Curves

The curve file describes the time series attribute curves that are referenced by the features and products files. The curve file contains three fields: CURVE, X, Y. The file is an ordered list of curve inflection points that describe the numerical relationship between time on the X axis and attribute value on the Y axis.

See Table 9, “Curve file format description” for a detailed description of the curve file format.

Table 9. Curve file format description

Field nameData typeDescription
CURVEString An alphanumeric identifier that uniquely identifies each curve definition.
Xfloat The X-value of a single inflection point on a curve.
Yfloat The Y-value of a single inflection point on a curve.

In the example below, three curves are shown (curves 4, 5 and 6. Curves are composed of one or more points. Curves are always used to obtain an attribute value (Y) for a given time value (X).

Example 13. Sample curve file records

	  CURVE   X       Y
	  ...
	  4       0       0.49
	  5       0       0.51
	  6       0       0
	  6       70      0
	  6       75      0.49
	  6       80      0.49
	  6       85      1
	  6       130     1
	  6       135     0.51
	  ...
	

A simple linear interpolation is used to obtain attributes for X values that fall between two vertices. Extrapolation of values below the minimum (or above the maximum) are simply assigned the first (or last) Y value. This means that a flat line may be represented by only one X,Y pair!.

Curve 4 is represented by only one record in the curves.csv file. This record defines a single point with the x,y pair (0,0.49). Static attributes, such as stratification codes are represented by curves with a single vertex at (0,1) to indicate that 100% of the block is classified as that particular stratum through time.

[Tip]Tip

For example, a static curve could represent an unmanaged riparian buffer, the classification of which will not change over time. When multiplied by the appropriate block area, this attribute will describe the area that is in the riprarian buffer.

Curve 9 on the other hand contains a number of inflection points to describe a shape that changes through time. There is a straight line interpolation between inflection points, and extrapolation of first and last values beyond the minimum and maximum values.

Curve definitions can be shared by one or more features. For example, a single yield curve might describe the growing stock attribute and also the clearcut harvest volume attribute. When building a track database there are significant opportunities for data set compression by sharing common curve definitions.