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Introduction to spatial joins with QGIS.Get your feet wet with geographic data analysis and visualization using QGIS, an open source, extensible, capable-rich GIS tool.Subscribe now.Get the highlights in your inbox every week.QGIS is a free and open source geographic information system (GIS) that is extensible, interoperable with other GISes, and used by a ton of people (including me) who have geographic data to analyze and visualize. It's a great platform with an enormous set of capabilities, which can seem daunting on first approach. If you're interested in getting your feet wet in geographic data analysis and visualization, the following basic tips will help you get started with QGIS.Exploring the spatial join problem.Relational database users are familiar with the concept of a table join, which is a way of associating data in one table with data in another. For example, suppose I have one table that lists employees ("Employee") and another that lists branches of the company ("Office"). I can add a value to the Employee table that indicates the office where the employee works:qgis_1_related-tables.png.Fig. 1: Related tables.The field OfficeId in the Employee table "points to" the correct row in the Office table. In the database world, OfficeId is said to be a primary key in the Office table and a foreign key in the Employee table.Since the office is at a location, there is a spatial thing happening here—I know the address of the office, so I can find it on OpenStreetMap, for example. What if, instead of an OfficeId field, I had a field specifying the office location on both the Employee and Office tables. Therefore, my relational database would be extended with an operator that is used to verify that two locations are equivalent; for example, that the Cartesian distance between those two points is less than some small number. This kind of spatial relationship problem comes up in all sorts of interesting problems.For example…I'm working on a writing project with some colleagues in Chile to explore issues related to hydroelectric power development there. One of the items we want to include in the project is a map showing the location of existing hydroelectric generation facilities. Along with that map, we want to summarize the information related to those facilities by watershed . A watershed is the area of land that drains into a river system and eventually to the sea (or another body of water that may be landlocked). Watersheds are important for all sorts of land management reasons, as they tend to define ecosystems, climatic zones, and even traditional practice areas. The figure below, made with QGIS, shows parts of two watersheds (the areas delimited by the thick blue lines, with their names—Río Itata and Río BíoBío—in blue italic boldface) and the power generation facilities (symbolized by blue diamonds):qgis_2_map.png.Fig. 2: A portion of the area of interest showing watersheds and generation stations. Map data © OpenStreetMap contributors.In the same way that my relational example showed two tables of related data, this map shows two spatial datasets: watersheds and hydroelectric generation stations. The watersheds are represented as features that have spatial extent (or area) and location; the generation stations are represented as points, which have location only. Both datasets include attributes that help identify each feature defined in the dataset; for example, the name of the watershed or the amount of power the generating station produces.Suppose this summarizing task requires determining how much power is generated in each watershed. One way to do this is to go through the generating station dataset and assign a value to each record that points to the watershed that contains the point. I can carry out this task manually because I can observe which points lie within which watersheds. This is pretty laborious. But, given that the two datasets already define the spatial nature of the watersheds and points, and given that QGIS can read this information and generate a map, can QGIS figure out this relationship for me?Installing QGIS.In order to use QGIS, it must be installed. The versions offered in various distros' repositories can be quite old, to the point of being unable to load various useful plugins because of incompatibilities between the plugin dependencies and the libraries offered by the distribution (e.g., Qt libraries).There are often newer alternatives to the ones in the repos. For example, both Fedora and Ubuntu offer GIS projects that incorporate all sorts of useful spatial analysis tools. Another alternative is to download it from the QGIS site (as I'm writing, both the new long-term release, version 3.4.4, and the previous long-term release, 2.18.28, are available). In my experience, it's better to select the new long-term release to avoid problems similar to those in the older versions in the repositories. However, currently much of the online content available for QGIS references the older QGIS 2 versions, and it may take some puzzling to determine how to accomplish things in QGIS 3.I downloaded the latest long-term release from the QGIS site and started it up:

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