# Quick Review: Spatial Data

Spatial data adds a geographic dimension to a qualitative and/or quantitative data set, situating it in a particular location within a coordinate system relative to other data points. (The coordinate system can be a real-world system or a locally created one used to meet the needs of a particular project.) 

Spatial datasets, in general, come in two distinct forms, [**vector data**](https://docs.qgis.org/2.8/en/docs/gentle_gis_introduction/vector_data.html) (points, lines, and polygons) and [**raster (or pixel data)**](https://datacarpentry.org/organization-geospatial/01-intro-raster-data/)**.** Raster and vector data can come together in the creation of a wide variety of mapping projects, from a traditional figure with an explanatory legend and caption, such as might appear in an academic text, to an online interactive platform that allows for the searching or filtering of thousands of pieces of spatial data or hundreds of historical maps.  

### **Vector Data**

**Vector** **data** includes points, lines, or polygons (shapes made up of straight lines) containing spatial information that represent some sort of feature or event in a physical or imagined landscape and may contain other types of qualitative or quantitative information, called **attributes**. A point may represent a tree, a city, or a moment in time. Lines might indicate the street grid of a town, the path someone traveled across the world, or a social link between two communities. Polygons can mark the boundaries of a country or voting district, the catchment area of a river, or a single city block. 

For example, the relatively simple and ongoing [World Travel and Description](https://dhprojects.bc.edu/s/travelanddescription/page/welcome) project from the Burn's Library collection pictured below uses vector point data to offer a selection of images and accounts from individuals and their observations about how the cities and landscapes they visited appeared. Users can filter the point data by data or search for particular location names in the search bar. 

![Screenshot of World Travel and Description](/files/SS3NCG8yeXBzWi0j0a5R)

### Raster Data

**Raster** consists of "cells" of data covering a specific area (its **extent**), with attribute values in each cell representing a particular characteristic. It may still consist of points, lines, and polygons, but these shapes are themselves composed of pixels (the way a jpeg or other image file type is). 

Data of this type may take many forms, such as satellite imagery containing vegetation or elevation data, precipitation maps, or even an historical map, which has been given a spatial reference. Unlike vector data, raster data has a particular **resolution**, meaning each pixel represents a particular geographic region of a specific size. 

![Vector vs Raster data](/files/CVOPtTzK16bXaUqFHKEC)

Most projects combine various forms of vector and raster datasets.  


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