The discrepancy occurs because the Copy Parallel tool in ArcGIS Pro works in a projected coordinate system (planar) rather than using geodesic calculations. As a result, when dealing with large-scale data or working in coordinate systems prone to distortion (such as UTM), the actual geodesic distance may differ from the intended offset.
How to Ensure a True Geodesic Offset
1. Use the Geodesic Buffer Tool
Instead of the Copy Parallel tool, consider using the Buffer tool and setting the Method to Geodesic. This approach accounts for the Earth's curvature, ensuring that the offset distance remains accurate in geodesic terms.
2. Apply Geodetic Construction with COGO Tools
If your dataset includes COGO-enabled features, you can use geodetic construction tools to generate an exact geodesic offset. These tools are designed to maintain geodetic accuracy.
3. Reproject the Data to a Geodetic Coordinate System
If your data is currently in a projected coordinate system (PCS), try reprojecting it to a geodetic coordinate system, such as WGS 1984 (EPSG: 4326). Then, run the Copy Parallel tool and measure again to check for accuracy.
Why This Happens
The reason your 25m offset appears as ~20m when measured geodesically is due to the distortion effects of projected coordinate systems. Since the Copy Parallel tool does not use geodesic calculations, offsets in projected space may shrink or expand based on latitude and projection properties.
For precise geodesic offsets, it’s best to use the Geodesic Buffer tool or geodetic construction methods instead.
