Operating Notes:

1. Small Scales, where the instrument cannot fit because of trackside structures, etc.

Raise the unit using a suitably-thick board whose top & bottom edges are completely parallel. Be sure to ensure that the track-center ridges on the ends of the instrument line up with the track center being measured or the grade reading will not be accurate.

2. Large Scales, where the instrument is too small to lie on the rails.

Use a board or similar item to span the rails. Make sure that the track-center ridges are parallel to the track center being measured or the grade reading will not be accurate.

3. Sharp Curves in the Smaller Scales.

Since, on sharp curves, the superelevation varies rapidly over short distances, the instrument will only be supported at three points and will rock in position. The problem is that the instrument is proportionally too large for the situation.

If all you want are the average values over that area, depress one end of the instrument onto the track/subroadbed, take the readings, and average them with the readings when you depress the other end. If you want an exact reading over a length that is shorter than the instrument, raise the unit on a suitably-short block of wood or other material whose top & bottom edges are completely parallel. Be sure that the track-center ridges on the ends of the instrument line up with the track center being measured or the readings will not be accurate. Also see the next note on Superelevation and Sharp Curves.

4. Superelevation and Sharp Curves.

Superelevation varies throughout a curve, and is often specified as specific values at different distances along the curve. You will find these specifications in civil engineering handbooks. In sharp curves, you cannot just place the instrument and take readings because there will be a different superelevation value at each end of the instrument. To get an accurate superelevation reading at particular spot, align one edge with a tie, perpendicular to the track centerline. Ignore the position of the opposite edge - it has no meaning. Take readings at many points along your curve to make sure that the superelevation matches the specifications and that it smoothly changes along the curve in between.

Specifications GM-20 Main Page