Using decimals in QGIS raster calculator

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

So, you’re diving into the QGIS Raster Calculator, huh? It’s a seriously powerful tool for tweaking raster layers pixel by pixel. Think of it as your digital scalpel for geospatial data. But here’s the thing: to really wield that power, you gotta get cozy with how it handles decimals. Trust me, it can get a little wonky if you don’t.

At its heart, the Raster Calculator lets you cook up new raster layers by throwing mathematical expressions at your existing ones. We’re talking adding, subtracting, multiplying, dividing – the whole shebang. But the secret sauce? Understanding data types.

Raster data comes in different flavors, like integers (whole numbers) and floating-point numbers (numbers with decimals). Imagine trying to pour a liter of water into a half-liter bottle – some of it’s gonna spill! Similarly, if your raster is an integer type, and you try to cram a decimal result in there, it’ll just chop off the decimal part. Not ideal.

That’s why, when you’re playing with decimals, you need to make sure your input rasters are floating-point types. If they’re not, QGIS has your back! Just use the Float() function in the Raster Calculator to convert them. It’s like magic, but with code.

QGIS gives you a couple of floating-point options: Float32 and Float64. Float32 is usually a good compromise between accuracy and file size. But if you’re dealing with super-sensitive data and need every last bit of precision, go for Float64. Just remember, bigger files!

Now, let’s get down to brass tacks. How do you actually use decimals in your calculations? Simple! Just type them in. Want to convert meters to feet? Multiply your elevation raster by 3.28: “elevation@1” * 3.28. Boom! Elevation in feet.

One little gotcha: if you’re using a constant, make sure it looks like a decimal. Use 1.0 instead of just 1. Otherwise, the Raster Calculator might get confused and treat it as an integer, potentially messing up your results.

Okay, so you’ve done your calculations, and now you’ve got a raster with a million decimal places. Not exactly user-friendly, right? You probably want to round those numbers off. Now, QGIS doesn’t have a direct round() function built into the Raster Calculator, which is a bit of a bummer. But don’t despair! There are workarounds.

My favorite trick involves the gdal_calc module, which you can find in the Processing Toolbox. It’s like a Swiss Army knife for raster manipulation. With gdal_calc, rounding is a breeze: round(A, 2) will round raster A to two decimal places. Easy peasy.

Alternatively, you can roll your own rounding function using a bit of mathematical trickery. Multiply by a power of 10, convert to an integer, then divide by the same power of 10. Sounds complicated, but it works! For example, to round to one decimal place: toInt(“raster@1” * 10) / 10.0.

Don’t forget to tell QGIS what kind of data to expect in the output raster! When you’re saving your results, make sure you choose a floating-point data type like Float32 or Float64. Otherwise, you’ll be back to square one, with those pesky decimals getting chopped off.

Here’s a word of warning: floating-point numbers aren’t perfect. They can’t represent every real number exactly, which can lead to tiny rounding errors. These errors can accumulate, especially if you’re doing a lot of calculations in a row.

To keep things accurate, use the right data types, be careful with long calculations, and round strategically. Also, QGIS sometimes estimates the min and max values of a raster using a sample. To get the real min and max, go to the Layer Styling panel and set the accuracy to “Actual (slower)”. It takes a bit longer, but it’s worth it for the accuracy.

The QGIS Raster Calculator is a fantastic tool. But sometimes, you might need something a little different. That’s where the SAGA Raster Calculator comes in. It’s another option in the Processing Toolbox, and it has some cool features that the QGIS calculator doesn’t, like the ability to use X and Y coordinates in your calculations.

So, there you have it: your guide to conquering decimals in the QGIS Raster Calculator. It might seem a bit daunting at first, but with a little practice, you’ll be a raster-wrangling pro in no time! Just remember to choose the right data types, round carefully, and watch out for those pesky rounding errors. Happy calculating!