Qgis Raster Calculator

An interactive tool to test and understand map algebra expressions without opening QGIS. Perfect for learning, debugging, and complex analysis.

Interactive Expression Simulator

Define pixel values for your symbolic raster bands below. Then, write a valid expression to see the calculated output for a single pixel. This simulates how the qgis raster calculator processes each pixel in your layer.

Calculated Output

0.60

Formula Evaluated:

( 0.8 – 0.2 ) / ( 0.8 + 0.2 )

What is the QGIS Raster Calculator?

The QGIS Raster Calculator is a core, powerful tool within the QGIS software that allows you to perform mathematical calculations on raster datasets on a pixel-by-pixel basis. Think of it as a calculator that doesn’t just work with single numbers, but operates on entire images (rasters) at once. For every pixel in the output raster, its value is determined by a formula you provide, which can reference the values of corresponding pixels in one or more input rasters. This functionality, often called map algebra, is fundamental for virtually all forms of spatial analysis. Whether you are a GIS analyst, environmental scientist, or urban planner, mastering the qgis raster calculator is essential for transforming raw data into meaningful insights.

Common uses include converting units (e.g., meters to feet for an elevation model), classifying data based on thresholds, or calculating complex spectral indices like the Normalized Difference Vegetation Index (NDVI) to assess vegetation health. The tool supports a wide range of mathematical operators, trigonometric functions, and conditional logic, making it incredibly versatile for data manipulation and analysis. For advanced workflows, you can explore advanced raster analysis techniques.

QGIS Raster Calculator Formula and Explanation

The syntax of the qgis raster calculator is straightforward. Raster layers are referenced by their name and band number, enclosed in double quotes, in the format "layer_name@band_number". For most single-band rasters like a Digital Elevation Model (DEM), you will use @1. You can then combine these raster references with mathematical operators to form an expression.

For instance, the classic formula for calculating NDVI, a measure of vegetation health, is:

( "NIR_Band@1" - "Red_Band@1" ) / ( "NIR_Band@1" + "Red_Band@1" )

In this formula, "NIR_Band@1" refers to the pixel value from the Near-Infrared band, and "Red_Band@1" is the value from the Red band. The calculator applies this exact formula to every single corresponding pixel pair in the input rasters to generate a new NDVI raster. This is a prime example of why the qgis raster calculator is such a crucial tool in remote sensing.

Common Operators and Syntax

This table shows common operators and functions used in the qgis raster calculator.

Element	Meaning	Unit (Auto-Inferred)	Example Expression
"raster@1"	Value of a pixel in band 1 of ‘raster’	Pixel Value (unitless number)	"elevation_dem@1" * 3.28084
+ - * /	Standard arithmetic operators	N/A	"band4@1" + "band5@1"
> >= < <= = !=	Comparison operators	Boolean (0 or 1)	"elevation@1" > 1000
AND, OR	Logical operators	Boolean (0 or 1)	("slope@1" > 15) AND ("aspect@1" = 90)
sin(), cos(), sqrt()	Mathematical functions	Varies	sqrt("rainfall@1")
if(condition, true_val, false_val)	Conditional statement (QGIS 3.22+)	Varies	if("dem@1" > 0, "dem@1", 0)

Practical Examples

Example 1: Calculating NDVI

The Normalized Difference Vegetation Index (NDVI) is a critical indicator for assessing vegetation health. It uses the principle that healthy vegetation reflects a lot of near-infrared (NIR) light and absorbs a lot of red light. Our calculator is pre-filled with this example.

• Inputs:
  • "raster1@1" (NIR Band): 0.8 (high reflectance, typical for healthy plants)
  • "raster2@1" (Red Band): 0.2 (low reflectance, as plants absorb it)
• Expression: ( "raster1@1" - "raster2@1" ) / ( "raster1@1" + "raster2@1" )
• Result: 0.6. This high positive value (typically ranging from -1 to +1) indicates dense, healthy vegetation.

Example 2: Creating an Elevation Mask

Imagine you want to isolate areas in your study region that are above a certain elevation, for example, 1000 meters. You can use a simple comparison in the qgis raster calculator to create a binary mask.

• Inputs:
  • "raster3@1" (Elevation DEM): Set this to 1500 meters in the calculator above.
• Expression: "raster3@1" > 1000
• Result: 1. The expression evaluates to 'true', which the calculator represents as 1. If you had set the elevation to 900, the result would be 0 ('false'). This creates a new raster where all pixels with an elevation above 1000m have a value of 1, and all others have a value of 0. Learn more with our qgis tutorials.

How to Use This QGIS Raster Calculator Simulator

Using this tool is a simple way to get comfortable with map algebra before applying it to large datasets.

1. Enter Pixel Values: Input numerical values for the symbolic rasters. These simulate the value of a single pixel at a specific location in your real raster layers. The units are abstract pixel values—they could represent reflectance, temperature, elevation, or any other continuous data.
2. Write Your Expression: In the text area, write the expression you want to test. Ensure you use the exact layer names provided ("raster1@1", "raster2@1", "raster3@1").
3. Calculate: Click the "Calculate Output Pixel" button. The tool will substitute your pixel values into the expression and compute the result.
4. Interpret Results: The primary result is shown in large green text. Below it, you'll see the exact formula that was evaluated. The bar chart provides a visual comparison of your inputs and the final output. If there's an error in your expression, a message will appear. This is a key feature for anyone learning spatial analysis techniques.

Key Factors That Affect QGIS Raster Calculator

The success and accuracy of your work with the qgis raster calculator depend on several factors:

• Data Type: The data type of your input rasters (e.g., integer vs. floating-point) can affect precision. A calculation involving division will almost always require a floating-point output format to retain decimal values.
• Cell Size and Alignment: For calculations involving multiple rasters, QGIS will resample data if the cell sizes or alignments don't match. This can introduce slight inaccuracies. It's best practice to ensure all input layers share the same grid system.
• NoData Values: Pixels with no data can propagate through calculations. If a pixel in any input raster is NoData, the corresponding output pixel will also be NoData for most operations.
• Expression Syntax: A tiny syntax error, like a missing parenthesis or quote, will cause the calculation to fail. This simulator is excellent for catching such errors early. Common errors include mistyping layer names or using single quotes instead of double quotes.
• QGIS Version: Functionality can change. For example, the user-friendly `if()` function was introduced in QGIS 3.22. Older versions require a more complex boolean-based approach for conditional logic.
• Coordinate Reference System (CRS): While the calculator operates on pixel values, the underlying spatial context is crucial. Performing analysis on layers with different projections without proper alignment can lead to meaningless results. It's a key part of all gis data processing.

Frequently Asked Questions (FAQ)

1. Why is my qgis raster calculator output empty or all black?

This is a common issue. It often happens if there's a syntax error in your formula, if you've renamed a layer in the Layers Panel but the calculator still expects the original filename, or if NoData values are propagating. Double-check your expression and ensure layer names are correct.

2. How do I handle units like meters or feet?

The raster calculator itself is unit-agnostic; it only sees pixel values. It's your responsibility to know the units. To convert an elevation raster from meters to feet, you would use the expression: "dem@1" * 3.28084. The numbers are just numbers; the meaning comes from your understanding of the data.

3. Can I use IF statements?

Yes, as of QGIS 3.22, you can use the `if(condition, value_if_true, value_if_false)` syntax. For older versions, you must use boolean logic, like `("my_raster@1" > 10) * 25 + ("my_raster@1" <= 10) * 50`. Our simulator supports both styles.

4. What does the `@1` in `"raster@1"` mean?

It refers to the band number of the raster. Most single-purpose rasters (like a DEM) have only one band. Multi-spectral imagery from satellites will have multiple bands, so you might use `"landsat_image@4"` for the red band and `"landsat_image@5"` for the NIR band, for example.

5. How do I reference a raster with spaces in its name?

You must use double quotes around the layer name, for example: "My Elevation Data@1". The calculator is very strict about this syntax.

6. Is the qgis raster calculator case-sensitive?

The layer names within the quotes are generally case-sensitive and must match what you see in the QGIS Layers panel. The operators and functions (like `AND`, `sqrt`) are not.

7. What's the difference between a floating-point and integer output?

An integer raster can only store whole numbers (e.g., 5, 10, 153), while a floating-point raster can store decimals (e.g., 0.6, 12.548). If your calculation involves division or results in fractional values (like NDVI), you must save the output as a float type (e.g., Float32) to avoid losing precision.

8. Can this calculator process entire files?

No, this web tool is a simulator that processes single-pixel values based on your inputs. Its purpose is to help you build and debug a valid qgis raster calculator expression that you can then copy and use on your full raster files within the QGIS desktop application.