Combine Raster Using Raster Calculator Arcmap

What is Combining Rasters using the Raster Calculator in ArcMap?

To combine raster using raster calculator arcmap is a fundamental process in Geographic Information Systems (GIS) known as Map Algebra. A raster is essentially a grid of cells (or pixels), where each cell has a value representing a specific characteristic, such as elevation, temperature, or land cover type. The Raster Calculator tool in ArcMap’s Spatial Analyst extension allows you to perform mathematical calculations on these rasters on a cell-by-cell basis.

This process is not about merging or stitching images together into a larger mosaic. Instead, it’s about creating a completely new raster layer where the value of each cell is derived from a mathematical or logical expression applied to the values of the input rasters. For example, you could add an elevation raster to a soil depth raster to create a new output raster that models a combined suitability index. To learn more about advanced data manipulation, you might explore our guide on {related_keywords}.

Raster Calculator Formula and Explanation

The syntax used in the Raster Calculator is a form of Map Algebra, which follows rules similar to standard mathematics and Python scripting. Expressions are built using raster layers (which must be enclosed in double quotes), operators, and numbers.

For a simple combination of two rasters, the formula is:

Output_Raster = "Raster1" [Operator] "Raster2"

For a conditional operation, the syntax is typically:

Output_Raster = Con("Raster1" > [Value], [Value_if_True], [Value_if_False])

The Con tool is a powerful way to {related_keywords} by reclassifying values based on a condition.

Key Components of a Raster Calculator Expression
Variable	Meaning	Unit (Auto-Inferred)	Typical Range
“Raster_Name.tif”	An input raster dataset. The name must be in double quotes.	Unitless in the expression; inherits the meaning of the raster’s cell values (e.g., meters, degrees Celsius).	N/A (Layer name)
+, -, *, /	Standard mathematical operators for addition, subtraction, etc.	Operator	N/A
&, \|, ~	Boolean operators (AND, OR, NOT) for logical operations.	Operator	N/A
Con()	A conditional function that tests a condition and returns a value based on whether it’s true or false.	Function	N/A
Numeric Value	A constant number used in the calculation (e.g., 100, 3.28).	Scalar (Unitless number)	Any real number

Practical Examples

Example 1: Simple Addition

Imagine you have a raster representing average rainfall in millimeters (`rainfall.tif`) and another representing irrigation water applied (`irrigation.tif`). You want to find the total water an area receives.

Inputs: `rainfall.tif`, `irrigation.tif`
Units: The cell values in both rasters represent millimeters.
Expression: "rainfall.tif" + "irrigation.tif"
Result: A new raster where each cell value is the sum of rainfall and irrigation for that specific location.

Example 2: Conditional Analysis for Site Selection

You need to find areas suitable for a certain crop. The criteria are: elevation below 500 meters and slope less than 10 degrees. You want to mark suitable areas with a ‘1’ and unsuitable areas with a ‘0’.

Inputs: `elevation.tif`, `slope.tif`
Units: Elevation in meters, slope in degrees.
Expression: Con(("elevation.tif" < 500) & ("slope.tif" < 10), 1, 0)
Result: A boolean raster showing suitable (1) and unsuitable (0) locations. Understanding how to {related_keywords} is key to this kind of analysis.

How to Use This Raster Calculator Expression Builder

Enter Raster Names: Type the names of your input rasters into the fields. Ensure they match the layer names in your ArcMap project exactly.
Select Operation: Choose the desired operation from the dropdown menu. If you select 'Con (Conditional)', specific fields for the conditional logic will appear.
Fill in Values: Provide the second raster name or a numeric value. For conditional statements, fill in the threshold, true, and false values.
Generate and Copy: Click "Generate Expression". The correct Map Algebra syntax will appear in the result box. Click "Copy Expression" to copy it to your clipboard.
Use in ArcMap: Open the Raster Calculator tool in ArcMap (Spatial Analyst > Map Algebra > Raster Calculator) and paste the copied expression into the expression box. Set your output raster location and run the tool.

Key Factors That Affect Raster Combination

Cell Size (Resolution): For accurate analysis, input rasters should ideally have the same cell size. ArcMap will typically resample one of the rasters if they differ, which can alter the data.
Spatial Extent: The analysis is usually performed on the area where the input rasters overlap. You can control this in ArcMap's environment settings.
Coordinate System: All input rasters must be in the same coordinate system to ensure that cells align correctly.
Data Type: Rasters can be integer or floating-point. Mixing types can affect the output; for example, dividing two integer rasters will result in a truncated integer raster unless you convert one to float first. This is crucial when trying to {related_keywords}.
NoData Values: Cells that have no data can impact calculations. By default, if any input cell for an operation is NoData, the output cell will also be NoData.
Operator Precedence: Complex expressions are not always executed left-to-right. Parentheses `()` are crucial to control the order of operations, just as in standard math.

Frequently Asked Questions (FAQ)

Q: Why are my raster names in double quotes?
A: The Raster Calculator requires raster layer names or full paths to be enclosed in double quotes to distinguish them from numbers or field names.
Q: What happens if my rasters have different cell sizes?
A: ArcMap will resample one of the rasters to match the other, based on your Geoprocessing Environment settings. This can affect the precision of your results. It's best practice to manually resample your rasters to a common resolution before performing map algebra.
Q: Can I use more than two rasters in one expression?
A: Yes. You can build complex expressions like ("raster1" + "raster2") / "raster3". Using parentheses is highly recommended to ensure the operations execute in the correct order.
Q: What does the "Con" tool do?
A: "Con" stands for Conditional. It performs an "if-then-else" evaluation for each cell. If a cell meets the specified condition, it gets one value; if not, it gets another. It's fundamental for reclassification and suitability modeling.
Q: My output is just 0s and 1s. Why?
A: You likely performed a logical operation, such as "elevation.tif" > 1000. Such expressions return a boolean raster where 1 represents 'True' and 0 represents 'False'.
Q: How are NoData values handled?
A: Typically, if a cell in any input raster is NoData, the corresponding output cell will also be NoData for that calculation. You can use tools like `IsNull` within a `Con` statement to replace NoData values if needed. For instance: `Con(IsNull("my_raster"), 0, "my_raster")`.
Q: What's the difference between this and merging rasters?
A: Merging (or mosaicking) stitches multiple rasters together to form a larger, single raster image. Combining with Raster Calculator creates a new raster based on mathematical operations between the cell values of overlapping rasters.
Q: Do I need a special license for this?
A: Yes, using the Raster Calculator tool requires the ArcGIS Spatial Analyst extension to be installed and enabled. It is a core part of advanced {related_keywords}.

Related Tools and Internal Resources

For more advanced spatial analysis, explore these resources:

Analyzing Spatial Patterns: Dive deeper into how data is distributed across your study area.
Working with Vector Data: Learn the fundamentals of points, lines, and polygons to complement your raster analysis.
Advanced Geoprocessing Models: Discover how to chain multiple tools together, including the Raster Calculator, in ModelBuilder to automate your workflows.

Raster Calculator Expression Builder for ArcMap

Explanation of Operation

Conceptual Value Distribution