GastFarm2016_Photogrammetry_ProcessedOutputs_DEM

Project Name: Gast Farm
Project Location: Iowa, United States
Project Date: 2016

Gast Farm is a 13-hectare archaeological site located in the Mississippi River valley in southeast Iowa. Gast Farm is one of the largest and best-preserved Woodland sites in the region, with both Middle Woodland (Havana Hopewell, ca. 50 B.C.–A.D. 250) and initial Late Woodland (Weaver, ca. A.D. 350–500) deposits. Additionally, a large mound was once present, and remnants of geometric earthworks also suspected. The PIs research seeks to is to understand the interanal structure and layout of the Woodland communities at Gast Farm, which can contribute to knowledge of Middle and Late Woodland domestic and corporate-ceremonial spheres, i.e., residential as well as sustainable and symbolic communities. PI William Green collaborated with SPARC researcher Adam Barnes to produce orthorectified images and a digital elevation model (DEM) of the Gast Farm site from aerial imagery collected in 1990, using AgiSoft Photoscan. The orthophotos reveal distinct soil discolorations because the original photos were taken after the field had been disked, planted, and rained on, and only a few days after row crops (corn) had emerged. Soil discolorations indicate large-scale cultural features such as middens (dark colors) and a mound and possible geometric earthworks (light colors). The orthophotos also show a series of narrow linear discolorations exactly 10 m apart that represent the paths created during the controlled surface collection that was underway at the time the photos were taken. The DEM produced by orthophoto rectification has a 50-cm resolution, which is unfortunately is not sufficiently sensitive to detect possible mound or earthwork features after they have been leveled and plowed.

This work is part of a larger 2016-2018 project at Gast Farm, which combined GIS development, aerial imagery analysis, and geophysical survey. Defining community organization, locating any subsurface traces of the mound, and determining the reality of the suspected earthwork constituted key research objectives. Over the course of the project, the Gast Farm team (1) identified the Middle Woodland (Havana-Hopewell) community plan, (2) determined no geometric earthworks were present but discovered six additional mounds, and (3) confirmed and expanded the Late Woodland (Weaver) community plan. In the process of obtaining these results, the project also accomplished several methodological advances: it (1) demonstrated the viability of magnetic gradiometry for identifying Woodland residential and mortuary features in Mississippi Valley alluvial fans, (2) showed how to incorporate legacy oblique aerial photography in a georeferenced GIS, (3) indicated the promise of drone-based photogrammetry in identifying cultural features beneath crop cover, and (4) modeled the virtual reconstruction of leveled mounds. 

This upload contains the digital elevation model created during the Gast Farm project in 2016. See the Index file for a list of files and folders.

Photogrammetry Details: AgiSoft Photoscan
Operator: Adam Barnes (SPARC)
Imagery type: Oblique Kodachrome slides
Year collected: 1990
Camera model: Nikon SUPER COOLSCAN 5000 ED
Resolution: 5400 x 3700 resolution
Focal length: unknown
Pixel size: unknown
Precalibrated: no
Number of images: 24
Flying altitude: 987 meters
Ground resolution: 9.13 cm per pixel
Coverage area: 5.95e+05 sq meters
Camera stations: 23
Tie points: 8295
Projections: 17912
Reprojection error: 2.13 pix

DEM Model Details
Resolution: 18.3 cm per pixel
Point density: 29.9607 points per sq meter

Further metadata:  GastFarm2016_Report/GastFarm2016_Report.pdf