Remote Sensing: LIDAR: Digital Camera

Digital Camera Sub-System

As one of the first innovative pioneers in the industry, Spectrum Mapping began acquiring airborne LIDAR data combined with directly georeferenced digital imagery in 1996. The integration of these two technologies and the fusion of their complementary datasets has been vital to the production of accurate bare earth surfaces and the classification of laser returns to support a wide range of mapping and natural resource applications.

Spectrum’s NexVue™ digital camera sub-system consists of a medium format 4096 x 4096 color pixel array with 10-bit dynamic range, a high-speed iris shutter system and precision mounts for a 50mm or 90mm lens. It can collect clear (smear free) stereo imagery with better than 6-inch ground resolution. Both natural color (RGB) and color infrared (CIR) imagery can be collected.

Color Digital Imagery		Color IR Digital Imagery

The digital camera is integrated with a laser altimeter, relative position GPS and Inertial Measurement Unit (IMU). The system is typically flown in a fixed-wing aircraft with a flight management system to facilitate maximum collection rate and to minimize costs. The information from these systems is combined to calculate the exact position and orientation of the sensors as related to the earth surface at the precise time of data acquisition. The system eliminates the need for extensive ground control panels, film development, film scanning and stereo plotting for DEM generation and image rectification.

Other advantages of Spectrum’s NexVue™ digital camera are:

• Aerial Film and Image Quality - Digitally collected imagery ensures the highest quality imagery available. From the time of image acquisition, there are no external sources for contamination or degradation of image quality. There is also no opportunity for scratches or other physical damage, as in film processing. The histogram and exposure for each frame is monitored during acquisition to maximize image quality. The exposure time, aperture, gain, and black-level will be adjusted to generate the clearest and most crisp imagery possible for time of flight conditions.
  
  With film, proper exposure parameters (i.e. aperture and shutter speed) are difficult to model. Final quality is unknown until film is processed. The Spectrum NexVue system displays both real-time imagery and histogram information, which allows the operator to adjust for changing conditions during flight. This ensures high quality radiometry, maximizes dynamic range and therefore contrast, and minimizes the number of re-flights.
  
  
• Reduced Relief Displacement - The NexVue digital camera yields a flatter or less distorted image when compared to the traditional wide-angle lens. This results in a significant reduction in relief displacement of taller features that might otherwise obscure the ground. This is especially advantageous for the production of orthorectified images and for planimetric feature extraction with mono- or stereo editing processes. Spectrum’s NexVue™ digital camera has two lens options dependent on project requirements. The lens is generally selected such that its cross-track field of view matches that of the LIDAR system for greatest efficiency of data capture. With 60% forward overlap, the 50mm lens has a base:height ratio of 0.29, which is the approximate equivalent of a traditional 12-inch focal length film camera with 9-inch film format. The 90mm lens has a base:height ratio of 0.16, which approximates that of a 24-inch focal length film system and produces imagery with minimal relief displacement for close to “true ortho” results.
  
  

  Conventional 6” lens 1:12,000	Digital 90mm lens 1:16,600
  Note building "lean" on film image obscuring street below Digital camera has a smaller field of view corresponding to a significantly better incidence angle

• Improved Radiometric Performance - Spectrum’s NexVue™ cameras feature10-bit radiometric resolution, resulting in imagery with a wide dynamic range that lets you see into the shadows while preserving detail in even highly-reflective surfaces. Stereo viewing and automated image matching are also enhanced by the absence of film grain noise, yielding more accurate and reliable photogrammetric measurements. Radiometric calibration of the sensors guarantees consistent illumination across the image format without the need for dodging. Ortho mosaics have uniform tonal balance and color.

Corrections to imagery can only be done on traditional camera film at the development or scanning stage. If shadows exist and obscure any areas after this point, there is nothing that can be done to eliminate the problem. The digital camera provides a much greater dynamic range than is available with traditional scanned film.

The image on the left is the original imagery collected with the Spectrum digital camera. The shadow caused by the overhanging sand dune obscures the ability to see into the overhang. The image on the right has been gamma adjusted, reducing the effect of the shadow and allowing a clear view of the area. Notice that the image has not been degraded. There are no “hot” white spots and the ripples in the sand are still clearly visible.

• Calibration - There are currently no conventional calibration reports available from the USGS for digital camera systems. To guarantee the accuracy and repeatability of its digital cameras, Spectrum performs a rigorous camera calibration at least twice a year in a controlled environment at its terrestrial calibration facility.
  
  
• Direct Georeferencing – The camera is fully integrated with an airborne inertial navigation system to provide directly observed exterior orientation parameters for each frame of imagery. The camera is rigidly mounted with the LIDAR unit and is boresighted with the same high-precision IMU. Each exposure location is time-tagged and recorded as a GPS event. Post-processing of the differential GPS and IMU data yields position and orientation data of such accuracy that no ground control, aerotriangulation or stereomodel setup is required. The result is a smooth and rapid workflow for both LIDAR data QC and digital orthophoto production.

Image Processing

Spectrum’s NexVue™ camera system does not require a post-mission processing step. As a result, imagery is available in near real-time and project turnaround time is reduced. Raw image data is processed on the fly as it is read from the camera and converted to RGB or CIR image files in industry standard TIFF format. The imagery is monitored by the camera operator during acquisition to ensure optimal exposure settings and can be reviewed in the field to check coverage and overlaps. Final image data products can also be delivered in a client-specified format if desired. Various formats are supported such as TIFF, JPEG, COT, MrSID, etc. During this stage image header information will be generated containing exposure number, position (x/y/z), and orientation (omega/phi/kappa). A header format will be generated to support batch processing of all frames, which automatically prepares the files for quality assurance testing and 3-D (stereo) editing as required.


Flight Report

Flight reports are recorded during image acquisition. Beginning and ending frame numbers are marked for each flight line and GPS time tags are recorded as part of the image metadata for each frame. During post-mission processing the GPS time is combined with the precise inertial flight navigation data and the X, Y, Z, kappa, phi, and omega values are then computed for each frame. The exposure position for each frame is written to a shape file and imported, together with its projected ground footprint, into the working project in a GIS where project coverage is checked for QC purposes.


NexVue™ Digital Camera Specifications

Digital Camera Specifications Camera Array 4k X 4k Pixel Panchromatic, Color, CIR Recording Rate Per Frame Less than 2.5 Seconds Camera FOV 50mm Lens 40 Degrees Fixed 50 mm Lens Calibration Less than 0.5 Pixel RMSE Full FOV Camera FOV 90mm Lens 23 Degrees Fixed 90 mm Lens Calibration Less than 0.5 Pixel RMSE Full FOV Minimum Ground Projected Pixel Footprint 6 inches (15cm) Height and speed dependent Image Geopositioning Accuracy Better than 1 Foot (20cm) RMSE absolute

info@specmap.com