# 6 Local / metric X and Y coordinates

The plot centre is defined by geographic latitude and longitude coordinates (see Explanatory Item 4).

The position of measurement devices or trees are defined in relation to the local plot centre.

X and Y coordinates of local plot centre give the position of a local plot centre in relation to the plot centre.

They are specified with “0” if the plot centre is the origin of the local metric coordinate system.

In any case the orientation of the local metric coordinate system must be from west to east (X coordinate) and from south to north (Y coordinate).

E.g X (Y) coordinate “17.83” means that the local plot centre is 17 meters and 83 centimeters east (north) of the plot centre, “-5.61” means that the respective point is 5 meters and 61 centimeters west (south) of the plot centre.

Note: All geographic coordinates which are specified in the reduced plots files are the coordinates of the plot centre mentioned above.

All coordinates are defined in horizontal level (compare picture below). Thus, if a horizontal distance (coordinate difference or radius) from one point (A) to another (B) must be measured at a slope, the horizontal distance (h) has to be used for coordinate calculations instead of the slope parallel one (s).

The horizontal distance can be calculated from slope parallel distance and slope as follows: h = s * cos(α)

Adjustment of a distance between two points for slope: In analogy, the elevation difference (e) between both points can be calculated as:

e = s * sin(α)

Note: It is important that distance (s) and slope (α) are measured in the same direction!