensor, it must be mounted on the same plate as the LIDAR sensor. Today, some companies have integrated the camera directly into the LIDAR housing (e.g. Riegl Q1560), but in most cases, the camera can be found beside it. To integrate the camera, the position should be as close as possible to the LIDAR housing and in most cases, making a simple second hole in the metal plate, which holds the LIDAR is enough. All Phase One aerial cameras have four screw holes (M4 bolts) in thefront of the camera for easy connection to the plate.
The mechanical integration calls for the camera to be installed in a nadir position in the same direction as the laser. It is important that both the camera and the laser should be installed with a shock absorption system.
Installing the camera close to the IMU/GNSS is also recommended in order to minimize the length of the lever arm between the IMU and the camera.
The GNSS antenna is usually mounted outside the aircraft on top of the cabin or can be found at a distance from the sensors itself. The distance can be measured by a total station from outside the aircraft or during a boresight
calibration. The offset values are needed for the post processing of images. The LIDAR and the camera usually have a specially marked point that can be used to determine the offset correctly. Phase One provides the interior offset measurements in documents on the website.
On the Phase One iXA and iXA-R, the tripod thread can be used for measuring the offset. Correctly mounting the LIDAR and camera, together with the exact measurements, ensures a smooth mechanical integration of the camera.
The integration with an IMU/GNSS and a flight management system (FMS) can be a bit more challenging as there isn’t a standardized system used by different LIDAR manufacturers. Often they rely on their own custom-built
system, or turn to a third party to provide a solution. In both cases, the camera needs to be connected to several ports. There are three interface connections between the camera and the LIDAR system/FMS. The hardware connection is to the embedded IMU/GNSS system, which is used by the FMS as well.
• Trigger — the camera uses a trigger pulse, most often
from an FMS, to trigger each capture.
• Event — the camera sends an event pulse to the IMU/GNSS.
• Data — the IMU/GNSS sends a data sentence back to the
camera, where it is recorded to the EXIF of each file.
Before a project can be executed, the FMS is used in the office for mission planning. During the flight, the FMS is used by the pilot to receive the necessary information needed to fly the lines. The FMS uses the GNSS information to obtain the actual position of the aircraft and to guide the pilot over the lines. During the mission planning, values are input regarding the camera’s identification, sensor size, focal length of lens, required image overlap/sidelap and altitude.
This information is used to generate positions on the line so that when the aircraft passes over those points, the camera is triggered automatically. As it is nearly impossible to fly a line with 100 percent certainty, the positions are calculated as snap circles. Once an aircraft enters a snap circle, the camera receives a signal and the image is captured.
The camera is triggered by the FMS. All Phase One aerial cameras come with a comprehensive installation guide with complete wiring information that is needed to wire the cables. If you are doing the integration yourself, it is important to know which wires are used on the FMS side of the system. In most cases, a D-sub port is used and
the manufacturer provides the exact pin layout in their documentation. Once the connection is established, the flight management takes control of the triggering of the camera.
The IMU/GNSS system continually collects data during the mission from the moment the system is switched on until it is switched off. This data can be seen as a continous stream
of information and it is necessary to mark the correct moment in this stream when the camera is exposed. To mark this moment the camera can send a mid-exposure pulse (MEP) to the IMU/GNSS system.
The mid-exposure pulse is a simple TTL level connection between the camera and the IMU/GNSS system. To interface the camera with the IMU/GNSS system, a two line cable (event signal and common) needs to be connected to the IMU/GNSS system. The settings to receive the signal has to be set on the receiving system. Most IMU/GNSS suppliers offer the needed information in their manuals. Complete instructions on connecting an IMU/GNSS are available in the download section of http://industrial.phaseone.com. All Phase One aerial cameras have predefined settings for most popular IMU/GNSS receivers.
Some IMU/GNSS systems are able to send event data (NMEA data) to another system like the Phase One camera.Phase One cameras use an RS232 interface to receive the data. Complete instructions on connecting an IMU/GNSS are available in the GPS Installation Guide, which is available in the downloads section of http://industrial.phaseone.com. All Phase One aerial cameras have predefined settings for the most popular IMU/GNSS receivers.
The IMU/GNSS system stores this data in a separate EO file. If this file is lost or becomes corrupted, the project can take a lot longer to complete and opens up possibilities of errors.All Phase One aerial cameras have the ability to write the IMU/GNSS data directly to the EXIF of each file, creating geo-tagged images and thus reducing the chances of error.
Phase One medium format metric aerial cameras are a perfect match for LIDAR systems. LIDARs offer the highest accuracy 3D point clouds and Phase One cameras are able to provide the matching color and details of the highest resolution that do not exist in the LIDAR data. The cameras, are easily integrated with all popular LIDAR systems, often in a half day or less.
Phase One cameras have a proven track record worldwide and have been successfully integrated with all major brands of LIDAR. Adding a Phase One camera enables LIDAR owners to augment their revenue by providing new
products to end users, such as orthophotos and colored point clouds.