The next stage will be to send us a coordinate description of the route of the project. This is referred to as a set of “seed points” and simply means selected points along the route, spaced at about 0.5 – 2 kilometres, with three dimensional coordinates (latitude, longitude and height) in the WGS84 system or one of its equivalents (ITRF, ETRF89, NAD83, etc). These only need to be accurate to about 10 metres in plan and height.
Ultimately, we need to know the heights above the WGS84 ellipsoid, and not above a local land levelling datum or sea level. So ideally give us ellipsoidal heights, but if not then just specify the land datum (eg Ordnance Datum Newlyn) and we’ll do the transformation.
At the same time, let us know your chosen name for the grid and its associated parameter files. We can accommodate up to 16 characters (no spaces). Usual practice is to include an acronym of the major cities the route passes through and the date of creation, or make some reference to the name of the project. Examples might be, Route66_09 or BPV16 for a route that went from Berlin to Vienna via Prague. Or XRail09 for the Crossrail project, or TPEN11 for the Trans-Pennine route. This name then becomes a unique SnakeGrid identifier and the parameter file will have a name that includes it. Example: Snakeparams_XRail09.dat.
Once we have this information we then create a SnakeGrid parameter file that defines the grid system to be used on your project. We can usually turn this around in a few days, but it helps to establish the timescale when you first make contact.
We’ll send you a copy of the parameter file – you’re free to copy this as much as you like, and pass it on to colleagues, sub-contractors, clients. We also supply a report detailing the characteristics of the grid, the maximum scale factor distortion to be encountered, and so on.