Component Table Mapper

Conceptual Background

The Problem

Arc/INFO polygon coverages have in their feature tables (.PAT) a single record per polygon. In CanSIS data, attributes for soil polygons are stored in component tables which consist of multiple records for each soil polygon (for example the SLC version 2.2 Component Table).

In order to map values such as the dominant value or percentage distribution of a particular variable in each soil polygon, this value must first be summarized from the existing component table. The summary table can then be joined to the .PAT of the soil polygons, and the summary value mapped

Creating the summary tables, if done interactively in ArcView, is a complicated and time-consuming process, and easily prone to operator error.

The Desired Summary Values

This extension can extract three types summary tables from a component table: Dominant Value, Percent Distribution, and Summary Statistics. Dominant Value and Percent Distribution are two interpretations of the same data; Summary Statistics produces similar results to the ArcView statistics function for numeric fields, with the addition of a spatially weighted average. These tables are described in greater detail below.

Dominant Value

The Dominant Value summary table reports the class of the selected variable with the largest total percent occurrence in the polygon.

For example, given the following subset of a component table showing the Soil Development variable:

Component Table (portion)

Polygon (SL)	Component	Percent Occurrence	Soil Development (DEVEL)
1	1	40	Brown Solonetzic
1	2	30	Brown Chernozemic
1	3	30	Brown Chernozemic
2	1	75	Regosolic
2	2	25	Brown Chernozemic

the dominant value (where the Soil Development field is mapped) would produce: Dominant Soil Development

Polygon (SL)	Percent Occurrence	Soil Development (DEVEL)
1	60	Brown Chernozemic
2	75	Regosolic

Percentage Distribution

The Percentage Distribution summary table reports the total percent occurrence of a particular class of variable.

For example, using the same data in the component table above, the percentage distribution of "Brown Chernozemic" (where the Percent Occurrence field is mapped) would produce:

% Distribution of "Brown Chernozemic"

Polygon (SL)	Percent Occurrence	Soil Development (DEVEL)
1	60	Brown Chernozemic
2	25	Brown Chernozemic

Summary Statistics

The Summary Statistics table reports any combination of the following:

• Minimum
• Maximum
• Range
• Sum
• Mean
• Weighted Average (weights based on % occurrence)
• Variance
• Standard Deviation

As an example, given the following table subset (selecting those records with applicable values): Component Table (portion)

Polygon (SL)	Component	Percent Occurrence	SLOPE	SLOPE Representative Value
1	1	70	n/a
1	2	20	B	7
1	3	10	D	23
2	1	100	C	12

Choosing to report the minimum, maximum, mean, and weighted average for the SLOPE value would produce: Summary Statistics for SLOPE

Polygon (SL)	Count	Minimum	Maximum	Mean	Weighted Average
1	2	7	23	15	12
2	1	12	12	12	12

Note that the weighted average is a statistic not interactively available through ArcView. It is based on the percentage occurrence, calculated according to the following formula: sum (value x % occurrence) sum (% occurrence)

It is important to recognize that some statistics may not be meaningful on certain types of data ­ for example, calculating a mean on data such as pH values, which use a logarithmic scale. It is up to the user to determine which fields and statistics are valid.

Next: Obtaining, installing and loading the Extension

Source: Component Table Extension technical documentation (CAR018.19970529)
Contact: Head, CanSIS