BG Map allows the user to assign elevations for control points and to carry the datum to located objects, whether they are designated NPO's (Non Plant Objects) or not. The programming assumption for this calculation is that the instrument and rod are the same exact height, i.e. HI = ROD, as in illustration #1. If you work with elevations and the screen prompt asks for the vertical offset in this circumstance, then the correct value is 0.0 or the default <ENTER>.

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. In very difficult circumstances, where vertical accuracy is only needed to the nearest 0.5', I have attached the mirror to the top of a 25' fiberglass rod; however, most everyday work can be done with an 8.5' or 12.0' adjustable prism rod.

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.

The illustration #2 shows a mirror above the HI and the vertical offset is -1.2'.

When giving a high rod, try to use an even offset for simplicity. If the HI is 5.10' then try using 6.10', 7.10' or 8.10' heights so that the offset is whole foot (or half foot: 6.60', 7.60') intervals before using an arbitrary height which takes some mental calculation.

Illustration #3 shows an often-used trick. 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. Now you must add a 4.20' vertical offset to bring the rod height back up to the 5.10' HI.

Illustration #4 demonstrates the mirror below the target. Though shown as inverted, this arrangement can occur whether the rod is inverted or upright, as when for instance you have a target on a ledge well above your head. In any case you must add 5.10' and 1.30' for a 'plus' 6.40' vertical offset. Take some time to understand this in terms of always returning the mirror to the HI height above the target.