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Capable of producing a DTM to an accuracy of +/- 15-cm, the system is extremely useful for applications where a relatively high degree of accuracy is necessary. In addition, a laser DTM can be produced in a much shorter time frame than a similar product using conventional photogrammetric techniques. The ALTM system is comprised of a high frequency optical laser coupled with GPS and an Inertial Navigation System (INS). A 3-dimensional GPS solution (X, Y, Z) is used to position the laser scanner at each second or half second, while the INS data are used to determine the systems' orientation. The GPS solution is computed from differential kinematic processing, using data collected simultaneously at the aircraft, and at base stations on or near the project site. |
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From the airborne platform (fixed wing or helicopter), the laser emits pulses at frequencies of up to 5000 Hertz. These pulses are reflected off vegetation or man-made structures at different time intervals, so the varied distances between the emission and reception can be calculated. With such high pulse emission rates, the laser can obtain as many as 300,000 3-D points per minute. Depending on the flying height, the aircraft speed, and the frequency and width of the laser scan, the ground point density can be as close as 2 feet. When care is taken to ensure quality data collection and high percentages of laser returns, an accuracy of +/- 6 inches is commonplace. Depending on the accuracy requirements of the DTM, the aircraft can be flown up to an altitude of 3,200 feet, producing a 2,100 feet wide laser swath.With a higher flying height, however, there will be a decrease in the density of the points, which in turn will affect the accuracy of the resultant DTM. Typical flying heights to achieve 1 foot contours are around 1,600 feet above ground level (AGL). |
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Data processing
During each laser profiler flight, the raw laser data are recorded on 8mm cartridge tape. These tapes are capable of storing vast quantities of data at a high capture rate. At the same time the GPS and inertial navigation data are also recorded. These two sources provide high accuracy positional information for the system.
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The GPS and inertial data are processed in tandem to achieve the best positional result. Once the position and attitude of the aircraft are known at each epoch (1-second intervals), then these data are integrated with the laser ranges to provide a position for each data point on the ground. Up to 5000 laser ranges are acquired each second, thereby creating files with millions of data points. The data are then processed using the proprietary ALTM laser suite of software to produce an ASCII file of (x,y,z) coordinates. The data can then be transformed into formats compatible with numerous GIS and CAD software packages.
Quality control of the data is done at several stages in the processing cycle. The GPS is analyzed using FLYKIN™ kinematic GPS OTF (On The Fly) software, as well as ALTM kinematic GPS software to meet pre-determined statistical criteria. Since the GPS data are processed immediately after each flight, any re-flights that are required due to poor satellite constellation or insufficient returns can be integrated into a subsequent day's survey mission.
The inertial data are analyzed at the integration stage of the GPS and inertial data. Any peculiarities with respect to velocities or drift are noted and a determination made as to whether a re-flight is necessary.
The primary quality control tool for the laser ranges is the percentage of returns that are received back at the laser after it has emitted a signal. Again, the rate of emissions can be as high as 5000 hertz. An acceptable range for returns is typically between 95% and 98%. Lower percentages are normal over water and other poor reflectivity surfaces.
Even though all of the laser data are positioned using 3-dimensional GPS and inertial data, checkpoints are surveyed along the project corridor. These checkpoints take the form of either a profile along existing roads, or DTM "pads" surveyed by kinematic GPS and/or total station survey techniques. |
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Classification of the data is done at the end of the processing cycle. This task processes the data points through an intensive filtering process, and various classes of points are separated. For example, vegetation data can be removed from the ground layer data, and it is also possible to separate buildings and power lines using customized filtering routines. |
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The end-product is normally delivered to the client in digital form, in the requested file format and on the requested medium. Hardcopy renditions of the product can be delivered as well. A variety of data visualization and interpretation services are available that add value to the basic product. |
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© Copyright 1996-2004 Lasermap inc. - All rights reserved. |
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