Working with attributes in R (and QGIS)

These slides are not formatted and mainly consist of notes in their current form.

Using QGIS to work with attribute data

September 14 - QGIS notes

Where do people encouter expressions?

• Field calculator: Change the value of an existing or new field based on an expression

  • Interface includes:

    • Expression editor

    • Expression search/reference

    • Help reference pane

    • Preview/validation

  • Features

    • Autocompletion

    • Validation in preview section

    • Check boxes to create

      • a new field (equivalent to dplyr::mutate())

      • virtual field

    • Custom expression language modelled on SQL; most similar PostGIS

    • Functional language; Expression must be a single command of chained functions

    • Different types of values, but it is not strict about it, e.g. ‘4’ + 6 (more forgiving than R)

  • Watch out!

    • Trap: character literals must be wrapped in single quotes; double-quotes are reserved for field references

    • Trap: NULL values return NULL, e.g. 5 + NULL is NULL

  • Tips!

    • || is concatenate

    • Number fields are evaluated as expressions

    • @ references variables (variable are context dependent, e.g. scale)

    • Context-dependent variables have informative prefixes (e.g. map_... ) and only appear in widgets where they are available. Variable scopes include:

      • global

      • project (can override global variables)

      • map

      • layout or layout item

Expressions are first built for Field calculator but also appear as:

• Expression dialogue for custom labels

• Select by expression (very common use)

• Layer feature styling

• Tips on using expression builder with Layout

  • Enclose expressions in [% and %] to evaluate

Things to demonstrate

• View > New map view

• Use symbol_color variable to match label color to symbol color

September 21 - QGIS Notes

https://www.youtube.com/watch?v=a0cVLzbEOig

Stats Summary panel

• Starts with layer selection

• Nice for interactive exploration

• Option to limit stats to selected features

• Allows use of expressions in calculating stats

Kind of nice but... don’t use it

• Old school GIS technique

• Ad hoc data analysis and not reproducible

Instead...

Use processing toolbox

• Basic processing algorthims

  • Buffer

  • Centroid

  • Basic stats for fields

  • etc.

• Advanced/esoteric techniques

For exploratory analysis

• Processing

  • Basic stats for fields
  • Stats by category

• Notes

  • Not quite as interactive

  • Generates an HTML report

  • End up with a bunch of files (HTML and spatial data) in folder w/ no way to keep track of it

But still... don’t do that either.

Instead use:

• Processing modeler to create a graphical analysis

  • Allows someone else (including future you) to see analysis process

  • Option to save models inside a project

  • Models will appear within processing toolbox whenever you open the project file

• Data-defined layer styling

  • See equivalent to mapview w/ zcol

# test code
library(dplyr)