Airborne LiDAR Data Processing and Analysis
Light detection and ranging (LiDAR) is fast emerging as a cost-effective method of archaeological reconnaissance and assessment. Airborne LiDAR surveys illuminate the earth's surface with pulses of light and measure the time it takes for reflections to arrive back at the aircraft. Location information is provided by an onboard GPS system. Time data are converted to distance, enabling calculation of the elevation of the target. Reflections occur from trees, vegetation, buildings, the ground surface, and other surface phenomena. After careful parsing of this information, it is possible to construct an extremely accurate digital elevation model representing the surface of the earth.
Imagery constructed from high-resolution LiDAR data is useful for detecting topographic patterning that may be associated with archaeological features. Geophysical data processing methods may be employed to reduce noise and enhance the visibility of subtle features. Data are typically presented as shaded relief images in which a digital elevation model is illuminated by an inclined light source from different angles. Careful processing and display of LiDAR data can reveal topographic patterning not readily perceptible to the naked eye at ground level.
It should be noted that although LiDAR imaging is an effective method of visualizing archaeological landscapes, the technology can only detect features with topographic expression. Thus LiDAR is not an exclusive investigatory technique, but may be included as part of a comprehensive research design that also includes methods of detecting subsurface features that lack surface expression.
The examples presented below represent a few selected examples of LiDAR data processing and imaging conducted by Archaeo-Physics. Click each image for more information.
19th Century Railroad Roundhouse
This sequence of images presents LiDAR and GPR data from the location of buried 19th century railroad roundhouse foundations.
Image 1 = discrete response LiDAR data with ground surface and near surface vegetation shown in grey-scale and trees, brush and buildings shown in reds and yellows.
Image 2 = bare earth LiDAR. Note: outline of the roundhouse and central turntable are visible after removing tree canopy returns.
Image 3 = bare earth LiDAR image with GPR data from 73 cm below surface
Northeastern Plains Village Complex Sites
||LiDAR was used to help assess the archaeological resources at five Northeastern Plains Village Complex enclosure sites located in North Dakota and Minnesota. The detailed imagery produced by this study graphically illustrate LiDAR's potential for cost effective mapping and evaluation.
The Orwell Site (21OT7)
||Analysis was conducted at a burial mound group and enclosure in Western Minnesota mapped by Theodore Lewis in 1891. The study was conducted as part of an effort to assess the integrity of archaeological resources at this site. It was previously thought much of the site was destroyed by large-scale mechanical excavation in the 1960's. Examination of the evidence shows these excavations mistakenly targeted an nearby unrecorded mound group, but left the Orwell site relatively undisturbed. Data suggest the northern portion of the enclosure is currently threatened by erosion associated with an abandoned gravel mining operation.
Wood Lake Battle Site
LiDAR analysis at this Dakota Conflict battle site in Minnesota was used to assess the archaeological resources present on a one-acre state owned parcel and to guide a cost effective geophysical survey design. A "U-shaped" topographic feature was identified in the resulting imagery. This feature likely represents a defensive breastwork constructed by the 3rd Minnesota Regiment. The feature has not been noted in the past, which suggests it is not readily visible at ground level.
The U-shaped topographic feature is most visible in the shaded relief image illuminated from an azimuth of 225 degrees and is only partially visible in other displays. This illustrates the importance of analyzing LiDAR data using more than one visualization method.
||LiDAR analysis was conducted as part of a National Register of Historic Places nomination process at Fort Juelson. The remains of this 1876 defensive earthwork are clearly visible in the LiDAR data. The analysis also identified what appears to be previously undocumented burial mounds on the hilltop. A subsequent subsurface geophysical investigation provided additional evidence supporting interpretation of these features as burial mounds.
LiDAR point cloud data were processed using LAStools by Martin Isenburg.
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October 8, 2012