ArcGIS Python Field Difference Calculator

Generate Python expressions to calculate the difference between two fields in ArcGIS Pro & ArcMap.

Generated Python Expression

Use the following expression in the ArcGIS Field Calculator.

What is an ArcGIS Calculation of the Difference Between 2 Fields using Python?

Calculating the difference between two fields is a fundamental geoprocessing task in ArcGIS. It involves subtracting the values in one column (field) from another for each row (feature) in an attribute table. This operation is crucial for change detection, comparative analysis, and data normalization. For instance, a GIS analyst might calculate the difference between population fields from two different census years to analyze population growth or decline. Using the Python expression engine within the ArcGIS Field Calculator tool provides a simple yet powerful way to perform this task.

The primary tool for this is the Field Calculator in ArcGIS Pro or ArcMap, where you can select ‘Python 3’ as the expression type. The syntax is straightforward and allows for direct mathematical operations on your data fields. This calculator simplifies the process by generating the exact expression you need to copy and paste.

ArcGIS Python Formula for Field Subtraction

To find the difference between two fields in ArcGIS using a Python expression, you use a simple subtraction formula. The field names are enclosed in exclamation points ! to denote that they are variables representing the values from those fields for the current record.

This expression tells ArcGIS to take the value from FieldName1 and subtract the value from FieldName2 for every feature in the layer. To learn more, see the ArcGIS Basics Tutorial.

Variables Explained

The formula’s components are straightforward and directly map to the fields in your attribute table.

Description of variables for field subtraction.

Variable	Meaning	Unit	Typical Range
!FieldName1!	The minuend. This is the field containing the base value from which you are subtracting.	Numeric (Integer, Float, Double)	Dependent on data (e.g., population counts, monetary values, measurements).
!FieldName2!	The subtrahend. This is the field containing the value that is being subtracted.	Numeric (Integer, Float, Double)	Dependent on data. Must be the same general unit as FieldName1 for a meaningful result.

Practical Examples

Understanding through examples makes the concept clearer. Here are two common scenarios where you would calculate the difference between fields.

Example 1: Population Change Analysis

A demographer wants to calculate the population change in a set of counties between 2010 and 2020.

• Inputs:
  • Attribute table has fields: POP2020 and POP2010.
  • A new field named POP_CHANGE of type ‘Double’ is created to store the result.
• Expression:
• Result: The POP_CHANGE field is populated with the difference, showing which counties gained or lost population. A positive value indicates growth, while a negative value indicates a decline.

Example 2: Property Value Appreciation

A real estate analyst wants to determine the value appreciation of properties by subtracting the last sale price from the current assessed value. This is a common step in GIS analysis for real estate.

• Inputs:
  • Attribute table has fields: ASSESSED_VAL and LAST_SALE_PRICE.
  • A new field named APPRECIATION of type ‘Double’ is created.
• Expression:
• Result: The APPRECIATION field shows the change in property value. This can help identify areas with significant market growth.

How to Use This ArcGIS Difference Calculator

This tool is designed to quickly generate the correct Python expression. Follow these steps:

1. Enter First Field Name: In the first input box, type the name of the field you are subtracting from (e.g., POP2020).
2. Enter Second Field Name: In the second box, type the name of the field you are subtracting (e.g., POP2010).
3. Generate Expression: Click the “Generate Expression” button.
4. Copy and Paste: The correct Python expression will appear below. Click the “Copy” button to copy it to your clipboard.
5. Use in ArcGIS: In ArcGIS Pro or ArcMap, open the attribute table, create a new numeric field to store the result, right-click the new field’s header, and choose “Calculate Field”. In the tool window, ensure the “Expression Type” is set to “Python 3” and paste the copied expression into the expression box. Click “OK” to run the calculation.

Key Factors That Affect Field Calculations

When you perform an ArcGIS calculation of the difference between 2 fields using Python, several factors can influence the outcome. Understanding them is key to accurate analysis.

• Data Type: Both fields must be of a numeric data type (e.g., Short, Long, Float, Double). Attempting to subtract text or date fields without proper conversion will result in an error.
• Field Name Syntax: Field names in Python expressions are case-sensitive and must be enclosed in exclamation marks (!). If your field name has spaces or special characters, it’s best to use the field’s true name, which can be verified in the layer’s properties.
• Handling NULL Values: If either field in a given row contains a NULL (empty) value, the result of the calculation for that row will also be NULL. You may need to handle these cases by first calculating NULLs to zero or using a more advanced Python script with a code block.
• Expression Type: Ensure you have selected “Python 3” (or PYTHON_9.3 in older ArcMap versions) as the expression type in the Field Calculator. Using another type like Arcade or SQL will require different syntax.
• Selection and Filters: The Field Calculator will only operate on selected features if a selection is active. Similarly, if a definition query or filter is applied to the layer, the calculation will only affect the visible records.
• Integer vs. Float Division: While not directly related to subtraction, it’s a critical concept in Python field calculations. In older Python 2 versions, dividing two integers resulted in an integer (e.g., 5 / 2 = 2). In Python 3 (used by ArcGIS Pro), it results in a float (5 / 2 = 2.5). This awareness is crucial for more complex calculations involving division. You can learn more about advanced scripting in our Python scripting for ArcGIS guide.

Frequently Asked Questions (FAQ)

1. What happens if I subtract a text field?

You will get an error. The Field Calculator’s Python engine cannot perform mathematical operations on fields of type ‘Text’ or ‘String’. Both fields must be numeric.

2. How do I handle NULL values in my data?

A simple subtraction will result in NULL if any input is NULL. To treat NULLs as zero, you can use a more complex expression in the “Code Block” section of the Field Calculator. Explore our data management tips for more info.

3. Why are my field names wrapped in exclamation points `!`?

In the ArcGIS Field Calculator, the Python parser uses exclamation points to identify and access the values of a field for the current row being processed.

4. Is the expression case-sensitive?

Yes. The field names inside the exclamation points must exactly match the case of your actual field names in the attribute table (e.g., !POPULATION! is different from !population!).

5. Can I subtract more than two fields at once?

Absolutely. You can chain operations together, for example: !FieldA! - !FieldB! - !FieldC!.

6. What’s the difference between using this expression and using the Arcade language?

Both can achieve the same result. Python is a general-purpose programming language widely used in GIS. Arcade is a newer scripting language developed by Esri designed specifically for use across the ArcGIS platform. The syntax is different; Arcade would use $feature.FieldName1 - $feature.FieldName2. For simple subtraction, both are equally effective.

7. Does this calculator work for ArcGIS Pro and ArcMap?

Yes, this simple Python expression format is compatible with the Field Calculator in both ArcGIS Pro (using Python 3) and older versions of ArcMap (using Python 2.x).

8. How do I create the new field to store the results?

In the attribute table view, there is typically an “Add Field” button or option in the table’s menu. When creating it, ensure you select a numeric data type like ‘Float’ or ‘Double’ to accommodate decimal values. Check out some field operations tutorials for more details.

Related Tools and Internal Resources

For more advanced GIS tasks and learning, explore these other resources:

• GIS Buffer Calculator: A tool to help with buffer analysis parameters.
• Understanding Coordinate Systems: A foundational guide for any GIS professional.
• ArcGIS Basics Tutorial: Perfect for beginners looking to get started with ArcGIS.
• Advanced Python Scripting for ArcGIS: Take your automation skills to the next level with ArcPy.