When working with LIDAR data there is no standard for normalizing intensity return values. Hence, there are times when the intensities within LIDAR data do not match between different scanners, or even different altitudes. This disparity can lead to issues when trying to use the LIDAR data intensities to extract features or during calibration. TerraScan incorporated a Scale Intensity tool, which allows users to scale the Intensity values of different scanners, lines, and/or classes so that they match. The Scale Intensity tool has three different options with respect to scaling the intensities: Direct Scale, Stretch Spread and Angle of Incidence.
In order to make the best use of the Scale Intensity tool it is useful to review the intensity histograms using TerraScan’s View Histogram tool. As an aside, this View Histogram tool is an example tool provided by Terrasolid for how one may use the programmer’s interface to add your own tools to the toolset, hence, it can be found within TerraScan under the TerraScan Main Window menu Addon/View Histogram. The histogram shows the median intensity values, the intensity spread and the average intensity value.
The information in the histograms will be used to help calculate the new intensity values. In this example, Scanner 1 has an average intensity value of 55.5, a median of 33 and a spread of 37. Scanner 2 on the other hand has an average intensity value of 109.7, a median of 100 and a spread of 78.
Direct Scale uses the linear equation for b+m*x to determine the new intensity value, where m is the scale and b is the y intercept. In this example if we want scanner 1 to match the range of intensities in scanner 2 the average of scanner 1 is 55 and the average of scanner 2 is 110, which is almost twice the value of scanner 1. Thus, the new value would be 0 + 2 times the old value.
Stretch Spread uses the current median and spread values for the scanner that needs to have the intensity values changed, and the new median and spread values represent the values to be matched. In this instance, scanner 1 is has a current median of 33 and a spread of 37, while scanner 2 has a current median of 100 and a spread of 78. Thus the input for scanner 1 to match scanner 2 would have the current median and spread for scanner 1 listed under Current and the median and spread for scanner 2 as the New. The tool uses these values to autocompute the equation, much as one could do manually using the Direct Scale option.
Angle of Incidence is used only with a phase shift sensor and is not applicable to a time of flight sensor. In order to use this option the normal vectors must be computed first in order for the angle of incidence to be computed. Note, there will be no adjustment to any points that are non-planar, meaning that only the ground surface and building wall points will see any intensity correction.