(Q)GIS Basics

Rachel Franklin

Newcastle University

Goals for today’s session

• Understand basic QGIS functionality: what is where, what it does, and why
• Get comfortable with QGIS projects and data management
• Know how to change layer symbology

First, a bit of QGIS housekeeping

There’s a variety of kinds of spatial data

1. Data in explicitly spatial format: shapefile, feature classes in a geodatabase, KML (google earth), geoJSON, and more – plus raster file formats
2. Non-spatial data with variables for latitude and longitude
3. Non-spatial data with geographic identifiers that allow “joining” to data from category 1.
4. Non-spatial data with addresses or place names
5. Georeferencing of paper or digital maps and digitization of features. i.e., create your own data

QGIS common files formats

• Vector data are typically stored as shapefiles or as elements of a GeoPackage, although other formats exist
  • Shapefiles are a classic spatial data format, typically recognized by other GIS software applications and this is the term often used to refer to spatial data
  • GeoPackages are containers that can include layers, but also raster data and even spreadsheets (e.g. csv) data
• Neither shapefiles nor GeoPackages will be recognizable as spatial data in Windows/Finder

Some basic GIS rules

Rule #1: Don’t use Windows/Finder to copy or move individual files!

• You can copy and paste folders, but pay attention to file directory paths

Some basic GIS rules

Rule #2: When you do work in QGIS, your efforts are saved as a project

• Projects link to the data layers you’ve added, but do not actually include those layers—those stay in their original location and can be used by multiple projects at once
• If you copy your project to another location, without moving the individual layers as well, your project will be empty when you open it

Some basic GIS rules

Rule #3: How we name variables, layers, and folders can matter

• No spaces in folder or file names
• Instead use _
• Don’t start variable names or file names with numbers
  • So: pop_1990 instead of 1990_pop
• Avoid strange symbols in file names and folders (& or *, for example)

Additional GIS functionality

Yesterday we explored some basics

• Adding layers
• Changing symbology

Other basic stuff

• QGIS pulldown menus and toolbars
  • Especially navigation
• Panel and toolbar visibility is controlled via View>
• Save often!

Querying a database

• Attribute queries create a subset of records or features that meet the query criteria
  • Can be simple, like “Find L’Aquila”
• This can be done by sorting, browsing, or searching
• Or selecting features that meet particular criteria (like minimum city size)
• Once we’ve created our selection we can:
  • Generate summary statistics
  • Create new layer or shapefile
  • Create new variables that label features as having a particular characteristic

Attribute queries

• Remember: vector data have attribute tables, often containing useful information!
• Structured Query Language (SQL)
  • Common database query language
  • A logical expression that generates a selected set of records
    • For example, all cities larger than 50,000 inhabitants
    • Or all “highly active” volcanoes
  • We can then “summarize” information based on our selected set of cities — e.g. median income for all cities greater than 50,000 inhabitants
• In QGIS: “Select features by expression”

Spatial queries

• Spatial queries also generate a subset of features, but not based on their attribute characteristics
  • Instead, we’re interested in their spatial characteristics
  • For example, all housing units inside a neighborhood
• Common types of spatial queries:
  • Containment – All the businesses in a particular region
  • Intersection – All the Italian provinces that touch the coastline
    • Can also be features that overlap or cross features from another layer
• Interactive selection – Can be done via table or map
• In QGIS: “select by location” and “select within distance”

Tabular joins

• Used A LOT to work with spreadsheet data containing information for geographical areas
• Tables are joined based on a common attribute
• This is how one table’s information is matched to the corresponding table’s
• In QGIS, the column you use to join tables needs to be identical and the same data type—but the same variable name is not necessary
• In QGIS: Joins interface accessed from layer properties
• Standardized codes are our savior: using names to join data tables can be problematic if name spellings vary

Tabular joins

Spatial joins

• Best thought of as a spatial analysis method: combines layers to create new information, based on location
  • Number of schools in a neighborhood, for example
• Output is a new layer or feature class
• What type of spatial join we do depends on what types of features we’re working with

Next up: Tutorial 1!