Contour lines and the slope line

In elevation projection a plane is given by its contour lines – the lines along which the plane keeps one constant height. They are its level lines, and because a plane rises uniformly they are parallel to each other and evenly spaced.

The distance between two neighbouring contours differing by one unit is the plane’s interval. The same proportion holds as for a line: a steeper plane has its contours CLOSER together. That is exactly what you read off a map – tightly packed contours mean a cliff, sparse ones mean flat ground.

The slope line (spádnica) is the line of steepest descent in the plane. In plan it is always perpendicular to the contours – perpendicular precisely because distances are measured in the image plane. A plane’s slope is the slope of its slope line, so the plane’s interval is measured along that line and not in some arbitrary direction.