ArcGIS Pro Raster Calculator Simulator
A hands-on guide on how to use the Raster Calculator tool with a live Map Algebra expression simulator.
Raster Calculator Simulator
Enter numerical values for the sample raster pixels below and write a Map Algebra expression to calculate a new value. Raster names must be enclosed in double quotes in the expression, e.g., "RasterA".
A sample numerical value for a pixel in the first raster.
A sample numerical value for a pixel in the second raster.
A sample numerical value for a pixel in the third raster.
Example:
("RasterA" + "RasterB") / 2 or Con("RasterC" == 21, 1, 0)
Values Chart
What is the ArcGIS Pro Raster Calculator?
The Raster Calculator is one of the most powerful and commonly used tools in ArcGIS Pro’s Spatial Analyst toolbox. It allows you to perform cell-by-cell mathematical calculations on one or more raster datasets. Think of it as a calculator that, instead of working on single numbers, operates on entire grids of numbers (pixels) simultaneously. The language used to create expressions is known as Map Algebra.
This tool is essential for anyone doing spatial analysis. You can use it to combine data from different layers, reclassify values, apply mathematical formulas like NDVI (Normalized Difference Vegetation Index), and implement conditional logic to identify areas that meet specific criteria. For example, you could use the Raster Calculator to find all locations that have an elevation above 1000 meters AND are within 500 meters of a river. Learning how to use the Raster Calculator is a fundamental skill for advanced GIS work.
Map Algebra Formula and Explanation
The Raster Calculator doesn’t have one single formula; instead, it provides an interface to build your own custom expressions using Map Algebra. An expression combines raster layers (which act as variables), numbers, and mathematical operators and functions.
Raster layer names in an expression must be enclosed in double quotes, like "Elevation". The calculation is then applied to every single cell in the raster(s). For a cell at a specific location (e.g., row 5, column 10), the tool takes the value from that cell in each input raster and performs the calculation to generate the output value for the same location.
This process is an example of a local operation in map algebra because the output value for each cell is determined only by the input values at that same cell location.
Common Operators and Functions
The Raster Calculator supports a wide range of operators and functions. Here are some of the most common ones:
| Operator / Function | Meaning | Example Expression |
|---|---|---|
+, -, *, / |
Basic arithmetic (Addition, Subtraction, Multiplication, Division) | "raster1" + "raster2" |
>, <, ==, != |
Logical comparison (Greater than, Less than, Equal to, Not equal to) | "Elevation" > 1500 |
& (And), | (Or), ~ (Not) |
Boolean logic to combine conditions | ("Slope" < 15) & ("Aspect" == 180) |
Con(condition, true_value, false_value) |
Conditional statement. If the condition is true, assign true_value; otherwise, assign false_value. |
Con("Landuse" == 42, 1, 0) |
Float(raster) |
Converts raster pixel type from integer to floating-point. Essential for division. | Float("Band4" - "Band3") / ("Band4" + "Band3") |
IsNull(raster) |
Checks for NoData cells. Returns 1 (true) for NoData cells and 0 (false) for valid cells. | Con(IsNull("DEM"), 0, "DEM") |
For more advanced analysis, explore tools for ArcGIS Pro spatial analysis.
Practical Examples
Here are two realistic examples demonstrating how to use the Raster Calculator for common GIS tasks.
Example 1: Calculating NDVI (Normalized Difference Vegetation Index)
NDVI is a common index used to measure vegetation health. It requires two raster bands from a satellite image: Red and Near-Infrared (NIR). The formula is: NDVI = (NIR - Red) / (NIR + Red).
- Inputs: A raster for the NIR band (
"NIR_Band") and a raster for the Red band ("Red_Band"). - Assumed Pixel Values: NIR = 180, Red = 30.
- Expression:
Float("NIR_Band" - "Red_Band") / ("NIR_Band" + "Red_Band") - Result: (180 - 30) / (180 + 30) = 150 / 210 ≈ 0.714. This high value indicates healthy vegetation. The
Float()function is crucial to ensure the division results in a decimal value, not an integer.
Example 2: Reclassifying Elevation with Conditional Logic
Imagine you need to create a raster that shows only areas at high altitudes. You want a new raster where all pixels with an elevation above 2500 meters are given a value of 1, and all others are given a value of 0.
- Input: An elevation raster (
"DEM"). - Assumed Pixel Value: A cell in the DEM has a value of 2800 meters.
- Expression:
Con("DEM" > 2500, 1, 0) - Result: Since 2800 is greater than 2500, the condition is true, and the output cell is assigned the value 1. If the input cell was 2000, the output would be 0. This creates a binary raster highlighting high-elevation zones. Learn more about Map Algebra basics to build more complex queries.
How to Use This Raster Calculator Simulator
- Enter Input Values: In the fields labeled "Raster 'A' Pixel Value", "Raster 'B' Pixel Value", and "Raster 'C' Pixel Value", enter any numbers. These simulate the values of a single pixel from three different raster layers at the same location.
- Write an Expression: In the "Raster Calculator Expression" text area, write a formula using
"RasterA","RasterB", and"RasterC"as your variables. Refer to the table of operators above for ideas. - Calculate: Click the "Calculate" button or simply type in the input boxes. The result of your expression will appear in the green "Calculation Result" box.
- Interpret the Result: The main result is shown prominently. The "Intermediate Values" section shows how the simulator substituted your numbers into the expression.
- Review the Chart: The bar chart provides a simple visual comparison of your input values and the final calculated output value.
- Reset: Click the "Reset" button to restore the calculator to its original default example.
Key Factors That Affect Raster Calculations
When you use the Raster Calculator in ArcGIS Pro, several factors can influence the result and performance. Understanding them is crucial for accurate analysis.
- Cell Size: For an operation involving multiple rasters, ArcGIS Pro will typically resample the rasters to a common (often the largest or "coarsest") cell size. This can lead to a loss of detail from higher-resolution rasters.
- Spatial Reference: Rasters must have the same coordinate system for their cells to align properly. If they differ, one must be reprojected, which can introduce minor inaccuracies.
- Processing Extent: The analysis extent defines the rectangular area for the output. It's usually the intersection (common area) or union (total area) of all input rasters. This choice affects the size of your output raster.
- NoData Values: Cells with no data can significantly impact results. For example, in an addition operation, if one of the input cells is NoData, the output cell will also be NoData. The
IsNullandConfunctions are vital for managing these. - Data Type (Integer vs. Float): Performing division with two integer rasters will result in a truncated integer output. To get a decimal result, you must convert at least one of the inputs to a floating-point type using the
Float()tool. - Expression Syntax: The order of operations matters. Use parentheses
()to explicitly control how the expression is evaluated, especially when mixing different operator types like arithmetic and boolean.
For automating complex workflows, consider a ModelBuilder tutorial.
Frequently Asked Questions (FAQ)
1. Why am I getting an error in my expression?
The most common errors are syntax-related. Check that all your raster names are enclosed in double quotes (e.g., "My Raster"), that functions have matching parentheses, and that you are using valid operators. This simulator will highlight the expression box in red if it detects a syntax error.
2. My division result is a whole number (0 or 1), but I expected a decimal. What's wrong?
You are likely performing integer division. If both rasters in your division have an integer pixel type, the result will be an integer. To fix this, convert one or both rasters to a floating-point type using the Float() function: Float("Raster1") / "Raster2".
3. How do I combine multiple conditions?
Use the boolean operators & (for AND) and | (for OR). For example, to find cells where slope is less than 10 degrees AND aspect is south-facing (e.g., 180 degrees), you would write: ("Slope" < 10) & ("Aspect" == 180). Always enclose each condition in parentheses.
4. What is the difference between a single equals sign = and a double equals sign ==?
In Map Algebra, the double equals sign == is a logical operator used to test for equality within an expression (e.g., Con("Landuse" == 41, 1, 0)). A single equals sign is used in other contexts, like in Python scripting, to assign a value to a variable and is not used for comparison inside the Raster Calculator tool itself.
5. Can I use values from a specific cell in the Raster Calculator?
No, the Raster Calculator is designed for global, cell-by-cell operations on entire rasters. It does not have a function to directly query the value of a specific x,y coordinate within the Map Algebra expression itself.
6. What does the `Con` tool do?
The Con tool is a conditional function. It evaluates a condition for every cell. If the condition is true for a cell, it assigns a specified value; if the condition is false, it assigns a different value. It is one of the most useful tools for reclassifying data. The syntax is Con(condition, value_if_true, value_if_false).
7. Why are my output cells all NoData?
This often happens when your input rasters do not spatially overlap, or if a NoData value is part of a mathematical operation. For example, `5 + NoData` results in `NoData`. Use the `IsNull` function within a `Con` statement to replace NoData values with a valid number (like 0) before performing calculations.
8. Does ArcGIS Pro need a special license to use the Raster Calculator?
Yes, the Raster Calculator tool requires either the Spatial Analyst or Image Analyst license extension to be active in ArcGIS Pro.