Mapping weighted averages from the component table

Weighted averages of interval and ratio data (and coded interval and ratio data) can be calculated according to the following formula:

   sum( value x % occurance)
   -------------------------
       sum( % occurance)

The following portion of the component file (relevant records and items for one polygon) is used to illustrate these instructions.

SL	CMP	PERCENT	SLOPE
690251	1	70	#
690251	2	20	B
690251	3	10	D

Step 1: Identify the relevant records

• # means non applicable, so component 1 is excluded from the calculation.

SL	CMP	PERCENT	SLOPE
690251	2	20	B
690251	3	10	D

Step 2: Convert data to a linear scale (if data is coded)

• For slope class B (4-9%), use a representative value of 7%
• For slope class D (16-30%), use a representative value of 23%

SL	CMP	PERCENT	SLOPE	SLOPE value
690251	2	20	B	7
690251	3	10	D	23

Step 3: Calculate weighted values for each component

• Weighted slope value = slope value * percent

SL	CMP	PERCENT	SLOPE	SLOPE value	weighted SLOPE value
690251	2	20	B	7	140
690251	3	10	D	23	230

Step 4: Sum percent occurances and weighted values by polygon

SL	SUM PERCENT	SUM weighted SLOPE value
690251	30	370

Step 5: Calculate weighted average

• Weighted average slope value = sum weighted slope value / sum percent

SL	SUM PERCENT	SUM weighted SLOPE value	weighted average SLOPE value
690251	30	370	12

Source: Peter Schut
Contact: Head, CanSIS