Datum Levels

Water levels and gate positions must be measured relative to a consistent fixed reference point or datum.

Reference Datum

The vertical datum (horizontal plane) beneath the gate from which all heights (levels) are given. For example the height of water or the height of the gate tip is given as a height above this datum. The reference datum is chosen to be a convenient height below the site datum - usually the factory default of 10 m. If a site has more than one gate, the reference datum for the site is determined from the master gate.

Site Datum

Sites use a local datum called the Site Datum that is located at the machined top surface of the short legged side of the FlumeGate, or the right side of the SlipMeter or SlipGate internal frame (that is, the side opposite the pedestal). The site datum is an easily identified point from which to take measurements of gate dimensions and level sensor datums. Water levels and gate positions are corrected to measure relative to an appropriate standard reference datum level.

Level Sensor Datum

Level sensors have their own internal datum from which they measure water level. The distance from the site datum to the level datums is given as a negative distance (because it is below the site datum). The level from the reference datum is calculated from this raw measurement. If a site has more than one gate, the level datums for each gate should be given with reference to the site reference datum. This means that the if the slave gate is 5 mm higher than the master gate, you would add 5 mm to the level sensor datum at the slave gate.

Note:

For integrated level sensors, the level sensor datum is fixed for the gate model and is determined by the location of the sensor assembly within the gate frame.
  1. Site datum
  2. Reference datum
  3. Level datum
  4. Distance from site datum to level datum (fixed for the gate)
  5. Distance from site datum to reference datum (variable but usually 10 m)

Site, reference and level datums