Conclusions drawn from each stage of this study are summarized as follows:
6.1 Laser System
A vehicle-based laser measurement system for generating high-resolution DEM data was developed. The system consisted of five units:
a laser line scanner to measure the surface elevation,
a flexible frame-rail mechanism to support the sensors on the bed of a small
utility vehicle,
a gyroscope sensor to measure the roll, pitch, and yaw angular displacements
of the vehicle,
a RTK GPS to provide the geographic position of the measured surface, and a data-acquisition and control unit.
A user interface program was developed to control the laser system and to collect the sensors data through a field laptop.
6.2 Components Tests
A test on the rotating mirror showed that the DC motor required a transient period of about 20 seconds to reach its steady-state speed. The laser scanner should not start the measurement during the transient period.
After testing the laser scanner on three types of objects – white paper, black paper, and sand surface, it was found that the white paper gave the most consistent distance measurement. The laser scanner was then calibrated on the whiter paper for distance measurement.
The spatial resolution of the DEM models developed by the laser system was affected by the rotational speed of the mirror, the sampling rate, the movement speed of the laser-carriage, the height of the laser line scanner from the ground, and the size of the scan area.
A test conducted on the gyroscope sensor showed that errors in pitch and roll
or roll rotations. Within ±30° of single pitch or roll rotations, the measurement errors in pitch and roll angles were within 0.8° and 0.4°, respectively.
A modeling study on the effects of pitch and roll measurement errors on elevation
measurement demonstrated that: a) Errors in roll angle measurement did not significantly affect the elevation measurement; b) For positive pitch angles, especially those approaching 30°, errors in pitch angle measurement had a significant effect on the elevation measurement; c) For negative pitch angles in the ranges of 0~-18°, errors in pitch angle measurement did not significantly affect the elevation. For those in the ranges of -27~-30°, errors in pitch angle measurement had a significant effect on the elevation measurement. Outside these ranges, the effect depended on the measured distance.
6.3 Accuracy Tests
DEM models were created by interpolating the raw laser data using a two- dimensional, three-nearest neighbor, distance-weighted algorithm.
Test results of surfaces with known geometric shapes indicated that the laser system can describe individual objects more accurately than a flat surface. Shapes of different objects can be identified from the DEMs.
The accuracy of the laser system in elevation measurement was evaluated by
comparing the DEM models generated by the laser system for a sand-stone surface with a reference DEM model generated by a more accurate, laser-based profile meter for the same surface. The correlation coefficient between the DEMs was calculated. Within four replications, the highest correlation coefficient between the measured and reference DEMs was 0.9371. The correlation coefficients among the four replications were greater than 0.948.
A median threshold filter and a median filter applied to the raw laser data before and
after the interpolation slightly improved both measurement accuracy and repeatability. The correlation coefficient between the measured and reference DEMs was improved to 0.954. Correlation coefficients of greater than 0.988 were achieved among the four replications.
Differences in materials and colors affected the grayscale images, which were
created from the intensity data provided by the laser scanner.
6.4 Noise Tests
Results of the ambient light test indicated that neither sunlight nor fluorescent light affected the elevation measurement of the laser system. The system provided consistent elevation measurements under both indoor and outdoor lighting conditions.
A test on the linear rail when the laser-carriage moved with and without the mirror rotation indicated that the mirror rotation did not significantly affect the roll and pitch measurement.
A test on the linear rail when the mirror rotated with and without the laser-carriage movement indicated that the laser-carriage movement did not significantly affect the roll and pitch measurement.
A test on the linear rail when both ends were fixed indicated that the rigidity of the
aluminum frame to support the rail was not sufficient.
Tests on the linear rail showed that the rail slightly tilted to the direction where the
laser scanner was located because of the change in weight balance. The variation range of the roll angle was small (<0.16°), which resulted in no practically significant effect on the elevation measurement. A consistent trend in yaw angle variation was observed when the laser scanner moved along the rail. It may have been caused by changes in the magnetic field.
Errors in the elevation measurement caused by the variation of the pitch angle
needed to be compensated.
6.5 Preliminary Test in the Field
A leave-one-out cross-validation algorithm was used to remove the outliers observed in the raw laser data.
The DEM data measured by the laser system was converted from a local coordinate system to the UTM geographic coordinate system by using the yaw angle measured
by the gyroscope, the known positions of the sensors, and the latitude, longitude, and altitude information provided by the GPS receiver.
Results of the preliminary outdoor experiment indicated that the laser system was