Geometry: line, text.
Every fifth contour shall be drawn with a thicker line. This is an aid to the quick assessment of height difference and the overall shape of the terrain surface.
An index contour may be represented as an ordinary contour line in an area with much detail. Small contour knolls and depressions are normally not represented using index contours.
The index contour level must be carefully selected in flat terrain. The ideal level for the index contour is the central contour in the most prominent slopes. An index contour may have a height value assigned. A height value should only be inserted in an index contour in places where other detail is not obscured. It shall be orientated so that the top of the label is on the higher side of the contour. The index value (label) shall be 1.5 mm high and represented in a sans-serif font.