Geographic Information systems and QGIS

To make the most of all the amazing satellite imagery and other remote sensing data that is freely available, we will need to use Geographic Information Systems software.

A geographic information system (GIS) is a database for geospatial data – that is, data that includes a geographic location. Combined with appropriate software, a GIS allows you to capture, manage, and analyse geospatial data, and to create maps to display it.

GIS data

There are two main types of data that you will encounter when using GIS: raster data and vector data. Let’s explore these together!

Raster data

Raster data consists of a continuous grid of squares, with each square containing data. This is just like pixels on a digital image or electronic device, but while in a digital image the pixels contain colour information telling a screen what to display, raster cells can represent a wide variety of different kinds of data.

Rasters best represent surfaces, with each grid square ultimately reduced to one or more numerical values. Satellite imagery is a good example of a raster, but the numbers could represent lots of other sorts of data – for example, elevation or temperature. Patterns or features within the data are not stored in the raster itself however, so the computer does not ‘know’ that the features represented in a raster – such as a house or an archaeological site – are a house or a site. Rather, rasters must be interpreted to identify any features they show.

_{Raster data ultimately reduces to a grid of numbers. Imagery courtesy of the USGS.}

Vector data

Vector data consists of features made up of points, lines or polygons. Rather than continuous surfaces, vector data consists of distinct features.

• Point: a point is a location with specific coordinates. These represent individual, isolated features – for example, the location of a tomb or house.

_{They can also represent larger features; if you were mapping the distribution of Bronze Age urban settlements, each settlement might be represented by one point, roughly in the middle of the site. This would tell you where the site is, but wouldn’t give you any information about the size or layout of the settlement.}

• Line: a line is a representation of a linear feature, such as a road or a river. They have a start point and an end point, though other points in between may give more information about the shape of the line.
• Polygon: a polygon represents an area, like a settlement or a country. They are two dimensional features showing the layout of a feature, and can give information like the area and perimeter of the feature.

_{This simple map of the Middle East includes major cities as black points, large rivers as blue lines and countries as polygons in various shades of grey. Data courtesy of Natural Earth.}

_{Can you think of other examples of vector and raster data that we haven’t mentioned?}

QGIS

In order to work with raster and vector data you need appropriate software. There are loads of great GIS software options to choose from, but in this course, we will use QGIS. QGIS is completely open source, which means that it is free to download and use. This course has been written using version 3.22, but it should be possible to follow along with any version of QGIS 3. We recommend downloading and installing the software using the “QGIS Standalone Installer” for the “long term release” version from the QGIS website download page here.

_{We are going to be using QGIS which is free and open source. Courtesy of QGIS.}

You can download this now if you like, but as we will introduce the software in more detail next week you can also wait and download it then. We’ll introduce you to QGIS as we go along, but in the meantime, if you are interested in learning more about GIS, we recommend reading the QGIS Gentle Introduction to GIS. You can also find an overview of the QGIS user interface here. This isn’t required though – it’s optional extra reading!

Congratulations on reaching the end of Week 1! We hope you’ve enjoyed it so far, and that you’re excited to learn more with us over the next few weeks! If you have any questions or comments, leave them in the Comments Section. While we respond to as many comments and questions as possible, this course is primarily unfacilitated and peer-to-peer learning is encouraged – so make sure to check out the other comments and join the conversation!