Decoding the Cloud
As surveyors, we take great care in understanding sources of errors in measurements, and strive to find ways to decrease or eliminate the effects of errors. With each instrument we use, we need to have an understanding of the sources of errors that can come from it. Most people know the sources of errors when using GPS, total stations or levels. In this article we will look at the types of errors that can be found in a relatively new technology: laser scanning.
There are four common error sources that can appear in laser scanner data (point clouds): mixed pixels, detector saturation and blooming, multipath and incidence angle.1 These errors are systematic and appear as noise or as bad measurements. Often, these error sources can be detected when you know what your subject point cloud is supposed to look like and you can weed out the bad information. It is beneficial to know what types of errors to be aware of when it is time to process your data.
Mixed PixelsMixed pixel errors occur when you are scanning a surface that has another surface closely behind it. When the edges get scanned, the laser beam gets split and part of it hits the front surface and part of it hits the rear surface. The energy that is bounced back to the scanner comes from two different distances; the scanner interprets them together and the result is a point in space that is not on either surface. This error is more pronounced with systems that have wide laser spot sizes. Some software makers have routines that will trim the edges of point clouds, which helps to remove points that trail away from the edge. If you are aware of this error, you can create more accurate models by ignoring the mixed pixels when fitting an object to a point cloud. Figure 1 and figure 1, part 2 show a group of pipes that were scanned and the cross section of two of them that have points trailing off from the edge of the pipe.
Detector Saturation and BloomingDetector saturation occurs when something is scanned that is highly reflective and the returned energy exceeds the dynamic range of the detector.² A range error occurs and the data points either come nearer or farther to the scanner than where they should be. I have seen this happen especially when reflective targets for total stations get scanned. There is a stream of points that come out at the scanner as seen in Figure 2 where a 1" reflective target was scanned. A simplified way of picturing how this happens is to imagine the laser bouncing back really fast to its source, which results in a shorter range measurement when a time-of-flight scanner is used. Another instance where this often occurs is on reflective lane lines on roads. The lines can sometimes slightly rise off the road surface. Sometimes there are surface reflectivity issues that cause an increase in the range measurement. I experienced this when I was scanning a 1-ft square white plate that had an orange cross painted on it. The orange appeared to be inside the plate, even though it was really on the surface (see Figure 3 and Figure 3, Part 2).
Blooming is similar to detector saturation. This error happens when very reflective objects are scanned and energy from the surrounding area of the laser beam is returned. Often this is seen when a retro-reflective target is scanned. This means that while prisms are ideal for use with a total station, they do not work well with a laser scanner.
Although there is no way to prevent detector saturation or blooming, surveyors should avoid using retro-reflectors and be aware of these errors when processing the data.