Technical Explanation

In surveying and civil engineering engineers face a common problem in that the earth is an ellipsoid with a surface that has a surface topography producing local variations in height. When it comes to representing this situation on a map or flat grid the information shown is in fact distorted from reality

On small sites it used to be common for there to be a locally defined coordinate system that was divorced from any other coordinate system which achieved 1:1 fit between the coordinates and the reality on the ground. The problem with this solution is that it is very difficult in incorporate GPS data and any national mapping used for services.

When working on projects that extend for a few kilometres, engineers and surveyors are accustomed to the concept of a local site grid that effectively makes a flat Earth assumption. In effect the inaccuracy introduced by the curvature of the earth is so negligible that it can be ignored.

However as projects increase in size past a few kilometres the curvature can not be ignored and to overcome these problems transformations called scaling factors are used to produce a local coordinate system that is accurate. However such scaling factors are only accurate over a limited area from a defined point before the curvature of the Earth means that distances and angles calculated from coordinates no longer match those measured on the ground. At this point the conventional approach is to define new grid with all the attendant problems of converting from one coordinate system to another at the boundary point. A further complication is that variations in height also affect the accuracy of the grid.

Thus projects which extend for hundreds of kilometres and which have substantial variations in height over the length of the project are faced with multiple grids that all have to be managed and with the transition from one grid to another offering substantial possibilities for error.

The Snake Projection solution uses the facts that railway lines, pipelines or roads are very narrow compared to their length and that the project corridors in such cases have gentle curves. Thus the system defines a local coordinate system that has a unity scale factor along a line that follows the trend of the route. The system also extends into three dimensions removing the need for height scale corrections. The Snake, or localised grid, for each project is unique to the project depending on the particular horizontal and vertical trends that the route takes.