LIDAR (Light Detection and Ranging)

SBG has pioneered and extended the range and capability of LIDAR technology in the Pacific Northwest. This system has been designed to offer performance far beyond any other airborne LIDAR system operating today and provides exciting technology applications for terrain mapping and remote sensing.

LIDAR is the technology of using pulses of laser light striking the surfaces of the earth and measuring the time of pulse return. The LIDAR laser scanner is mounted photogrammetrically in the bottom of an airplane (similar to an aerial camera) along with an Inertial Measuring Unit and Airborne GPS. Robust data storage is required to process the return time for each pulse returned back to the sensor and calculate the variable distances from the sensor, or changes in terrain/land cover surfaces. As with any photogrammetric GPS activity, the LIDAR system requires a surveyed ground-base location to be established in the project area. LIDAR scanning can occur day or night, as long as clear flying conditions are present.

In addition to rapid pulsing, modern systems are able to record up to five (5) returns per pulse (These data sets are then available for orthophoto development, high-resolution contour production, and bare-earth surface evaluations). This demonstrates the ability of LIDAR to distinguish not only the canopy and bare ground but also surfaces in between (such as a forest structure and under story). For example, in urban areas, the first pulse return (or 1st return) of LIDAR data measures the elevations of the canopy, building roof elevations, and other unobstructed surfaces. Depending on the surface complexity (variable vegetation heights, terrain changes, etc.), the data sets can be remarkably large: 200,000 points per square mile sub-urban, 350,000 points per square mile forestland.

LIDAR light intensity data. Click for larger image

LIDAR systems also have the capability to capture intensity reflectance data in addition to the x-y-z coordinates. Reflectance percentage values differ depending on the type of surface they hit (i.e. snow may reflect 90%, black asphalt 5%), and are called “lidar intensity”. This data may be processed to produce a geo-referenced raster file, which is ortho-metric and looks somewhat like a conventional image. These images are useful for identification of broad land use and serve as ancillary data for post-processing.

A distinct benefit to LIDAR is that all the data is geo-referenced from inception, which directly interfaces to GIS applications.

LIDAR intensity data Willapa Bay,Washington. Click for larger image.

The advantages of using LIDAR, instead of traditional photogrammetry for topographic mapping, pushed research to develop high-performance systems. LIDAR technology offers the opportunity to collect terrain data of steep slopes and shadowed areas (such as, the Grand Canyon), and inaccessible areas (such as, large mud flats and ocean jetties).

These LIDAR applications are well suited for making digital elevation models(DEM), topographic mapping, and automatic feature extraction. Applications are being established for forestry assessment of canopy attributes, and research continues for evaluation of crown diameter, canopy closure, and forest biometrics. Additional uses for wireless communication design, coastal engineering and survey assessments, and volumetric calculations are demonstrating the value of LIDAR data collection.

To find out how our LIDAR terrain mapping can help your project please call or send us an information form below:

 

For more informaton call 503-646-1733 or Click Here to fill out an information form:

 

The Leica GeoSystems ALS-40 LIDAR System Technical Description

This airborne LIDAR (Light Detection and Ranging) system is composed of a laser sub-system consisting of the source, scanning assembly and timing electronics, a positioning and orientation sub-system consisting of the differential GPS and inertial measuring units, a data storage unit and processing software.

The laser/scanning assembly will allow operation up to 20,000 feet AMT, with a 40 kHz repetition rate. The laser source has a peak power of 11.7kW @ 15KHz and a pulse width of 11.8 nanoseconds. The scanning assembly is composed of a Beryllium mirror, galvanometer and encoder providing a variable field of view, up to a maximum of 75 degrees to match the ground footprint of a conventional camera system.

The controller unit contains the primary system interface and timing modules. These include a serial interface for the positioning and navigation unit, a 1 pulse per second port input for time synchronization, and a serial data port for data storage. Actual system control of the positioning unit and the laser system is through a Windows NT graphical user interface, that operates on a ruggedized computer with Ethernet and RS-422 interface to the positioning and laser subsystems.

All data is written to large, high throughput, removable hard drives. Data from both the laser subsystem and the positioning subsystem are tagged with the GPS PPS clock, and the laser data stream also contains the GPS time word for subsequent time matching and processing.

The custom developed and integrated positioning system for our LIDAR projects is the APPLANIX POS/DG that elimates the need to perform aerotriangulation. The core of the system is the Inertial Measurement Unit (IMU) and the POS computer system with an embeddedl GPS card and data logger. The IMU measures the translation and rotational dynamics of the sensor. It is entirely solid state for high reliability, containing a triad of high quality silicon accelerometers, and three low noise Dry Tuned Gyros. The embedded GPS receiver can be operated either stand alone or with differential pseudorange corrections applied in real time or post processing.

Real time operation requires data transmission from a base station. The dual frequency unit is initialized with a similar ground station is used to compute precise positions of the sensor on board the aircraft. Two software processes are used to assist quality control of the positioning solution. The POS software package (PosProc) utilizes a 19 state Kalman filter for blending the inertial and GPS measurements. The tight integration is achieved by processing the satellite pseudo ranges and range rates and by implementation of closed loop control on the strapdown inertial computations. Final position and orientation parameters (X, Y, Z,w,f,k) and time (t) are provided for subsequent merging with the laser ranging information.

Performance: Our system has been designed to offer maximum performance. The system develops a sinusoidal scan pattern on the ground with a variable field of view from 1° to 75° . The system operates at altitudes from 2000’ to 20,000’ giving a swath width of 350’ to 30,000’. The achievable point density may vary between 1.5 to 12 meters with a horizontal range of 15 cm to 1 m and a vertical accuracy of 15 to 60 cm.

Data Processing and Quality Control: Once the GPS positions are determined, the scanner position and sensor orientation are used to compute the position of the laser spot on the ground. The appropriate transformations are employed to derive the final data product in the user-specified horizontal and vertical datums. Obstructions and vegetation are removed during the post-processing phase, if required, and the data is closely examined for anomalies. An important phase of our process is to correct and check the LIDAR data by setting stereo pairs from SBG Geo-Data covering the project, and photogrammetrically adjust the data. The resulting LIDAR image is a DEM, although the data will not be at a regular grid spacing on the ground. The final DEM is then formatted to any user defined system, or may be delivered as ASCII point data (x, y, z). Final data sets are usually written to CD-ROM.

For more informaton call 503-646-1733 or Click Here to fill out an information form:

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