While attending East Carolina Universitys Department of Geography (I graduated last summer) I traveled all over the country with my advisor Thad Wasklewicz (Ph.D.) acquiring high resolution terrain data. Many of the projects I was involved in used terrestrial laser scanners to map the surface of the earth in order to study the spatial and temporal dynamics of sediment transport and the growth of drainage networks within a particular basin, or location.
The scanning often occurred in remote areas on rugged terrain where mass movements were common and thus control networks for repeat surveys were difficult to establish. And yet we still had a reputation for consistently great data. In topographically complex environments we could monitor surface changes of a few centimeters over 1000’s of square meters.
Not all TLS surveys are equal, and if you are going to be creating authoritative measurements in complex terrain you need to understand the shortcomings of your particular instrument with respect to what you are trying to accomplish. There are many mandatory requirements for an accurate TLS survey regardless of your purpose; Low registration errors, thoughtful scan placement, cleaning the lens, etc… However for every purpose there are unique requirements that facilitate the best possible TLS survey, and capturing terrain is no exception.
Less than ideal terrain data is captured often. And perhaps for good reason; it either wasnt the focus of the survey and/or only a coarse resolution terrain survey was thought to be needed. There are, however some applications that need accurate high resolution terrain data, and if/when you collect this data here are a few tips to keep in mind.
Spot Size – Laser spot size is the size of the laser beam when it contacts a surface. Generally speaking, the longer the distance the greater the spot size. If you a measuring a planar surface spot size is unimportant. For high level terrain mapping it is very important because:
1) It increases unwanted, non-ground points.
If vegetation is present, the larger the spot size, the less likely it will be that the laser beam will sneak through vegetation making contact with a true ground surface.
2) It unknowingly reduces horizontal resolution.
You cannot record data accurately at a resolution less than your spot size. If the width of your laser beam is 5cm, you will not be able to record data at a 2cm resolution accurately. The secret here is to know what your instrument is capable of. Your spot size should be fine if you are using a phase based scanner or a time of flight scanner at close range (<50m). If you are over 50m you should be careful if you are trying to measure at high resolutions.
Case in point. I have witnessed terrain data captured in steep environments with an Optech IRLIS scanner from the safety of a ridgeline road, ~2,000 meters away from the area of interest. The Optech is designed to work at these distances, but it also projects a spot size of 0.5m at 2,000m. The data I have seen reflects this; when you’re TLS DTM (Digital Terrain Model) of difference calculation (amount of change in surface elevation between surveys) errors are +/- 20cm, something is wrong.
Shadowing – Shadowing is a data void within the area of interest. Shadowing can drastically reduce the level of detail in a high resolution terrain survey, thereby creating error and uncertainty in the modeled surface. Two things to watch for are:
1) Vegetation
Vegetation is difficult to filter and time consuming to remove in the field. Vegetation causes an overestimation in the elevation of local terrain and a significant reduction in local measurement resolution. In many cases vegetation is unavoidable, but you can reduce its effect by scanning from multiple sides, filling in the data voids from different perspectives.
2) Complex topography
If the terrain you are measuring is complex (ex. boulders, rocks, convexity’s and concavities in landscape, etc…) you’re going to introduce shadowing into the point cloud. Its hard to notice but it will affect the accuracy of your DTM. To avoid terrain shadowing, register the point clouds in the field, and look for shadowing then. More scan positions can be inserted if you see shadowing, but you will need to identify it in the field since you can do nothing about it once you are back in the office.
Laser Attenuation – Laser attenuation decreases scan resolution as distance from the scanner increases. In the built environment, full of vertical surfaces, attenuation is minimal. For mapping terrain, attenuation is significant.
To keep resolutions consistent across your area of interest it is important to; disperse scan positions evenly, increase the scanner resolution when scanning from long distances and preform basic point cloud measurements in the field to verify the resolution you are capturing are satisfactory.
Thad Wester is a GIS contractor at Sandhill Telephone Cooperative in South Carolina. In his personal time he consults local engineers, architects and surveyors on the focused use of terrestrial laser scanners in their business. For more information visit thadwester.com.