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Spiro Mounds Survey

 

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Spiro Mounds Microtopographic Survey

Image of Brown Mound - Spiro Spiro Mounds is one of the most important Mississippian sites in North America. Located in eastern Oklahoma, the site is characterized by numerous ceremonial plazas; three types of mounds; one burial mound, two temple mounds, and nine house mounds;  and supporting city environs.

This study focuses on Brown Mound, a temple mound which has been both looted and excavated in the past. A goal of the project was to generate a higher resolution contour map than the existing 25 cm contour interval map shown below. Additional objectives of the project included generation of a five cm resolution digital elevation model (DEM) and creation of a 3D model of the mound.

 

 


Spiro site contour map showing location of 1979, 1980, 1981 excavation units. 
Adapted from Rogers, 1982

The equipment used for this study include the Optech ILRIS 3D Scanner and the Genie TZ-50 boom unit.  Brown Mound is approximately five meters in height and is covered by grasses and sapling trees.  In early April 2004, trees were cleared and the site was mowed, but due to wet weather conditions fieldwork could not resume until after substantial regrowth had occurred. The existence of ground cover such as tall grasses, or visual obstacles such as trees, is a fundamental problem to the  in microtopographic surveys and in the production for accurate surface models for sites such as Spiro.

Since the survey area lacked identifiable ground control features which are essential for the scan and data alignment processes, scan targets were created.  The targets were 16 x 16 x .5  inch matte black boards with unique white characters. The design of the targets was chosen to produce high contrast between the target and the grass making the targets easily discernible in any scan.  The targets were distributed across the mound and surrounding area to ensure necessary ground control.


Distribution on scan targets across the survey area

To attain complete scan coverage of the mound, five boom locations were chosen. The average scan was taken approximately 45 m above the mound, and the scan times ranged between 15-30 minutes. Nine scans at a resolution of 2-3 cm were required to achieve complete overlapping coverage of the survey area. The scan file sizes ranged from 20 to 30 MB. Field notes where taken for the azimuth (direction) of the scan and the tilt of the scanner head.


Genie Lift Boom with Optech scanner acquiring scans of Brown Mound

Upon returning from the field, the scan files where all parsed and cleaned.  The image below denotes the cleaning process where data not pertaining to the scan target, Brown Mound, were removed.   


Process of cleaning (removing unwanted data) a scan. 

Following data cleanup, all of the nine scans were aligned using the Polyworks processing suite.  Following alignment, the data were merged into a polygonal mesh and were reduced from 14 million points to seven million.  This was done to ensure that the data would be manageable in subsequent processing software. 


Process showing alignment of the dataset

 Because the scans were taken from the boom unit, the axes of the data were tilted. To correct this problem, the merged product was rotated along the x-axis using the tilt angle (relative to the horizontal plane) of the scanner head.  The axes of the resulting data set were oriented so that the new z axis was 'true z' or vertical.


Process showing the rotation of the data coordinate system

After the axes rotation was complete, the data was exported into an ASCII point cloud file. The ASCII file was imported directly into Golden Software’s Surfer 8 program, where a five centimeter grid was created. Triangulation with linear interpolation was the chosen algorithm in the gridding process.  Because of the vegetation on the mound, the DEM results were coarse and noisy.   A low pass moving average filter (Surfer’s Lowpass 3 utilizing a 3 x 3 neighborhood) was ran on the data in attempt to remove the vegetation effects.  A 0.5 m contour maps was generated on the filter results producing the data shown in the image below.   It should be clearly understood that the elevations shown in the figure below are relative to the ground plane chosen in the axes rotation step. Any deviations of this ground plane from the plane normal to the local vertical will result in incorrect contours. 

The next step was to import the grid created in Surfer into the Idrisi Kilimanjaro. A number of Idrisi grid modules produce output that can be of help to archaeologists and others interested in these types of studies. Another contour map was created, this time with a 15 cm interval. Slope and aspect maps were also created within Idrisi.


Final (.5 meter) contour lines of Brown Mound produced in Surfer

Overall, it took approximately 25 hours to complete the project from start to finish. The process can be broken down into:
6-7 hours to set up and obtain the scans
4-5 hours to clean and align scans
4 hours to create and reduce polygonal mesh
2-4 hours to grid data in Surfer (depending on interpolation algorithm)
2-4 hours to create grid and other derived surfaces in IDRISI (slope etc…)

We believe that the applications of the scanner/boom combination can be of great help to people that are interested in a multitude of studies. These include, but are not limited to, archaeology (predictive modeling, historic preservation), geology (weathering studies), and architecture and engineering (identification of structural anomalies, change detection and monitoring). However, several questions remain and require further research. First, the periodic movements of the boom mounted scanner due to wind, for example, need to be somehow minimized. Possibilities include securing the platform with tension lines and monitoring movement during the scan so that it might be removed in postprocessing. Second, the effects of vegetation need to be investigated further with an eye toward minimizing its affect on the resulting surface. Third, a better method and procedure for “leveling” the scans is need to ensure that contours, i.e. lines of constant elevation, are relative to the plane normal to the local vertical. One promising method is to use geodetic grade GPS technology to observe the 3D coordinates of each scan center and rotate the aligned scans using the points. This would replace the rotation step explained above.