preacher’s cave high definition survey and 3dlaser scanning project, eleuthera, bahamas

8/4/2019 Preacher’s Cave High Definition Survey and 3DLaser Scanning Project, Eleuthera, Bahamas

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Preacher’s Cave High Definition Survey and 3D

Laser Scanning Project, Eleuthera, Bahamas Introduction:This report presents the results of a High Definition Survey and three-dimensional laser

scanning of the Preacher’s Cave site located in Eleuthera, Bahamas. This project wasundertaken at the direction of Robert Carr, Executive Director of the Archaeological and

Historical Conservancy, who is conducting on-going archaeological investigations at thecave site.

Description of Global Positioning System and Laser Scanning Workflow: Data

was collected during two field days on February 2nd and 3rd, 2006. A site visit was made

on February 1st, which allowed for reconnaissance and choice of scan set-up locations to

maximize the site coverage. A Leica 3000 HDS Scanner was used in the current study to

perform a terrestrial laser scanning survey of Preacher’s Cave. Accuracies of +/-6mm at50m in the x, y, and z dimensions were attained. The scanner captures data in a 360° x

270° field-of-view, and a total of six set-ups were utilized to obtain coverage of the cave

site. The real world coordinates are tied into controls established using GlobalPositioning Systems Real-Time Kinematic Surveying (GPS-RTK) techniques. The point

clouds gathered for the future study would be registered in the Cyclone software version

5.4.

Laser scanning was chosen as a method of recording the Preachers Cave site as it is a

nondestructive technique that offers three-dimensional documentation which includesvisualization and measurement capabilites. The scanned data can be tied to real world

coordinates and used for virtual recording and analysis within a geographic information

system (GIS). Using GIS, pattern analysis, detailed topographic mapping and spatialcorrelations can be analyzed. Previously collected data and location information such as

established benchmarks and coordinates, field specimen and archaeological test areas

and denoted features can be added to the developed GIS and included in the current

analysis, with a spatial database that can be updated as information at the site continues tobe collected. Scanning was an ideal method for the cave setting, as the laser can be used

in all lighting conditions and can even be performed in full darkness. Unlikeconventional line-of-sight total station measurements which can be a time consuming

way of collecting x, y, and z locations, the laser scanner collects millions of

measurements with a high degree of speed and accuracy and can be viewed in three

di i



3) Extensive photographic documentation of the cave site to be utilized to ensure accurate

interpretations of the laser scan data, and to use as photo drapes when appropriate forcontextualizing the scan data.

4) Set-up of the targets to ensure maximum overlap and registration of the scan data with

known locations. Five targets were used in the current study, with each scan view‘seeing’ a minimum of at least three targets in each of the six scan set-ups performed.

5) Post-processing of the data that was recorded by the laser scanner. This includes a

reduction of the dataset to include just the areas identified as being of interest. Duringthe post-processing, a quality control analysis is also performed, which assesses the

accuracy and precision of the recorded data. Finally, the data is exported to a variety of

formats for analysis in other GIS and 3D modeling software. Two dimensional images of the scan data and areas of interest were prepared for inclusion in this preliminary report,

and post-processing continues on the data to analyze areas of investigative interest. As

new findings occur at Preachers Cave, the model can be updated to include spatiallocations, and can also be utilized for a number of visualization, pattern analysis and

measurement needs. The three-dimensional data can also be processed and provided in a

viewable format for interpretive development and understanding for the use and

modification of the landscape through time.

The Scanning Field Procedure: Scanner set-ups consisted of six locations, two

outside of the cave and four set-ups in the interior portion of the cave. These set-ups

utilized five laser targets, which were established inside the cave structure, with eachscan set-up in view of a minimum of three targets (Figure 1). The laser targets are used

in the locational registration of the scan data, and provide spatial control for the scanning

and georeference to a real-world position when combined with the GPS data collected.Target set-up is crucial to the quality of the post-processing by allowing each scan set-up

to be registered in reference to the other scan set-up positions. Sub-meter, differential

GPS was utilized to collect locational data of features on the site including intial scan setup position outside of the cave, previous benchmark locations (Figure 2), and features

such as solution holes located on the ceiling of the cave and accessed from above thecave (Figure 3) and areas of planned improvements to the site (Figure 4). A Trimble

ProXR unit with a TSC1 datalogger was utilized in the current project, utilizing a manual

3D position mode, and taking a minimum of 90 positions on each acquired point location.

Scan set up locations were chosen to work in conjunction with spatial documentation



Figure 1. Target

placement.

Target 2

Target 3

Tar et 5Target 4

Tar et 1



Figure 2.

PreviousBenchmark Locations.



Figure 3. GPSDocumentation of solution

holes on ceiling of cave.



Figure 4. Proposed Improvements.



from archaeological excavation. The developed topographical model of the floor surfacealso portrays and preserves an exact representation using a Cartesian coordinate system

which can be georeferenced to a real world location. The previous mapping of the floorand archaeological features was performed using an arbitrary established coordinatesystem for the site, and was not able to capture the floor detail with the same degree of

accuracy, precision and detail, but is useful in depicting feature locations which will be

used in the current study (Figure 5).

Figure 5. Previous mapping of the cave floor compared to the current study.

Other areas of site documentation included the front ‘pulpit area’ and the front facingrock area with concentration on a section containing several historic period cut nails and

having a feint area of engraving (Figures 6 and 7). Nail pattern analysis from this area

was performed during the preliminary data post-processing and the engraved area was



Figure 6. Nail and engraved area pattern analysis.



Figure 7. Photodrape of front ‘pulpit’ area of cave showing nail and engraving features.



Scan set-ups were also performed in the back and mid-point portions of the cave,including a close range scan on a rock face under a large solution hole which may have

other feint engraved areas. These scan views are still being post-processed and are notavailable for review in this preliminary report. Additionally, a scan from outside the cavewas performed on an area with possible rock art dating to the earlier Taino period. These

scan views will be made available for analysis as post-processing of the data continues.

The scans will also provide three dimensional visualizations, which will be madeavailable in digital format upon completion of the post-processing and can be utilized to

view the cave in a spatially accurate platform and can assist with interpretive

development and archaeological spatial pattern analysis. The overall raw or largely un-

processed data depicts the cave configuration in highly accurate detail, and will be of future use to the on-going heritage management and archaeological research at Preachers

Cave (Figure 8).



Figure 8. Raw data collection from interior scan set-up location. Note that vegetation (palm trees) at cave entrance have been

‘digitally removed’ from the scan to allow view into the cave.

preacher’s cave high definition survey and 3dlaser scanning project, eleuthera, bahamas

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