So, in areas with low vegetation the resulting should be reflected in the metadata, which therefore DTM will often not be a satisfactory representation of need to be considered when trying to archaeologically the terrain surface.
Peer-reviewed Journal Papers
This makes the interpretation of evaluate the resulting DTM. In the following some of micro-topographical archaeological features difficult the parameters will be discussed. With the analysis of the FWF signal a separation of these echoes objects can be feasible. Choice of sensor At the time of writing , there are two different types Archaeologically speaking, the DTM derived from a of sensor systems available: discrete echo scanners conventional scan may show considerable problems in so called conventional scanners and full-waveform areas with low and dense vegetation, as can be seen in recording systems.
While conventional systems use Figure 5. While the left and middle parts Figure 5. All scans have a comparable original point beam typically with an interval of 1 ns. In previous density 4 points per m2, see Table 5.
Automated Classification of Airborne Laser Scanning Point Clouds
In Figure 5. Tuning the filter-settings, the low and dense the laser pulses poorly toward the sensor. However, the quality of the DTM is the optimal timeframe for data acquisition can vary highly reduced — only a few of the pits can be discerned from year to year and region to region.
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For example, and the remaining terrain point density is quite low. Important metadata concerning data acquisition are date, point density and field of view. There is still a The point density is a function that mainly relies on the lack of research on limitations due to the type of tree pulse rate, field of view, flying height above the terrain, cover and the seemingly short time-frame available overlap between two neighbouring scan stripes and for scanning wooded areas. In a forest, only part of the laser collection is the dormant period, when deciduous pulses will pass through to the ground surface.
To get a trees and most of the understory have lost their leaves. Using a wide scan which in Central Europe can be between October angle and a large overlap e.
Uni Trier: Umweltfernerkundung und Geoinformatik - Airborne Laserscanning
Only when the leaves have subsided after some rainfall or snow will micro Although it is often argued that scanning a forest topographic features to become visible again. There demands a narrow opening angle of the scanner to Table 5. The parameters listed here are considered by the authors as a minimum standard.
April March — April Point-Density pt per sq m min. The quality of the DTM could be improved significantly. This results in Furthermore, the large overlapping area of two doubled objects, noise and formation of non existing neighbouring strips will be an advantage during structures sinusoidal curves, but also edges.
During the process of strip-adjustment for advanced geo- the process of archaeological interpretation these can referencing. With a large overlap it will also be certain be considerably irritating. This is demonstrated by an that every object on the ground would be hit at least example from the Iron-age hillfort in Schwarzenbach twice from two view points, and that there is a good Figures 5.
In the displayed area a large number of chance that some of the oblique laser pulses will hit roundish depressions can be observed, which resemble the ground below conifers where the almost vertical archaeological features as for example lime kilns. These laser pulses of a system operating with a narrow scan features are, however, non-existent. They are errors angle might not get through. Also in steep terrain, a resulting from an inadequate geo-referencing. These inaccuracies can be improved by doing a simultaneous 3D strip adjustment Kager For A possible drawback of a wide opening angle with a this, tying features between the overlapping stripes large overlapping area is an increasing number of are used analogously to tie points in aerotriangulation.
Especially near the borders of the scanning lines with low noise from covering vegetation are strip, a high number of last echoes will be returned used e. Geo-referencing Geo-referencing of the laser data is usually done by The method of geo-referencing used in most cases the data providers, where a calibration over asphalt or is not communicated via metadata. Therefore it is flat horizontal areas with sparse, very low vegetation important that an interpreting archaeologist at least is usually performed Pfeifer et al.
The geo- knows about the problem and takes care in areas with referencing process still causes a few problems Kager strip overlap. In that way the roundish depressions introducing inaccuracies into the resulting depicted in Figure 5. These inaccuracies will lead to problems can be identified as virtual features due to discrepancies within the overlapping area of two strips, where between overlapping strips. The sinusoidal curve crossing the area horizontally in the DTM shading on the left image a is removed after strip adjustment b.
For further information, refer to the text. Choice of filtering software and parameters Another important piece of information which usually does not find its way into metadata is the method used for filtering the ALS data i. As stated above, a single laser pulse will result in several echoes. Only the last echo may typically originate from the terrain surface.
However, very often, the laser pulse will not be able to reach the ground surface. In these cases, the last echo will be returning from cars, tree-trunks, dense vegetation or even animals, for example. Therefore, any last echo can either be a terrain or an off- terrain point.
Since the archaeological interpretation of an ALS campaign is typically based on the derived DTM, it is critical that all off-terrain points are removed from the data set used for DTM generation. Therefore, the last echo data has to be classified typically the term filtering is used into terrain and off-terrain points. Although filtering the vegetation is of prime importance in forested areas, only a few projects take care to get their hands on the filtering process Cifani et al. These are Even if data acquisition was commissioned for the high-quality data fit for the purpose of topography.
Also, parameter settings which removes building structures the method of filtering used is mentioned only in a very and to a certain degree flattens micro-topographic few publications. Above all, the DTM is delivered with a grid www. Gabriele Cifani et al.
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Each of them was produced for a special parameters. Finer archaeological detail, such as pit- purpose and each is best in certain environments. The same area can, however, reveal more features, like round barrows and lime-kilns, flattened information when the original ALS point cloud is and removed e. The examples filtered in an archaeologically adapted way or special clearly show that the process of filtering is of crucial purpose ALS data is used.
Besides the DTM. All images display DTMs filtered using the same clearer view due to the higher resolution 0. Similar results can be seen in the comparison of al. While both building and derivates of hill-shading from different directions hill- lime kilns can be identified both in the general and shadings range, mean of hill-shadings, hill-shadings special purpose data, details become only distinct in PCA , composite of hill-shaded relief and elevation the latter one.
Therefore, at the very Apart from these more sophisticated techniques, least, a second shading with perpendicular light-rays we have particularly good experiences using a very has to be provided.
Bathymetry by Fusion of Airborne Laser Scanning and Multi-Spectral Aerial Imagery
While Koreen Millard et al. This is demonstrated problem of displaying and interpreting archaeological in the depiction of a field of hollow ways crossing the ALS data so far. Bewley et al. The shaded DTM on the left map with dense line interval to identify topographic part gives a good impression of the large number of relationships. Michael Doneus and Christian Briese paths running parallel to and partly intersecting each b were discussing slope and the subtraction other.
Since the light source is placed in the northwest of a resampled DTM 5m grid width from the high upper left corner , those tracks running exactly resolution DTM to get a model of the micro-relief. Combining this Astrid Humme et al. Ralf Hesse b recently published while the image is still comprehensible. For extremely a similar but advanced approach to derive local relief low relief features and for outlining the extent of the models from ALS-based DTMs, which seems to be individual structures, local relief models as described particularly promising.
Other techniques comprise the use of a principal While country-wide LiDAR derived DTMs become component analysis PCA creating an rgb-image from available, systematic archaeological interpretation the three main components derived from altogether of large datasets has started in various institutions 16 shaded reliefs with varying illumination angles e. Schmidt et al. In a recent poster presented at al. Tracks running exactly northwest-southeast are invisible. All hollow ways are clearly visualized independent of their direction.
Doneus et hardly detectable and comprehensible when surveying al. In several cases, previously unknown a; Poirier et al.