In OptiSurface Designer V2.6, there's a new feature that allows you to export to Trimble Field Level .gps file from an OptiSurface Design file that has the original coordinate system as UTM. The UTM coordinate system is the default in importing data files with latitude, longitude, elevation including .ags file.
For more information about the differences between Trimble Field Level coordinate system and UTM coordinate system, see this article: Coordinate System Differences with Different GPS Systems
The figures below show the examples of the rotation and scaling difference between UTM coordinates system and Field Level coordinate system.
Figure 1. Example survey before correction there is a small rotation and scaling error.
Figure 2. Example survey after correction with rotation error fixed but small scaling error remaining.
The horizontal scaling error remaining is in the order of 0.1%. ie at 1000m from the Masterbenchmark the error is in the order of 1m.
Before relying on this your should do a check as explained in this article: GPS Coordinate System Check