In 2006 I created a map in an older version of Arc Map. On that map I showed a polyline feature representing the longest flow path in a watershed. Using Arc Hydro, the plolyline is automatically placed in a feature dataset in a geodatabase. This automatically calculates the length. That mxd is long gone due to "file management". I'm recreating the map using v10.1 and the latest Arc Hydro extension. This is a simple process.
The old map has a length on the printed map of 4.92miles. The new map calculates the length of 6.1 miles! Visually, the lines are practically the same with only minor differences which could be explained. Nothing explains the 1.2 mile difference. Comparing to older measurements using manual methods from old USGS maps are in the length is 5.25 miles.
The projection should be the same as I always load our SDE base layers into the mxd first to make sure our local standard coordinate system are used.
There is a second line that represents the distance along the longest flow path from a point projected from the centroid. It's length is not that different from the same line from the 2006 work.
Any thoughts on why the distance would be so different now?