Quantum GIS: Spatial Data

Vector data represent discrete features, which could have names or attributes. It is often used in social science research to describe things with clear boundaries.

Vector features can be of three types:

• point: a single pair of coordinates, like a cell phone tower or a crime scene
  
• line: something that has length but not width; often a road or a river
  
• polygon: an area with boundaries; often a political feature such as a state or county, but could also be a lake, building footprint, etc.
  

The most common file format for vector data is the shapefile (.shp).  The shapefile is actually a collection of files: (.shp, .shx, .dbf).

Attribute data are not really a separate kind of data, but rather the fields (numeric or text) that are attached to features.

ArcGIS can join together geographic data like shapefiles with tabular data in formats such as .xls, .mdb, .txt, or .csv, using the Join tool.  The columns in the table become the attributes of the geographic feature to which it corresponds. The trick is finding a unique identifier common to the features in the shapefile and the rows in the table.

Raster data represent continuous features as cells (pixels) in a grid, much like image files in which each pixel has a location in space plus a meaningful value. Raster files are frequently used in scientific studies for phenomena like rainfall, elevation, or things that vary continuously in space.

See What is raster data?

Common raster file formats include .img, .tif, and common image formats.

A huge amount of spatial data is available free online, particularly through government agencies. International data may be among the most difficult to find.