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?
By Daniel Slater, Sales Director, emapsite, OS Partner
One of the typical bugbears that field data collectors face every day is ensuring they can capture the essential information they need in the most cost-effective way.
Time on site can rapidly mount up when you can’t find fast, accurate answers to your queries straight off.
If you work for, say, a housing association, utility company or other organisation with large property holdings, the practicalities of data collection can prove costly.
It was with this common concern in mind that we came together with our friends at Leica Geosystems on a journey to help.
As you may know, Leica Geosystems make really powerful field control devices that are used all over the world. A prime example is the Zeno 20 handheld GNSS data collector. It’s a rugged, high-accuracy field device that everyone can use. It runs on an Android operating system and it’s designed to help users collect and manage GIS and asset data.
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.
Guest blog by Myrddyn Phillips, Hill Data & Mountain Surveys
Calf Top (SD 664 856) is a rather unassuming hill which is approximately 6km south of Sedbergh in the Yorkshire Dales. It rises above the deep cleft of Barbondale to its east and Dentdale to its north, and although not the highest hill in the area it is quite prominent above its surroundings.
However, it isn’t the hill’s prominence that is of interest, it is its height, and being a mountain surveyor those hills that are given a 609m spot height on Ordnance Survey maps are particularly interesting, as this height equates to just under 2,000ft, with 609.6m the metric equivalent of this all important imperial height. All important, as 2,000ft is generally regarded as the benchmark height in England and in Wales for when a hill is promoted to the dizzying ranks of a mountain.
If you were watching BBC Breakfast this morning, you may have seen their reporter, Graham Satchell, heading out with our Flying Unit and finding out how we survey Britain from the skies.
We’ve actually been using aerial photography to carry out our surveys for almost 100 years. Originally, this had the advantage of capturing information from areas that surveyors found hard to visit on foot. Today, it means we can keep on top of the data capture process – making continuous revisions of the whole nation’s landscape.
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.
Yesterday marked 80 years since the trig pillar was first used in the retriangulation of Great Britain on 18 April 1936. On that day, a group of surveyors gathered around a white concrete pillar in a field in Cold Ashby and began the retriangulation of Great Britain.
We’re celebrating by sharing the story of the humble trig pillar, still much loved by walkers today, and giving you the chance to join our celebrations with The Trig Pillar Trail Challenge. But what is the background to the trig pillar?
Talking about the 79th anniversary of the trig pillar this month has sparked a flood of reactions. Lots of our lovely followers on social media sent us pictures of them (or their dogs) with our trig pillars around Britain. Others expressed delight that they now know what those odd concrete pillars were for. Some wanted to adopt a trig pillar if we no longer used them (sorry, not something we offer). Still more people were amazed that we no longer use the vast majority of trig pillars and asked us what we use instead. The answer to that is OS Net.
Today’s guest blog is from Cristina Savian at Autodesk. If you were at last week’s GEO Business 2014 event, you may have seen our Acting Director General, Neil Ackroyd give his keynote and feature the image below, an infraworks model of Shrewsbury which was created using our new building height dataset. Cristina created the model and tells us how.
Customers using Ordnance Survey’s OS MasterMap Topography Layer can now access information on the heights of almost 20 million buildings across Great Britain with the alpha release of building height attributes. Released on 17 March, OS MasterMap Topography Layer – Building Height Attribute is a product enhancement to OS MasterMap Topography Layer, and available to licence holders at no additional cost.