on December 27, 2022

Multiply Rasters with Scalar Numbers as Input in QGIS Modeler

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Scaling Rasters Like a Pro: Multiplying with Scalars in QGIS Modeler

So, you’re working with rasters in QGIS, right? Think of them as digital maps made of tiny little squares, each holding a piece of information. Now, sometimes you need to tweak those values – maybe convert units, apply a weight, or just generally scale things up or down. That’s where multiplying your raster by a single number – a scalar – comes in handy.

Sure, you could use the Raster Calculator for this. But let me tell you, if you’re doing anything more complex, or if you need to repeat the process, the QGIS Modeler is where it’s at. Trust me, it’ll save you a ton of time and headaches.

QGIS Modeler: Your Geoprocessing Powerhouse

Think of the Modeler as your visual recipe builder for GIS tasks. Instead of clicking through menus and running tools one by one, you visually connect them, creating a workflow. It’s like drawing a flowchart for your GIS analysis. Then, bam! You run the whole thing with a single click. I wish I had discovered this tool sooner.

Why Bother with the Modeler for Simple Multiplication?

Okay, I get it. Multiplying by a scalar sounds simple. But here’s the thing:

  • Automation is King: The Modeler lets you automate the entire process. No more repetitive typing!
  • Reusability for the Win: Create a model once, and reuse it on any similar dataset. Talk about efficiency!
  • Crystal Clear Workflows: The visual nature of the Modeler makes it easy to see what’s going on. Perfect for documentation and sharing.
  • Complex Workflows, No Sweat: Multiplication is just the beginning. You can easily integrate it into larger, more complex models.

Let’s Get Practical: Building Your Multiplication Model

Alright, let’s walk through how to actually do this:

  • Fire Up the Modeler: In QGIS, go to Processing > Graphical Modeler. A new window will pop up – that’s your canvas.
  • Bring in Your Raster: Drag a “Raster Layer” input onto the canvas. Give it a clear name, like “Input Raster,” and make sure to mark it as required. This tells the model that you need a raster to run.
  • Introduce Your Scalar: Now, drag a “Number” input onto the canvas. Name it something obvious, like “Scale Factor,” and choose the right data type (integer or float).
  • Summon the Raster Calculator: Head over to the “Algorithms” tab and find the “Raster calculator.” Drag it onto the canvas. This is where the magic happens.
  • Configure the Calculator: Double-click the “Raster calculator” to open its settings.
    • Under “Layers,” select your “Input Raster.” QGIS will refer to it by name and band number (e.g., “Input Raster@1”).
    • Now, the expression. This is where you tell the calculator what to do. Use the names you gave your inputs. For example: “Input_Raster@1” * Scale_Factor.
    • Finally, tell QGIS where to save the result. Pick a location and a format (GeoTIFF is usually a safe bet).
  • Connect the Dots: The Modeler should automatically connect the inputs to the Raster calculator. If not, just drag from the little gray circles to create the connections.
  • Save Your Masterpiece: Hit the “Save” button and give your model a descriptive name (e.g., “Multiply Raster by Scalar”).
  • Run the Show: Click “Run”! A window will appear, asking you for the input raster and the scale factor. Fill ’em in!
  • Behold the Output: Once the model finishes, the scaled raster will appear in your QGIS map.

    • Pro Tips and Things to Keep in Mind

    • Data Types Matter: Make sure your raster and scalar have compatible data types. If not, you might need to use a conversion function in the Raster Calculator expression.
    • NoData Awareness: Think about how you want to handle NoData values (those empty cells in your raster). Use conditional statements in the Raster Calculator to set them to a specific value after multiplication, if needed.
    • Extent and Resolution: By default, the output raster will match the input. But you can tweak these in the Raster Calculator settings if you need to.
    • Virtual Rasters for the Win: For huge rasters, try the “Create on-the-fly raster” option. This avoids writing a new file to disk, which can save a lot of time and space.

    Final Thoughts

    The QGIS Modeler is a game-changer for raster processing. Once you get the hang of it, you’ll be automating tasks left and right. So, give it a try! You might be surprised at how much time and effort it saves you. Happy mapping!

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