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Articles

1978: Vol. LV, No. 2

Third Order Hydrostatic Levelling

Submitted
August 7, 2015
Published
2015-07-09

Abstract

The paper reports on the Australian Survey Office’s experience in using a hydrostatic levelling technique in the tidal mangrove swamps of Hinchinbrook Island, Queensland where conditions precluded the economical use of spirit levelling or trigonometrical heighting for proposed third order results. The technique employs repeated 300-metre bays with change points, along the banks of a watercourse. The watercourse is the means of transportation and provides the necessary temperature stability for the hydrostatic tube. The equipment used is cheap and readily available anywhere at short notice. Experiments performed in Brisbane to test accuracy are described. The Australian Institute of Marine Science is conducting an inshore productivity research programme in the tidal mangrove areas of northern Queensland. The initial and largest intensive study area (fig. 1) is at Missionary Bay, Hinchinbrook Island (north of Townsville) and it is here that much survey assistance is being provided by the Australian Survey Office. A basic mapping programme for the production of 1 : 5 000 orthophotomaps and contour sheets has been undertaken, and this included the establishment for two months of an automatic tide gauge to determine Mean Sea Level. Scientists have a need to understand tidal movements and depths through the mangrove system because biological productivity is presumed to be closely related to events associated with these water movements. To this end the Australian Survey Office has established a series of tide boards (fig. 2) spaced at one kilometre along two of the creeks which penetrate the mangroves, and has established bench marks throughout the area to act as level control points for the hydrographic contour survey of the mangrove system. Owing to the massive interwoven root system and dense canopy, photogrammetry was unable to provide the 0.25 metre contours required within the mangroves. Instead, high tide inundation of the area is being utilized for surveyors to wade or swim through the mangroves with waterproof watches, field books, compasses, tagged measuring rope and offset tapes (to measure “ soundings ” ). Photo identification and scaling from the orthophotomaps is used to control the terminals of these hydrographic mangrove traverses. Levelling on to a common datum of the tide boards and hydrographic survey bench marks (temporary tide board stations) presented a problem. Spirit levelling is impossible and trigonometric heighting was discarded in most cases, owing to inaccuracy and the difficulty and expense of setting up observing towers in the mud and above the canopy. Trigonometric heighting has been used as a check, where possible, and in a few shortdistance cases where a photo control point could be seen from a stable position on the creek/mangrove edge. The method of transfer of tidal datum by simultaneous tide watch was not employed, because of the large number of bench marks required and because of lack of knowledge of the Mean Sea Level (tidal) gradient. Hydrostatic levelling offered the only economic alternative. It could be carried out regardless of tide height ; the cost of equipment was low ; and expected accuracy was third order.