Ordnance survey grids8/11/2023 The ellipsoid was originally defined by Sir George Airy in 1830. If you are in the UK, this smaller ellipsoid would be the Airy Ellipsoid, part of the OSGB36 datum. Hence, multiple ellipsoids and the need to convert between them. Therefore, if you happen to be on this small part of the earth, using this smaller ellipsoid would give greater accuracy. However, the smaller ellipsoid can be seen to be a better fit for a small part of the earth. The large ellipsoid whilst not fitting perfectly gives us a general ellipsoid that can be used all over the world, think of this as WGS84. WGS84 is a standard datum used as the default in most GPS systems, so why not simply use that? The figure below shows a rough image of the earth with all of its lumps and bumps. Why So Many Datumsīefore moving on to discuss coordinate systems, it is worth explaining why we use different ellipsoids, and therefore different datums. Therefore, being able to transform coordinates to and from different datums is essential, the examples shown later in the article will show how to do this in C# using the GeoUK package. It can be shown that if a Latitude and Longitude based on the OSGB36 datum were entered into a GPS using a WGS84 datum, it would point to a different place. This means that Latitude and Longitude can only describe a point on the earth if we know which datum is being used. In some cases, several datums can be in use at the same location depending upon specific requirements. The thing is, different ellipsoids, and therefore different datums, are often used depending upon where in the world you are. These definitions are referred to as a Datum. The Latitude and Longitude coordinate system can describe any point on earth, however, it can only do that if we have first defined the details of the earth such as the origins of the Latitude and Longitude system and the size and shape of the ellipsoid to be used. The earth is ellipsoidal, albeit with a few lumps and bumps. If you took at an inflated beach ball, placed your foot on it and gently pressed, you would have a squashed sphere, referred to as an ellipsoid. Ordnance Survey provides detailed documentation in relation to coordinate systems used in the UK. One thing I should mention is that this article is a greatly simplified discussion of a complex topic. Net 4.5+, Windows Phone 8+, Xamarin.IOS/Android, Windows 8 Store Apps and so on. GeoUK was built as a Portable Class Library project (Profile 78) and as such can be added to Visual Studio or Xamarin and runs against. The freely available GeoUK NuGet package is used for these examples. The article will walk through some C# examples, including how to convert from GPS coordinates to British National Grid and back again. There is much more information at the Ordnance Survey website.This article aims to demystify the concepts of transforming from one coordinate system to another. The national, "OS", grid reference for Dean Royd Farm may then be written as 394026 421179. Note: sometimes numbers are used instead of the two-letter grid code.Įxample: the datum point of SD has an easting of 300km and a northing of 400km from the OS "origin". SD 94026 21179 gives a square metre inside Dean Royd Farm yard, 94026m east and 21179m north of datum. SD 940 212 gives a 100m square containing Dean Royd Farm, 94.0km east and 21.2km north of the same datum. SD 94 21 gives a 1km square containing Dean Royd Farm, being 94km east and 21km north of the south-western corner of grid square SD. References have a degree of accuracy dependent on the number of digits used. To locate a feature within a square on the map, the first part of the reference is distance traveled in an easterly direction from the datum (called the "easting"), and the second part is the distance traveled in a northerly direction (the "northing"). Grid references have their datum, or "origin", at the South West corner of the square, usually this is the bottom left hand corner (when you have north at the top of the map). Every 100km square has a unique pair of letters - for example, Todmorden can be found within the area named SD. The UK Ordnance Survey (OS) divides the British Isles into 500km squares designated S,N,H,O,T and subdivides these into 100km squares, allocating a second letter to each.
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