Laser scanning is an effective surveying method that gathers information from surfaces using laser light beams and time-of-flight calculations. Laser scanning systems, such as LiDAR, throw out thousands of light pulses per second, and each pulse of light is projected until it hits a surface. After hitting a surface, light pulses bounce back to the system sensor. The time taken for light beams to complete this journey and the distance travelled by each individual pulse is measured, and these measurements are used to determine XYZ coordinates for the exact spot that each light pulse hits before bouncing back to the sensor.
Once scans are complete, the millions of coordinates collected during laser scans are processed using CAD software and point cloud processing software, such as TopoDOT, and brought to life in the form of 3D digital models representing the scanned environment. So collectively, the coordinates (also known as point cloud data) form a complete picture of the surveyed area, which may be a building, a street, road, railroad, or natural landscape.
Laser scanning is used because it’s one of the most accurate land surveying methods and it can be used in almost any environment — indoors, outdoors, night, or day. And to an extent, laser scanning can also be used in different weather conditions.
However, wet surfaces can affect the performance and data quality of laser scans. Here’s what you need to know about using laser scanning on wet surfaces.
The Problem with Laser Scanning on Wet Surfaces
If possible, it’s best to perform laser scans in dry conditions, as wet surfaces can interfere with data accuracy and lead to false points being calculated.
During a normal laser scan of a dry surface, the light beam that reflects back to the sensor is only a partial reflection. But wet surfaces can act like a mirror and refract light pulses — preventing them from returning to the sensor — or cause light pulses to reflect back to the sensor in full. Sensors may not record data from these fully reflected pulses or they may calculate false reflective points when light pulses hit the mirror pane formed by water, rather than the actual surface. False points can reduce the accuracy of the point cloud datasets and create incorrect contours in the 3D digital models created with the survey data.
Because water forms a mirror pane over surfaces, this can prevent laser light pulses from reaching the actual surface. And because water will always form a flat surface, it can hide imperfections in the surface beneath it. For example, if you scan a wet road, the mirror pane created by puddles can hide potholes and road distress from laser scanning systems.
When scanning wet surfaces outdoors, it’s worth noting that standing water might appear still, but it actually moves ever-so-slightly. This can cause no returns and holes in your data, and if light pulses do capture points from wet surfaces with moving water, the resulting data and LiDAR imagery are likely to be very noisy.
Laser Scanning in the Rain
Scanning in the rain is also something you’ll need to consider if you’re facing the possibility of scanning wet surfaces. If outdoor surfaces are wet because of drizzly weather, you might get caught in a downpour during your survey.
The accuracy of laser scanning systems can be reduced by rain droplets, as light pulses that strike raindrops will then be refracted, leaving holes in your dataset. The scanning system itself can also be affected by rain. For example, in systems with a camera, raindrops on the lens can distort the image. This means laser scanning systems will need to be kept under cover during the survey.
While generally, you want to avoid laser scanning on wet surfaces and in rainy conditions, it’s not always possible, so you’ll need to make sure you have the right tools to make the most of point cloud data and correct inaccuracies. Some advanced point cloud data software, like TopoDOT, gives you the ability to correct point cloud outliers and assess data value.