Tanzania 2004
 

 

David’s entry for the Annual Young Navigators’ Competition run by The Royal Institute of Navigation won him a day out at RAF Cranwell, including a flight.

BSES Expeditions

Navigation in Africa:  from the Low-tech to the Ultra-modern

 Navigation covers a wide range of techniques for finding the way to where you are going. During an expedition to Africa during the summer of 2004, I encountered a wide variety of these methods; I also took part in an investigation into the accuracy of one navigation system: GPS. We were in Northern Tanzania, which boasts a diverse range of surroundings, from the vast open savannah of the Serengeti to the semi-tropical jungle in the Crater Highlands. We viewed one of nature’s most spectacular displays: glowing lava spouting from the last active volcano in the region, Ol Doinyo Lengai. 

 I was one of thirty-six young people on the expedition, which was organized by the British Schools Exploring Society, BSES. We were accompanied by nine voluntary leaders, and local support staff from our tour operators, Gane & Marshall. We undertook a range of scientific and environmental project work in the area including a gravity profile traversing a section of East African Rift Valley; a survey of the northern crater of Ol Doinyo Lengai, and an ecological investigation comparing flora and fauna on the active volcano an extinct one, Kerimasi.

 The basis for any navigation is a journey. During our journey we learnt a lot about our surroundings and about the culture of the local people, the Maasai, so absolutely alien from our own. The south of Kenya and the north of Tanzania together make up the homeland of the Maasai people. Some of our local staff had been born into Maasai tribes and had subsequently moved to the city; we were accompanied by Maasai guards, armed with spears and machetes to ward off dangerous animals. As they grow up, Maasai undertake rites of passage to mark the transition to a new status. Boys become warriors around the age of fourteen; the rite in this case is circumcision. Warriors let their hair grow long until they become elders later in life, at which point they cut it off. 

 With their knowledge of the land, passed down through the generations, the Maasai were our natural guides. Prior experience is the oldest and most reliable form of navigation: getting from A to B is easiest if you have done it before. It is not at all technical, but learning one’s local routes is nevertheless a skill. In remote Africa the routes were not well traveled, so the past experience of our guides was sometimes essential to our reaching our destination. Mount Longido, for example, is covered in patches of dense jungle; the paths are not mapped so to reach the summit we relied on our local guide. In places the path was swallowed up by the rain forest, at which point our guide alone knew which bit of greenery to hack through to find it again - in this situation, neither map nor GPS could help you to navigate.

 Without a doubt, the landscape in Tanzania was the most spectacular I have ever experienced. The Great Rift Valley is renowned for its extreme topography, and justly so. I had read about the steep escarpment at its side, about the panoramic views across hundreds of miles of open savannah, but seeing it for myself was truly awe inspiring. The geological activity in East Africa has resulted in geography totally unlike the UK. The mountains are mostly extinct volcanoes, making them in general cone-shaped, with craters at the summits where magma chambers have emptied and caved in. Some volcanoes imploded completely leaving sunken craters, including the incredible Ngorongoro crater 12km across. On others, water pouring down the mountainside in the rainy season has cut narrow ravines 15m deep into the lower slopes. And where mountains are absent, the plains are perfectly flat - the horizon is so far away that sometimes it is obscured by haze.

 This kind of environment leads naturally to another method of navigation: sight. Flat land means you can often see your destination. In Great Britain we have so many hills that this is an impractical method of navigation for long distances. However, while crossing a valley or climbing a hill you may be able to see a target you can aim for on the horizon. Cairns provide visual markings of a path that may otherwise be obscured by snow or fog. In Africa navigation by sight can be practical. Navigating from Lake Natron to Mount Lengai without a map was easy; the mountain was there in front of us. Unfortunately, the landscape in Africa exists on a totally different magnitude to what we were used to, and there were no buildings around to give a perception of scale. This lead to a bizarre situation where we simply could not judge distances by sight, so what looked like ten miles on the ground was actually twenty-five! It put it in perspective when we realised that a tiny dot on the side of the mountain was a tree.

 Although we could find our way to our destination with guides and visual markers, for our science projects we needed to pinpoint our precise location and altitude. This was essential for the Gravity Survey project: we were using a spring gravity meter that could measure the value of “little g,” the gravitational pull of the Earth to the seventh decimal place. In maths and physics, g is usually taken to be 9.8; however the true value varies slightly according to your distance from the centre of the Earth (i.e. altitude), and the density of rocks beneath your feet. We were measuring the value of g while traversing the East African Rift Valley, with the aim of finding out whether the Earth’s crust gets thinner near the boundary of the tectonic plates. In addition to knowing the coordinates of where our gravity measurements were taken, it was essential to get a precise and accurate measurement of altitude because, due to the “inverse square law” discovered by Newton, altitude has a greater impact on g than underlying geology. Thus, for each of our gravity measurements we needed to make an altitude correction. This is where the Global Positioning System comes in.

 Handheld GPS receivers are now commonplace navigation aids. These are just one application of the US satellite-based system that can be used to find your location anywhere on Earth. Between 1978 and 1994 the USA launched the 24 satellites that make up the GPS network; wherever you are on the Earth’s surface there are at least 6 satellites in range of your receiver. The mathematics and physics that calculate your position from these satellites are complicated, but they work by emitting high frequency radio signals with positional information encoded. The receiver decodes this information from at least three satellites to triangulate your position. In theory this is a flawless system of navigation; in practice it has several important weaknesses. Since the receiver is an electronic device, there is significant risk of a fault, or simply of running out of batteries; therefore GPS should never be your sole means of navigation.

 For the Gravity Profile we needed to use GPS at its most accurate. We used surveying GPS receivers (Leica GPS SR530), set up on tripods, even using a correction for the height of the antenna. One source of error in the GPS is changing atmospheric conditions, so to get a highly accurate reading in two positions we left the receivers on for twenty-four hours. This allowed the system to compensate for slight differences in signals at day and night. After this we knew the coordinates of these locations to within a matter of centimetres. We were then able to take simultaneous GPS readings in our known locations and in the field, with a second tripod based receiver. We used these readings to work out the position of the receiver in the field relative to the position we know exactly; thus we could work out the location in the field to within a matter of centimetres. This incredibly precise use of the Global Positioning System was an integral part of the Gravity Survey; an example of how important navigation is in the field of geology.

 Taking an absolute measurement of location and then using the relative readings to work out exact location precisely was a time consuming business. Now, though, we can compare the readings given by the surveying GPS receiver in the field with the corrected readings to get an idea of the error on the instruments. We also took readings off handheld eTrex units at each point to look at the accuracy of these. I have not subjected the results to rigorous statistical analysis, however I can confirm that both systems performed reassuringly well. The Leica surveying system was considerably more accurate than the eTrex, with the majority of readings having an error, horizontally, less than 3m and almost all the readings were within 5m of the true position. For the eTrex the majority were within 5m and almost all had an error less than 15m.

 These results are very promising for GPS navigation. Several sources I have looked at while background reading give much larger values for the possible error in reading, up to 100m. When you consider the many users of GPS, including the emergency services, search and rescue teams, and the military, it is comforting to know that they know where they are. Furthermore, with plans for forthcoming non-US satellite positioning systems, the future of global positioning looks bright. The European Union’s council of ministers has confirmed 2.1 billion funding to launch satellites for the Galileo navigation system between 2006 and 2008. When a second system comes online, satellite receivers will be able to compute signals from both satellite networks, giving even better accuracy. It is easy to imagine a world where satellite navigation becomes ubiquitous, with receivers built into every car and mobile phone as standard. And as navigation becomes ever easier, journeys become easier and the world becomes a smaller place.

 One weakness of handheld GPS was highlighted in Africa. The route we were walking involved going around Kitumbeine, an extinct volcano: the difficulty was in knowing how far we had walked. The GPS can very easily tell you the straight-line distance between your present position and your last GPS fix; after we had walked fifteen km round the mountain the GPS showed that we were five km from the camp we started at. It was necessary to put in several waypoints during the journey to get a better idea of the distance covered; however the value is still an underestimate. To find the true distance, we had to look at a map.

 Maps were probably the least used method of navigation on the expedition. In stark contrast, I have had meticulously planned routes on Ordnance Survey maps for every expedition I have been on in the UK. In Tanzania maps simply were not necessary. We were lead by a guide on most of the walks; in Britain this would have been too expensive. We could easily see where we were going across the open savannahs, quite unlike the hilly landscapes that characterise England. And we could pinpoint our position and direction with GPS units.

 The entire expedition experience was amazing. I learnt a lot about the world we live in, and the sights I have seen have given me a new perspective on it. My navigation skills were exercised and enhanced by the expedition, and now I see navigation as a broad spectrum of techniques rather than just setting off with a map and a compass. Finally, I would advise anyone considering such a venture to go for it; you only live once.

 

Sources

I have written most of this essay out of my background knowledge, which I attribute to the Scouting movement, and my experiences in Tanzania and elsewhere, which I attribute to the Duke of Edinburgh’s Award Scheme and the British School’s Exploring Society. My other sources are shown below. 

 

Books

Geography An Integrated Approach (third edition), by David Waugh, published by Nelson Thornes, 2002

The Macmillan Encyclopedia 2002 Edition

 

Articles

New Scientist, 11December 2004, p.21, “A Phone to Sniff Out Dirty Bombs,” by Jenny Hogan

New Scientist, 18 December 2004, p.5, “Galileo go-ahead”

Physics Education, July 1999, p.185, “‘Little g’ revisited: springs, satellites and bumpy seas,” by Roger Hipkin

 

People

Many thanks to:

Ewan Laws (BSES science leader), for teaching me about gravity anomalies.

Hugh Anderson (BSES science leader), for teaching me about Global Positioning Systems and for help writing about the expedition project.