JobXML Reductions vs FieldBook: why your parser matters
Trimble JobXML has two coordinate sources. Parsing the wrong one double-applies corrections and silently shifts your CAD export.
The short answer
Trimble JobXML files contain coordinates in either Reductions (already corrected for prism, scale, and sea level) or FieldBook (raw observations with a Reduced flag). A parser that treats both the same will double-apply corrections, shifting your entire project by centimeters or more.
What Reductions contain
When a crew reduces observations in Trimble Access, the JobXML Reductions section stores coordinates that already include:
- Prism constant correction
- Station scale factor (combined scale factor at the instrument)
- Sea-level (elevation) reduction
These are not raw ground coordinates. They are grid-ready values for the localization the crew applied in the field.
What FieldBook contains
The FieldBook section holds raw observation records. Each observation may carry a Reduced attribute. When Reduced="true", the coordinate has already been processed. Treat it like a Reduction. When Reduced="false", you have a true raw observation.
The double-correction trap
If your workflow applies localization scale and rotation to coordinates that already include station scale factor and sea-level reduction, you apply those corrections twice. The error is proportional to your combined scale factor deviation, often 20–100 ppm, which is 2–10 cm over a 1 km traverse.
How FieldToCAD handles it
FieldToCAD branches on the data source: it reads Reductions when present (the common Trimble Access export path) and respects the Reduced flag in FieldBook. It never re-applies prism, scale, or elevation corrections to already-reduced values.
What surveyors should verify
Before exporting to Civil 3D, confirm which section your JXL export contains and that your office workflow matches the field reduction. FieldToCAD's visual validation map catches gross localization errors, and parser correctness catches the subtle double-correction trap.