Levelling
BASICS OF LEVELLING
USES OF LEVELLING
In the context of tidal measurements, levelling is used for the following purposes:
• Referencing of Tide Gauges: To determine and check the vertical stability of the tide gauge bench mark (TGBM) with respect to reference points (benchmarks) in its immediate vicinity. In order to isolate any local movements, there should be at least three such benchmarks, and the levelling should be repeated on an annual or semi-annual basis.
• Connection to GPS Reference Points: To determine its regional stability and to separate sea level rise from vertical crustal motion, the TGBM should be connected via GPS to reference stations fixed in a global co-ordinate system. Generally speaking, the GPS antenna cannot be directly placed on the TGBM and a GPS reference point must be established a short distance away. This must be connected to the TGBM by levelling.
• Connection to National Levelling Network: Mean sea level is used to define vertical datums for national surveying and mapping - hence the TGBM must be connected to the national levelling network. Connection to the network will also allow all tide gauges to be connected to each other, providing information on spatial variations in mean sea level.
PRINCIPLE OF DIFFERENTIAL LEVELLING
Differential levelling provides a means of accurately measuring height differences between points some tens of metres apart. A level is set up on a tripod and levelled so that the line of sight is horizontal:
[pic]
A graduated staff is held vertically over the first point and a reading made of the intersection of the cross-hair with the image of the staff (backsight - b). The same (or an identical) staff is then held vertically over the second point and a further reading made (foresight - f). The difference between the two readings is the difference in height between the two points:
δh = b - f
If b is greater than f then δh is positive (i.e. there is a rise in
USES OF LEVELLING
In the context of tidal measurements, levelling is used for the following purposes:
• Referencing of Tide Gauges: To determine and check the vertical stability of the tide gauge bench mark (TGBM) with respect to reference points (benchmarks) in its immediate vicinity. In order to isolate any local movements, there should be at least three such benchmarks, and the levelling should be repeated on an annual or semi-annual basis.
• Connection to GPS Reference Points: To determine its regional stability and to separate sea level rise from vertical crustal motion, the TGBM should be connected via GPS to reference stations fixed in a global co-ordinate system. Generally speaking, the GPS antenna cannot be directly placed on the TGBM and a GPS reference point must be established a short distance away. This must be connected to the TGBM by levelling.
• Connection to National Levelling Network: Mean sea level is used to define vertical datums for national surveying and mapping - hence the TGBM must be connected to the national levelling network. Connection to the network will also allow all tide gauges to be connected to each other, providing information on spatial variations in mean sea level.
PRINCIPLE OF DIFFERENTIAL LEVELLING
Differential levelling provides a means of accurately measuring height differences between points some tens of metres apart. A level is set up on a tripod and levelled so that the line of sight is horizontal:
[pic]
A graduated staff is held vertically over the first point and a reading made of the intersection of the cross-hair with the image of the staff (backsight - b). The same (or an identical) staff is then held vertically over the second point and a further reading made (foresight - f). The difference between the two readings is the difference in height between the two points:
δh = b - f
If b is greater than f then δh is positive (i.e. there is a rise in