Yes, we have arrangements with many companies, including Topcon, Trimble, Leica, Atlas and Track Access Surveys. So if you’re interested in joining them, get in touch and we’ll take it from there.

The datum is defined by the original control survey.

Although a national mapping system may have been used in preliminary work such as route planning, the high accuracy control work for the project would normally be carried out using GNSS. This measures vectors in the WGS84 coordinate system, so what matters is the datum of the higher level control.

In the UK, for instance, it is probably the case that the starting point for the control was Ordnance Survey Active Stations, which are in the ETRS89 datum. So the coordinate system in use for Crossrail (for example) is XRail09, but more formally might be referred to as SnakeGrid XRail09 on the ETRS89 datum

Essentially, SnakeGrid can be used in association with any datum that uses the GRS80 ellipsoid or an equivalent – which really means any modern satellite-based datum such as ETRS89, NAD83, GDA94, etc.

It’s best just to contact us and ask, if you’ve not been able to find out from colleagues. Or if you’re not sure of the exact extents of an existing grid, or are trying to work out if it will be valid on a branch line or a neighbouring highway, for example, that was not part of the original design.

Give us as much information about the project as you can, as there are places where more than one SnakeGrid overlap, each covering a different extent. We’ll let you know if something already exists and can usually help to put you in touch with the people who originally commissioned it.

Most major manufacturers of GNSS equipment, such as Trimble, Topcon and Leica support the use of SnakeGrid, and several software companies also have the algorithms integrated into their packages. See the section on our partners for more information.

In most cases the third party software reads the SnakeGrid parameter file directly, but Leica equipment requires an intermediate step of preparing a separate parameter file in a different format. Contact your Leica supplier directly to ask about this.

All we’re doing with the seed points is defining a broad corridor, several kilometres wide, in which the scale factor distortion will be close to zero. It doesn’t really matter if this corridor moves a few metres one way or another – it will still encompass your project.

Similarly in height, we’re defining the level at which the scale factor is unity: above or below this by a few tens of metres the distortion will be very low.

Once the SnakeGrid has been defined then its definition is fixed, and its use will be consistent with whatever coordinate accuracy is required.

No.

With conventional coordinate systems there will be a scale factor corresponding to the projection, which is derived from an appropriate algorithm, and a scale factor corresponding to the effect of height on horizontal distance. With SnakeGrid, provided that you are within the zone of applicability, then the scale factor distortion from both these sources combined will be less than 20 parts per million, and so for conventional engineering survey accuracy it can be ignored.

No.

Give us the coordinates of each branch separately, and we’ll fit a grid that encompasses both branches. We need the original coordinates as we need to test that the distortion is within the acceptable limits where it will actually be used.

When a SnakeGrid is delivered it is accompanied by a report that describes where it can be used, what level of distortion can be expected, details of links to other grids, test values, and so on.

Nowadays we produce a map showing the level of distortion across the whole area of the project. We can also do this retrospectively for older grids. Just get in touch if you need one for a grid you’ve commissioned in the past.

See the details of how to commission a grid at SnakeGrid for your project.

In this case you need to tell us the name of the existing SnakeGrid that you want to merge into, with details of the point where you want the join to occur. This could be a set of coordinates, but it also helps to give a description – “at Claydon Junction”, for example. The new grid will have the same coordinates and orientation as the existing one at the junction, and the coordinates will generally agree within around 2 mm up to 2 km away, allowing through surveys and computations from one grid to the other.

You can only join at one point – you can’t tie into another grid at the far end, or come back into the first one at a different point.

Choose anything up to 16 characters long, no spaces. Provided that it doesn’t clash with an existing name you can have what you like.

Most people choose names that contain an acronym of the major locations on the route or that describe the project, or use the ELR, and give the year. Examples are XRail09 (for Crossrail), RBEPP12 (Reading, Bath, Exeter, Plymouth, Penzance), DCL_2015 (Didcot to Wolverhampton).

It helps to have something you can pronounce easily – you’ll get fed up referring to WDFJSA2014 after a while.

As indicated above, when you choose a name we make sure that it’s not been used before so that you are assured that each grid has a unique identifier. The exception to that is if you come across a SnakeGrid that has the prefix LOC, followed by the name. This is a SnakeGrid LOCAL, and therefore generated by a user and not by us – hence we can’t guarantee that the name is unique.