General GIS Concepts

What is GIS?	The acronym GIS stands for Geographic Information System. A GIS is constituted by the interaction between tabular and spatial data, where the tables contain information related to map features. For this reason, we often speak of Relational Databases when discussing GIS.
Tabular Data	Tabular data consists of attribute tables that define the parameters of the map features. There is really no limit to what the tables can contain, whether Boolean strings (True/False), Text, or Numeric data. For example, a Boolean entry in a cities table may define whether or not each city is a national capital. A text entry may have the city's name, or the archaeological period in which it flourished. A numeric entry could have population figures or lat/long coordinates. The advantage of the relational database system is that the different columns can be sorted and selected according to the user's need. These selections then appear highlighted on the map.
Spatial Data	Spatial data places the features on the map. The coordinates of a point are the most obvious example of this, but it also incorporates projection systems, line and polygon attributes, and other information. There are two main classes of spatial data: vector and raster.
Vector Data	Most work archaeologists do in GIS is based in vector data. This system of recording features is based on the interaction between arcs and nodes, represented by points, lines, and polygons. A point is a single node, a line is two nodes with an arc between them, and a polygon is a closed group of three or more arcs. With these three elements, it is possible to record most all necessary information.
Raster Data	Raster data is characterized by pixel values. Basically, a raster file is a giant table, where each pixel is assigned a specific value from 0 to 255. The meaning behind these values is specified by the user- they can represent elevations, temperatures, hydrography, etc. Satellite imagery uses raster data to record different wavelengths of light. Raster data is advantageous to vector data in constructing 3D images, as the values for every pixel are calculated through a process called interpolation. In ArcMap, it is possible to control what type of interpolation method is used when converting from vector to raster data. Other programs, such as Erdas Imagine, are tailored specifically to raster data and may be more appropriate for certain projects.