For this purpose, the survey engineer or Markscheider determined a fixed reference point from which measurements were made.
If the gallery lay above the Markscheider's reference point, it had a positive "vertical height difference" (Seigerteufe).
If the gallery lay below the reference point, it had a negative vertical height difference.
[5]With the emergence of mining engineering the upper surface of the terrain was used as the reference point, usually at the headgear.
[2] Today the reference surface for mining in Germany is standard sea level (Normalhöhennull, NHN, formerly Normalnull).