QGIS Version 3.6 Raster Calculator

Unleash the Power of QGIS 3.6 Raster Calculator: A Human’s Guide

So, you’re diving into the world of GIS and want to bend raster data to your will? The QGIS Raster Calculator is your trusty sidekick for pixel-based wizardry, letting you conjure up new raster layers with mathematical spells. Whether you’re a seasoned GIS guru or just getting your feet wet, mastering this tool in QGIS 3.6 can seriously level up your spatial analysis game. Let’s break it down, shall we?

Getting Cozy with the Raster Calculator Interface

The Raster Calculator’s interface is surprisingly friendly, like a well-organized workbench with all your tools within easy reach. To fire it up, just head to Raster > Raster Calculator in the QGIS main menu. The window that pops up is your command center, divided into a few key areas.

First, you’ve got your Raster bands list. Think of this as your inventory of raster layers loaded in your project. A simple double-click adds a layer to your expression, formatted as “layer_name@band_number”. For instance, “DEM@1” would grab the first band from a layer named “DEM.” Easy peasy!

Next up, the Operators section. This is where the magic happens! You’ll find all sorts of mathematical goodies like +, -, *, /, as well as trigonometric functions and logical operators. Just click an operator to add it to your expression.

The Raster calculator expression box is your canvas. This is where you’ll craft your calculation masterpiece. You can type directly or mix and match layers and operators from the lists.

Finally, the Result layer section is where you tell QGIS where to save your creation. You can specify the file path, format (GeoTIFF is a good choice), data type (Float32 and Int16 are common), and extent. Oh, and don’t forget to check the box to add the result to your project automatically!

Speaking the Language: Constructing Expressions

The Raster Calculator speaks a language that’s surprisingly easy to learn. Expressions are built from raster layers, operators, numbers, and a sprinkle of functions.

Basic Math

You can do all the basic arithmetic you’d expect: addition, subtraction, multiplication, and division. To add two raster layers, say “raster1” and “raster2,” the expression is simply “raster1@1” + “raster2@1”.

Conditional Logic

Now, things get interesting! Conditional statements let you perform calculations based on certain conditions. The classic syntax looks like this: (condition) * (value_if_true) + (NOT condition) * (value_if_false). Let’s say you want to flag all pixels above 50 in an “elevation” raster as 100, and the rest as 50. The expression would be “elevation@1” > 50) * 100 + (“elevation@1” <= 50) * 50.

Thankfully, newer versions of QGIS (3.22 and up) offer a much cleaner if function. The same logic becomes if(“elevation@1” > 50, 100, 50). Much easier on the eyes, right?

Function Power

The Raster Calculator boasts a bunch of built-in functions, from trigonometric (sin, cos, tan) to logarithmic (ln, log10) and even statistical (mean, max, min). Want the sine of a raster layer called “angle”? Just use sin(“angle@1”).

Taming NoData Values

NoData values are those pesky gaps in your raster data. Ignoring them can lead to calculation chaos! You can use conditional statements to wrangle them. To set all NoData values in a raster named “data” to -9999, try if(isnull(“data@1”), -9999, “data@1”).

Real-World Adventures: Practical Applications

The Raster Calculator is a Swiss Army knife for GIS tasks:

• Unit Conversion: Need to switch from meters to feet? No problem! “elevation@1” * 3.28 does the trick.
• Raster Masking: Want to hide certain areas? To mask out everything below 0 meters elevation, use (“elevation@1” >= 0) * “elevation@1”.
• Image Classification: Categorize your raster data based on pixel values. Classifying an elevation raster into two classes (1 for < 50, 2 for >= 50) is as easy as (“elevation@1” < 50) * 1 + ("elevation@1" >= 50) * 2.
• Index Calculation: Calculate indices like NDVI using band combinations. For example, (“NIR@1” – “Red@1”) / (“NIR@1” + “Red@1”).
• Terrain Analysis: While dedicated tools are great, you can even use the Raster Calculator for custom terrain analysis like slope and aspect.
• Change Detection: Spot the differences between rasters from different time periods by subtracting them.

Pro Tips and Troubleshooting

To keep your raster calculations smooth and accurate, here’s some friendly advice:

• Data Prep is Key: Make sure your input rasters play nice together. Same CRS, resolution, and extent are a must! Reproject and resample if needed.
• Mind Your Data Types: Choose the right data type for your output. If you’re expecting decimals, don’t use an integer type!
• NoData? No Problem!: Always handle NoData values explicitly to avoid weird results. The isnull() function is your friend.
• Double-Check Your Work: Before hitting “run,” give your expression a good once-over. QGIS will usually flag syntax errors.
• Memory Matters: Raster calculations can be resource-intensive. If you run into memory issues, try working with smaller tiles or boosting your memory.
• Name It to Win It: Use descriptive names for your output layers. Trust me, future you will thank you!
• Inspect Your Results: After calculating, take a peek at the results to make sure everything looks right. The identify tool is perfect for checking pixel values.

Final Thoughts

The QGIS 3.6 Raster Calculator is a powerhouse for anyone working with raster data. By understanding its ins and outs, you can unlock a world of spatial analysis possibilities. Follow these tips, and you’ll be well on your way to becoming a raster calculation pro!