How to Define Projection in ArcGIS Pro

Step 1: Start a New Map Document

To begin the process of defining projection, it’s essential to start with a clean map document. This helps ensure that your data is organized and manageable. Open ArcGIS Pro and create a new map document.

Step 2: Add Your Data

Add your dataset to the new map. For this example, we will use a file named “Rhode Island,” which is missing projection information. When you attempt to add it, you may encounter an error stating that the data source is missing coordinate system information.

Step 3: Identify the Issue

Upon adding the dataset, notice the warning at the top of the screen indicating an “Unknown coordinate system.” This means that the data can be drawn, but it might be misaligned. The system often places data without coordinate information at the intersection of the equator and the prime meridian, commonly referred to as Null Island.

Step 4: Use the Define Projection Tool

To rectify the issue, utilize the Define Projection tool. Right-click on the file, navigate to the spatial reference settings, and confirm that the coordinate system is indeed unknown. The Define Projection tool allows you to assign the correct coordinate system to your data.

Step 5: Input the Correct Coordinate System

In the Define Projection tool, you’ll need to specify the input dataset (the Rhode Island file). The tool will automatically populate the existing coordinate system, which is unknown. You need to determine the correct coordinate system for Rhode Island, which is the Rhode Island State Plane at eighty-three feet (NAD 83 State Plane).

Step 6: Analyze the Data Extent

If you’re unsure of the correct projection, examining the extent of the data can offer clues. For example, if the coordinate values appear excessively large, they are likely in a projected coordinate system, not degrees.

Step 7: Apply the Correct Projection

Once you have confirmed the correct coordinate system, run the Define Projection tool. This action will relocate the Rhode Island dataset from Null Island to its proper geographical location.

Conclusion

In this tutorial, we learned how to use the Define Projection tool in ArcGIS Pro to correct the projection of a dataset with missing information. Remember to only use this tool on files that have an unknown or incorrect projection to ensure accurate geographic data representation.