four-figure grid reference, such as ‘19 45’, indicates a 1 km by 1 km square on the map and.There are two main types of grid reference: The numbers going across the map from left to right are called eastings, and go up in value eastwards, and the numbers going up the map from bottom to top are called northings, because they go up in a northward direction.
There are four main first letters: ‘S’, ‘T’, ‘N’ and ‘H’ covering Great Britain, plus an ‘O’ square covering a tiny part of North Yorkshire that is usually below tide.Ī unique National Grid reference should have this two-letter descriptor followed by the grid reference numbers within that square. The first letter, for example ‘S’, denotes 500 km by 500 km squares and this is subdivided into 25 squares that are 100 km by 100 km within it, making ‘ST’, ‘SU’, ‘SO’ and so on. The two-letter codes can be found printed in faint-blue capitals on Ordnance Survey maps and can also be found in the map key. Ordnance Survey divides Great Britain into 100 km by 100 km squares, each with a two-letter code. The National Gridīefore we look at what the grid reference numbers mean, it’s important to understand the wider picture of the National Grid. While the correct term for these is ‘National Grid reference’, these terms all mean the same thing however, as we’ll see, grid references can be given in a number of different formats. You may find a variety of terms used to describe National Grid references, such as ‘OS grid ref’, ‘grid reference’, ‘OS map ref’ or simply just ‘map reference’. This is a simple way of finding points and places on a map, to give to others as a meeting location or to quote if you get into trouble and need to specify exactly where you are.
A series of faint blue lines on every map makes up a numbered grid that is used to create the National Grid reference. National Grid references are used to accurately pinpoint your location on an Ordnance Survey map. Despite the approximations made in applying the method, when compared against official estimates of the datum transformation parameters, this relatively simple procedure resulted in estimates that appear acceptable in regard to combining data sets at a nominal scale of 1:50 000.How To Read Ordnance Survey National Grid References The resultant parameters are applied to road survey data in Universal Transverse Mercator (UTM) format for overlay with the SPOT image. The method uses a single geodetic control point, available 1:50 000 topographic maps and a SPOT satellite panchromatic image geo-referenced to GNG. In this paper a practical solution for deriving the required transformation parameters to convert from the World Geodetic System of 1984 (WGS84) to the GNG system is demonstrated. Consequently, integrating GNG topographic map data within a GIS with data derived from other sources can be problematic.
The Ghana National Grid (GNG) is an example of a mapping system that is not defined in image processing and GIS software and for which the transformation parameters are not readily available in the literature. The problem of incompatible projections and conversion between mapping systems is of general concern to those involved in the collection of natural resources data.