As a mapping specialist at The New York Botanical Garden and a professional land surveyor for the last 20 years, I am offering consulting services on land surveying to any garden that is mapping or planning to map their collections or garden. In the real world of surveying, often we need to manipulate the rod height so as to allow more flexibility for sighting objects. Branches and other obstructions frequently dictate creative ways to get a shot from a given setup.
Most rods have a sliding sleeve for changing the mirror height.
The burden of keeping track of the vertical offset falls to the rod person. After confirming the HI with the instrument operator, the rod person must keep track of the mirror height, keeping in mind that the goal is always to calculate the offset to return the value to the HI of the setup. Rod heights above are always 'minus' values and heights below are always 'plus' values. When giving a high rod, try to use an even offset for simplicity. Low branches frequently interfere with tree locations, so shooting just below the leaf line at the base of the plant means just inverting the rod, perhaps snug against the trunk.