Situation:
A drainage culvert is in place on the edge of the field.  Initial survey was conducted but invert level of culvert was not taken into account.

Question:
How do we get the culvert invert level?  Do we remove the antenna and take readings and bench mark antenna at same place as previous survey?

Answer:
Create a new Field and Survey in the Trimble FMX. Use the same master benchmark as previous survey and create another benchmark at the culvert invert.  We can then see the difference. You should use a steel or wooden post as a 'staff' to get the antenna up away from the ground. As long as the antenna is the same vertical distance ('staff' length) away from the Master benchmark and Benchmark when measuring these two points then we can work out the relative difference.