Nadir
Nadir turns up in 2 fields: hydrography, farming. Here is what it means to the people in each one.
Nadir in hydrography
The point on the seabed directly beneath the sonar, where the beam looks straight down.
The vertical direction below the transducer and the seabed spot it hits. In multibeam data the nadir beams return a strong, mirror-like echo and carry the least refraction and roll error, so they anchor quality checks such as crossline comparisons. In side-scan data nadir is the blind zone beneath the towfish, where the sonar records only water until the first seabed echo arrives.
"Compare the crosslines at nadir only, the outer beams are smiling."
Nadir in farming
An empty hive box placed underneath an existing brood chamber to give the bee colony more space.
An older beekeeping practice involves adding an empty box, called a nadir, to the bottom of a hive rather than the top. This provides the colony with additional space to expand the brood nest or store honey downwards, following the natural building behavior of bees in a hollow tree.
"If that colony keeps growing at this rate, we'll need to slip a nadir under them to prevent a swarm."