In the vertical dimension, engineering levels have been measured with respect to Ordnance Datum Newlyn (ODN), the national height reference. Consequently, the HS2 Snake Projection is the horizontal coordinate system used for project design and construction. In fact, the length of the project on the ground did not change – it is just that the British National Grid is a best-fit for the whole country which means the map distortion is far in excess of what is appropriate for precision engineering required on projects like HS2. Measuring the line first in British National Grid, and then measuring again in HS2 Snake Projection would reveal an apparent increase in length of 60 metres. To demonstrate the reasons for using the HS2 Snake Projection, imagine a line approximating the route from London to Birmingham. It features scale factor distortion of under 20 PPM throughout, which equates to a distance distortion less than 2 cm per km (a map of distortion at ground level is seen in Figure 2). The HS2 Snake Projection applies to both Phase One between London and the West Midlands and Phase 2a to Crewe. The basic issue solved by the Snake Projection: projecting the curved surface of the Earth onto a flat plane. It is typically used on long linear infrastructure works, and effectively operates as a true-scale site grid that is valid for the entire length of the route. When tailored to a specific project the Snake Projection delivers a seamless coordinate system. It is a relatively new innovation in planar engineering grids that was developed through a collaboration between Network Rail and University College London. An example is illustrated in Figure 1, where the scale factor represents the errors in distance measurements within the projected grid. One of the fundamental issues of survey engineering is how best to represent the curved surface of the Earth onto a flat plane without introducing distortion of some kind. HS2 has led knowledge sharing sessions with the wider industry, with the revised Snake Projection methodology being increasingly adopted on further infrastructure projects. The HS2 Survey Grid transformations enable highest accuracy engineering survey for the project design, construction and operation fully validated and authoritative for use, with significant benefits realised to date. To ensure the project achieves positional stability and thus remains unaffected by these changes, both now and into the future, a description is presented of the development of a new geodetic transformation called HS2TN15 and a new equipotential geoid model, HS2GM15, which are applicable respectively to plan and height coordinates. For example, the update caused a 13mm horizontal shift and 25-30mm vertical shift in engineering coordinates. In 2016, Ordnance Survey updated the geodetic coordinates of OS Net and the implications of these changes on HS2’s track design are potentially significant. This approach has several advantages including minimal scale factor and height distortion at track level with seamless continuity over the project.įor HS2, the zero-order survey control is defined by the Ordnance Survey OS Net™ network of Continually Operating GNSS Reference Stations (CORS). The project’s topographical survey, design and construction use an engineering coordinate system defined by the HS2 Snake Projection together with the HS2 Vertical Reference Frame. This paper provides a summary of innovative approaches developed on Snake Projection coordinate transformations that maximise data interoperability and compatibility within CAD, GIS and survey software and systems. The six-figure grid reference for the church is 817098. In the diagram above the four-figure grid reference for the square marked X is 8109. In this instance, the tourist information office is located at 476334. You now have a six figure grid reference.Write this number after the last two digits. Next, estimate how many tenths up the grid square your symbol lies.Write this number after the first two digits. Estimate or measure how many tenths across the grid square your symbol lies.First, find the four-figure grid reference but leave a space after the first two digits. The distance between one grid line and the next is divided into tenths. In this case you can imagine that each grid is divided into 100 tiny squares. Sometimes it is necessary to be even more accurate.
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