5.9. Derivative indices

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Field and lab measurements acquired during the Upper Colca project were transformed, in some cases, into indices and other measures that are more widely useable than the original measurement data. The indices that were generated using GIS data, primarily the vector to raster conversion and lab measurements, such as the Bifacial Thinning Flake index, are described below.

Fieldwork was largely conducted using a GPS receiver connected to a mobile GIS unit, resulting in a variety of vector datasets, however some of these phenomena are better examined using the raster data model. The vector ? raster transformation is a basic concern for scientific fieldwork because it makes explicit the process of deriving spatial generalizations from detailed observations. For example, distributions of temperature or rainfall are interpolated from point-specific measurement locations where empirical evidence has been gathered by instruments. Similarly, in this project specific observations were generalized to produce continuous raster surfaces.

The spatial extent of surface archaeological features were recorded as bounded vectors using mobile GIS, as was described in the preceding chapter. Because mobile GIS involves the use of GPS to delimit features, all spatial features had to be recorded as discrete vectors, such as polygons, even when they are better investigated and represented as raster data. The following methodological explanations are all cases where field data were recorded in a vector form and then interpolated to raster for analysis and review.

The analysis of these raster surfaces took place in two contexts. The first was direct spatial queries against the raster in a GIS context. The second form was derived from the first: it was a tabulated query of all 1200 spatial features recorded during survey and the Mean, Standard Deviation, Min, and Max of the raster value for each feature. These tabulated data were linked using the ArchID reference number to their respective GIS and lab database records, and used in SPSS 12 and MS Excel 2003 for analysis and display.

Source

Data

Resolution

Destination Raster

GPS Vectors

Lithic Loci and collection: Density / material type variables

1m

Obsidian

Non-obsidian

Raster

ASTER Imagery:VNIR bands

15m

Distance from NDVI calculated large bofedal

Raster DEM

ASTER DEM(Spaceborne remote sensing platform)

30m

Elevation (masl)

Slope (degrees)

Aspect (8 categories)

Cumulative viewshed index

Raster DEM

SRTM DEM Topography

90m

Regional scale elevation

Table 5-10. Vector and raster layers used in analysis and derived raster output.

A variety of raster surfaces were created from source data layers used in the analysis. The creation of these non-standard raster layers is the subject of the next section.