The SnakeGrid approach

The SnakeGrid approach

Along the whole length of the route, we aim to keep the maximum scale factor distortion less than 20 parts per million, no matter how the height changes, and this will hold for several kilometres on either side. In general, we achieve much better than this with rmse values of 5ppm or less not uncommon. This low level of distortion is generally less than the practical survey accuracy, and so in effect the grid can be considered as distortion-free.

We do this by developing a set of algorithms that projects the curved surface of the Earth in such a way that the line of true scale follows a generalised trend line through the project. We then do the same in height to follow the generalised profile of the route. A bespoke projection that winds it way along the route following the changes in elevation to minimise the distortion. The result is a smooth sinuous corridor where the scale factor is close to 1.

All the benefits of a low distortion coordinate system with no compromise on access.

The benefits

Seamless coordinate system

• Surveyors and engineers only have one set of control and one set of CRS parameters throughout the whole project
• No change in scale factors or grid zones
• Designers can designer freely across the whole project without coordination issues created by multiple zones or CRS systems

Zero distortion

• Design space matches the real world
• Designs are constructed as they were drawn
• Contractors workflows are simplified reducing risk
• Setting out procedures for engineers are made simpler

Access and software

• As easy to access as a standard projection with the benefits of low levels of distortion
• One system throughout the projects removes the decisions about which zone to work in
• Easy to transfer between different packages

Reduced risk and lower costs

• Simplified workflows between project phases and partners
• Lower levels of rework due to incorrect scale factors or grid zones being used
• Easily relatable to external data sets making decision making and design coordination simple

The impact of the HS2 NTv2 revision was significant. The coordinate system became immediately compatible with virtually all spatially enabled platforms, in particular CAD and GIS. The first benefit was allowing the reprojection of design data to and from the HS2 grid to be undertaken in-situ, which is especially useful for CAD files which will now retain complex geometry.

Perhaps the most significant benefit is the use of reprojection ‘on the fly’ which facilitates the single source of truth. Previously the same piece of geospatial information may have been stored in multiple data containers; one for each of the coordinate systems required, which resulted in significant issues with data conversion and data management. Now a single version of data requires maintenance, and when the geometrical representation is required in an alternate coordinate system the reprojection can be performed automatically within software.

More complex cases

Diverging routes

Quite often we are asked to cover minor routes alongside the principal route. Where these are within a few kilometres of the main route, it is usually possible to keep them on the same grid.

However, where they diverge significantly with the second route heading in a different direction, or maybe a series of radial routes from a city centre, we may need a different approach.

In these cases, we will likely need to design multiple SnakeGrid projections before merging them together at a common point. We then deliver a single set of parameters that selects the correct grid depending on the location.

To the end user it appears as just one coordinate system maintaining the low distortion and continuous philosophy that underlies /guides the SnakeGrid approach.

Extending grids

In the cases where projects are extended beyond the original scope it is also possible to apply the same methodology to extend an existing grid. A new grid can be designed and merged to the existing grid to create one continuous coordinate system. As before, we then deliver a single set of parameters that selects the correct grid depending on the location.

To the end user it appears as just one coordinate system maintaining the low distortion and continuous philosophy that underlies /guides the SnakeGrid approach.

Network approach

By utilising this approach entire national network have been covered (NI rail) with a limited number of SnakeGrids all accessed via one set of parameters. Working on the rail in NI, then you are on NIR09 grid, no questions asked.

On the mainland UK, even though the network is much larger / complex, it has been possible to break down national networks into regions which can be covered by single coordinate systems reducing the number of coordinate systems needing to be managed.

A note on datums

The SnakeGrid coordinates (E/N) inherit the datum of the geographic coordinates from which they are derived. Since these geographic coordinates are ultimately derived from GNSS surveys, the GRS80 ellipsoidal parameters are assumed.

In practice, this means that SnakeGrid can be used with any modern datum that is related to WGS84 or ITRF. In Europe this will probably be ETRF89, in the USA it may be NAD83.

To identify your project grid it is important to specify the datum along with the projection, for example SnakeGrid EWR3 on ETRS89/OS Net v.2009.