Earthworks

The Kalahari occupies an exceedingly flat tableland taking up much of the central part of southern Africa.  When the national gravity survey of Botswana was mooted in 1970, absolute heights above seal level for this remote, dry and thinly-populated area were unavailable and precision primary levelling was still in progress.  Earlier levelling work in pursuit of potential engineering work on river courses (Du Toit, 1926; Brind, 1955) had proved inconclusive.  A by-product of the national gravity survey (Reeves & Hutchins, 1976) was the first rough topographic contour map of Botswana dervied from about 2500 heights determined by barometric altimeters tied to the few available reliable datum points.  The lowest point in the inland drainage basin in the Makgadikgadi salt flats was determined as 903 (+/- 5) metres.  Almost all of northern Botswana lay below 1000 m with the only escape for water from the annual flooding of the Okavango delta in the NW of the country at about 930 m, located near the administratrive centre of Maun.