Layer data aggregation and map development
is illustrated in the following example taken from the SSURGO data
users guide.
The comp table shares a one-to-many relationship
with the layer table. This relationship is reduced to one-to-one
relationship by calculating the sums of the weighted averages for the
available water capacity. The attributes required for this example are
in the mapunit, comp, and layer tables. Analysis
begins at the layer table that is at the lowest level in the
schema. The results of the data aggregation are moved to the comp
table, for map analysis purposes.
The dominant component is selected from the comp
table to reduce any one-to-many relationships that exist between the
mapunit and comp tables. The following steps outline the
GIS map development for "available water capacity" for each soil type.
(1) Calculate weighted averages: The data elements
needed to generate a map for available water capacity are shown in example
1. Only the records for the first (dominant) components were selected.
The available water capacity is calculated for the entire profile as
total inches of water. The laydepl and the laydeph are
the low and high values of the range for the layer depth reported in
inches. The awcl and awch columns contain the low and
high values the available water capacity reported in inches of water
per inch of soil. The column, wtavg in example 1, was added to the selected
layer table attributes to hold the user-calculated weighted averages
for the available water capacity calculation. Follow this method:
wtavg of some soil parameter = ( laydeph
- laydepl) * (( awcl + awch) / 2)
The wtavg is the total inches of available
water in each soil layer (horizon). The laydepl and laydeph
are the beginning and ending depths in inches of the soil layer measured
from the soil surface. The awcl and awch are the low and
high values for the range in the available water capacity reported in
inches of water per inch of soil.
(2) Calculate the sums of the weighted averages
of each layer. The weighted average of the available water for the soil
layers is summed for the dominant map unit components in the comp
table. The muid and seqnum columns in the attribute table
relate the comp and layer tables. The summation of the
weighted averages (sum_wtavg) is the total inches of water available
in the soil profile and is computed as follows:
Sum_wtavg = (SUM) wtavg
The summation is also an aggregation of the data
that reduces the one-to-many relationships between the layer
and comp tables to one-to-one relationships of soil polygons
to attributes. Map legend classifications (or source water model "weights")
are then added to the comp table for mapping and source water
analysis operations. Note: This methodology is found in the SSURGO
users guide, and may be found on the Internet at the following address:
http://www.ftw.nrcs.usda.gov/pdf/ssurgo_db.pdf
