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Laser scanning at the Ilvesheim gravel bar


This web site presents the results of the course ”Acquisition and Analysis of Laser Scanning Data in Physical Geography”, held by Jun.-Prof. Dr. Bernhard Höfle at Heidelberg University’s Institute of Geography in the summer term of 2013.

To get a first idea of what the project is about, watch the documentary video or read the short introduction below. For more information, you can read the entire project report online (see "Project Report" in the menu above) or download the project report in PDF format..

We also have a Google Maps plug-in and a WebGL-based point 3D point cloud viewer for interactive visualization of the laser scanning data this project is all about. Take a look at the help page to learn how to use these features.

A short Introduction

The object of interest

The lower Neckar river near Ilvesheim, a small town in the southern german state of Baden-Württemberg. The settlement is enclosed by an oxbow which is no longer used by ships, and the water is allowed to shape its bed mostly free of anthropogenic influences. This resulted in the formation of a gravel bar.

Since 2011, the gravel bar has been investigated by geography students from nearby Heidelberg University. Every summer, they perform a topographical survey in order to learn about the erosion and deposition processes that continuously change the gravel bar's shape and volume. Their most important tool: An expensive piece of 21st century technology called a laser scanner.

The technology

In modern geography, laser scanners are widely used to produce highly detailed 3D models of practically anything from the size of a small boulder up to entire landscapes. As the name suggests, laser scanners work by sending out pulses of infrared laser light. Whenever the light hits a surface, a portion is reflected back to the scanner, and the scanner measures the time it took for the light to travel to the obstacle and back.

Knowing the speed of light, it can then compute the distance to the reflective object with very high precision. By repeating this process for all directions in lots of very narrow-angle steps, the laser scanner can measure the exact shapes and positions of all surrounding objects. The result is a so-called point cloud – a dataset which contains millions of 3d coordinates that represent the shapes of the detected surfaces.

The laser scanner does not only detect the geometry of surrounding objects, but also their colors, using a digital camera that is mounted on its top.

The strategy

In order to acquire a complete model of the region of interest - without missing parts and in good resolution everywhere - it is not sufficient to perform only a single scan. Just like a human's eye, the scanner's optical sensor cannot see through objects. To capture a part of the scenery, it needs to be in line of sight from the scanner's position. Each obstacle in the field of view will produce a so-called "scan shadow" in the resulting data set - an area void of any recorded points.

To fill the scan shadows, multiple scans from different positions are made, so that each part of the scenery is visible from at least one position.

The data which is acquired duing each scan is transferred to a laptop computer which is connected to the scanner. The computer is used both to control the scanner and to perform post-processing and analysis of the acquired data. This includes the merging of data from different scanning positions to produce a shadowless point cloud for the entire region. The computer can do this mostly automatically, but it needs some hints. These are provided in the form of special reflectors which are placed in the surrounding area prior to scanning.

When a laser pulse hits a reflector, an especially large portion of the light is returned to the scanner and a very strong reflection is recorded for the reflector's location. The scanner's software is able to recognize the same reflector in different overlapping pieces of the data set and uses this information to put the pieces together.