QGIS, according to its website, is “a user friendly Open Source Geographic Information System (GIS) licensed under the GNU General Public License.” To draw an analogy, it is to making maps what Adobe Photoshop is to editing images. It gives you the ability to create maps from scratch or from imported data, or modify and add to existing maps. The software is set up so that each imported map or map feature is a “layer.” Layers can be made of point locations, background images, polygons such as country borders, or more complex types of vector data, like WMS maps or ESRI shapefiles. While QGIS can manipulate maps, it cannot create data. Users must do that themselves, by manually creating a table of information or (more commonly) importing data from the Web or other sources. The two main categories of maps that QGIS can assemble, along with the type of data required to make them, are below:
- Maps of event locations, such as lightning strikes, earthquakes, tornados, or the locations of tagged animals. These maps require importing a Comma Separated Variable (.csv) file or a text (.txt) file. CSV files are simply spreadsheets that have been saved in a form a computer can easily understand. Contrary to the name, they can be saved with tabs, spaces, or other characters separating the information, not just commas. If you are unsure how your CSV file is organized, simply open it in a text editor such as Notepad to determine what characters separate rows and columns. The information in a CSV file that could be used to create these maps would have to include separated latitude and longitude columns. If the coordinates are in the same column, see Finding/Manipulating Point Data
- Maps of regions colored according to data, such as a map of countries shaded by population. To create this map, you’ll need an ESRI shapefile that includes country borders and names or IDs, as well as a CSV file that has country population by country name or country ID
Usually, maps are made up of a base map (the world map, either an image or a vector map made of polygons such as continents) and overlaying layers of interesting data, such as locations of phenomena. Base maps can be created with vector or raster data. Vector layers can be scaled indefinitely, because they are made up of mathematical algorithms that tell the computer where to draw lines and shapes. Every time you zoom in, the shape just gets redrawn. Raster layers are images. They are composed of pixels, so will lose quality the more they are stretched or zoomed in on.