Converting Text Attribute Values to Numbers in QGIS: A Practical Guide

Working with geospatial data in QGIS often involves dealing with various data types, including text and numerical values. However, sometimes the data we receive isn’t quite in the format we need, such as having numerical data stored as text.

This can limit our ability to perform calculations, create visualizations, or run geoprocessing tools that require numerical input. In this blog post, we’ll explore how to convert text to numbers in QGIS, discuss common reasons for conversion failures, and provide practical solutions to ensure a successful conversion process.

Our conversation will cover a step-by-step guide on using QGIS’s Field Calculator to create new numerical fields or update existing ones with converted values. We’ll also delve into some of the pitfalls you may encounter during the conversion process, including non-numeric characters, extra spaces, inconsistent decimal separators, and data type limitations, among others.

By understanding these potential issues and learning how to preprocess and convert your data effectively, you’ll be better equipped to handle a wide range of geospatial data challenges in QGIS

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How to change a layer’s attribute value from text to number in QGIS

To change a layer’s attribute value from text to number in QGIS, you can use the Field Calculator to create a new field or update an existing field with the appropriate numerical data type.

Here’s a step-by-step guide on how to do this:

1. Open QGIS and load your desired vector layer.
2. Open the Attribute Table by right-clicking on the layer in the Layers panel, and then selecting “Open Attribute Table.”
3. Click the “Open Field Calculator” button, which looks like a small calculator icon, in the toolbar at the top of the Attribute Table.
4. In the Field Calculator dialog, you can choose to either create a new field or update an existing field.
5. a. If creating a new field:
   • Check the box next to “Create a new field.”Enter a name for the new field in the “Output field name” textbox.Select the desired numerical data type from the “Output field type” dropdown (e.g., Integer, Decimal, or Real).Set the “Output field length” and “Output field precision” as needed.
   b. If updating an existing field:
   • Uncheck the box next to “Create a new field.”
   • Select the field you want to update from the “Update existing field” dropdown.
   • Ensure that the existing field’s data type is set to a numerical type. If it’s not, you will need to create a new field instead.
6. In the “Expression” box, construct an expression to convert the text values to numerical values. If the text values are simple numbers stored as text, you can use the to_int or to_real functions to convert them. For example, if your text field is called “text_field”:
7. For Integer conversion: to_int("text_field")
8. For Real (float) conversion: to_real("text_field")
9. If the text values need further processing before conversion, you can use additional functions and operators in the expression to modify the text as required.
10. After entering the expression, you can check the “Preview” box at the bottom of the Field Calculator dialog to ensure that the conversion is working as expected.
11. Click “OK” to apply the changes. If you created a new field, it will be added to the attribute table with the converted numerical values. If you updated an existing field, the text values in that field will be replaced with the converted numerical values.
12. Finally, save the changes to the layer by clicking the “Save Edits” button (a floppy disk icon) in the toolbar at the top of the Attribute Table.

Keep in mind that if the text values cannot be directly converted to numbers, the conversion functions will return NULL values. Ensure that your text values can be correctly converted to the desired numerical data type before performing the conversion.

Troubleshooting text to number conversions in QGIS

There are several common reasons that text-to-number conversions might fail in QGIS. Some of these include:

1. Non-numeric characters: If the text values contain non-numeric characters, such as letters or special symbols, the conversion functions (e.g., to_int() or to_real()) may fail or return NULL values.
2. Extra spaces or leading/trailing whitespace: Whitespace characters can cause conversion issues, as they may not be interpreted as part of the number. You can use the trim() function to remove leading and trailing whitespace before converting the text.
3. Decimal separators: Inconsistent use of decimal separators (e.g., a mix of commas and periods) can cause conversion issues. You can use the replace() function to ensure a consistent decimal separator (usually a period) is used before conversion.
4. Thousands separators: The presence of thousands separators (e.g., commas) can cause conversion issues. You can use the replace() function to remove them before conversion.
5. Locale-specific number formatting: Some locales use different formatting for numbers, such as swapping the roles of commas and periods. You may need to adjust the text values to match the expected formatting before conversion.
6. Invalid field type: If you’re updating an existing field rather than creating a new one, ensure that the existing field’s data type is set to a numerical type. If it’s not, you will need to create a new field instead.
7. Complex expressions or calculations: If the conversion process involves complex expressions or calculations, errors in the expression syntax or logic can lead to conversion failures. Double-check your expressions for any mistakes, and use the “Preview” feature in the Field Calculator to verify the expected output.
8. NULL or empty values: If the text field contains NULL or empty values, the conversion functions may return NULL values. You can use the coalesce() function to handle NULL values or the if() function to handle both NULL and empty values during conversion.
9. Data type limits: When converting to integer or real (float) types, ensure that the values fall within the acceptable range for the chosen data type. For example, an integer data type cannot store fractional values, and exceeding the range of valid values for a data type can result in incorrect or NULL values.

To mitigate these issues, carefully inspect your text values before conversion, and use appropriate functions and expressions to preprocess the data as needed. By addressing these common reasons for conversion failures, you can increase the likelihood of a successful text-to-number conversion in QGIS.