Update: DVD released on 24 July 2017. We have one copy of the DVD to give away. Just retweet this message by 12 noon on Friday 28 July: https://twitter.com/OrdnanceSurvey/status/889486114536476674
March 24 sees the UK film release of Lost City of Z. It chronicles the South American adventures of British explorer, cartographer and archaeologist Lt Colonel Percy Fawcett. I joined a panel discussion in London last week, along with historian Dan Snow and Lost City of Z author David Grann, discussing how Percy would have explored and mapped a new land. Catch up on the podcast here.
A member of the Royal Geographical Society (RGS), Percy Fawcett first arrived in South American in 1906 to survey and map an area of jungle lying on the Brazil and Bolivian border. The border between the two countries was not fully mapped and it was agreed that an RGS survey and map would be accepted as an impartial representation of the border. Today we would complete this activity using satellite systems and sophisticated surveying technology, which obviously wasn’t available back then. So, how would Percy and his team have gone about making maps?
Last month we had a story about hills growing into mountains and now we blog about the opposite situation…Recent press stories about what *might* happen if rising sea levels lead to a change in the datum value used for mean sea level on OS maps, has seen some people thinking that the heights of hills and mountains might be about to shrink. They are not!
We have to measure height in Britain against a commonly agreed datum level and ‘mean sea level’ is a common value chosen in many countries. It becomes the ‘zero height’ which all other heights are measured from. As land-based creatures, it’s natural for us to think of the sea as being zero height and anything above it as being ‘high’, and consequently anything below it as having a ‘depth’. So, it’s very common to see heights of hills and mountains quoted as being ‘above mean sea level’.
What exactly is mean sea level?
Seeing as an improvement in our model that transforms height from GPS to one above mean sea level has caused a hill to “grow” into a mountain – we thought it would be a good idea to explain how positions and heights surveyed by our surveyors with GPS make it onto our maps.
All positioning and surveying, not just that from GPS, has to take place on a mathematically simplified model of the surface of the Earth. The surface that the model attempts to emulate is called the geoid. The geoid is a complex concept, but can be imagined as a hypothetical surface that would be formed if the water in the oceans, close to mean sea level, continued under the land and was only influenced by Earth’s gravity field. This surface is one we already refer to without perhaps thinking about it – we say oceans have “depth” (below the arbitrary zero height surface) and mountains have “height” (above the surface). The geoid is a complex shape since it is influenced by varying Earth gravity. It is too complex to act as our surface for the calculations involved in positioning and surveying, so we need to fit a simple model shape to it.
Australia has recently announced a 1.8m shift in its mapping coordinates, to compensate for the country’s 7.5cm shift north each year. Inevitably the question is why, and could the same thing happen here?
In Australia, the shift is to take into account the growing difference between maps (and the coordinate reference system they’re based on) and the system used by satellite positioning (GPS). It’s a fact that the world is constantly shifting on tectonic plates, but maps (and their users) like fixed coordinates that don’t change. Before GPS, this was simple to achieve as most positioning and mapping was created from fixed ground points in a coordinate reference system tuned to a particular country. In Great Britain our fixed points included the very familiar trig pillars and we have a mapping coordinate reference system called OSGB36 National Grid which is fitted closely to our little bit of the Earth. Tectonic plate movements had little or no impact on the mapping coordinates or fixed points because they all moved “as one” and generally stayed the same shape.
A battle is raging on the other side of the Atlantic.
It’s a battle about use of the airwaves, or more precisely “radio spectrum,” by two rival American technologies.
In one corner is the well established Global Positioning System (GPS), the enabler of location services the world over. Facing up in the opposite corner is a fourth generation (4G) mobile broadband network from a company called LightSquared.
LightSquared is currently seeking a Federal Communication Commission (FCC) licence to operate its network and proposes to ultimately install around 40,000 transmitting beacons across the USA. The network will also be supported by a satellite that is already in orbit.
The nub of the problem is that the band of radio spectrum allocated to the LightSquared network is adjacent to the band allocated to GPS. This causes interference to GPS because the LightSquared signals are much more powerful and therefore swamp out the weak GPS signals. The interference problem has been confirmed by two independent tests.
This is a major problem.
GPS and other satellite systems such as the Russian GLONASS and upcoming European Galileo are now heavily relied upon, not only for positioning (we use the service day-in day-out to help keep Great Britain’s mastermap up-to-date) but increasingly for providing accurate timing, for the mobile ‘phone networks for example.
They are also starting to be used as part of “Safety of Life” systems such as aircraft navigation and landing.
Both camps are lobbying the FCC and fighting their corners.
LightSquared have offered a change to the proposed network which involves using different frequencies and lower power at the base stations which they say limits the interference. However they are also arguing that GPS receivers should be forced to update so that they filter “out of band” signals such as LightSquared’s and also operate more closely in their allotted spectrum. It would obviously be a long an expensive process to implement this solution.
At the moment, the issue if confined to the United States, but commentators in Europe are also watching the debate with interest since the developing Galileo service uses some of the same frequencies as GPS.
We can expect this fight to rumble on for some time yet…
What is ‘space weather’? Well this generally means solar flares – or as you might have heard on the news recently – coronal mass ejections to give them their full title!
Solar flares are related to sunspot activity which tends to run in 11 year cycles. We’re now entering the period where sunspot activity is increasing to amaximum for the current cycle. On Tuesday there was a big flare – the biggest for 4 years – whilst tonight those of you in Scotland might even get to see the Northern Lights as a result, so keep your eyes on the skies!
So what’s all this got to do with mapping?