Elevation levels in Secrets of the Nethercity

One of the neat things about the map of the Nethercity is how, despite fitting onto a single piece of paper, it changes elevation frequently crisscrosses over itself. The map on page 23 is shaded by each section's relative elevation, which works well.

However, is it ever explained what these relative elevation levels correspond to in feet? (That is, is the height distance between elevation 0 and elevation -1 generally 10 feet? 20 feet?) The major example where this would be useful is if a party decides to slide or climb down the ravine from C2B to L1, but you could also imagine characters digging through floors to drop onto lower sections.