FLOOD09931

• County: Statewide
• Community: Urbanizing Areas of Colorado
• Basin: Statewide
• Title: A Guide for Erosion & Sediment Control in Urbanizing Areas of Colorado
• Date: 1/1/1973
• Prepared For: State of Colorado
• Prepared By: USDA Soil Conservation Service
• Floodplain - Doc Type: Educational/Technical/Reference Information

DETAILED VS. GENERAL SOIL MAPS

There are two kinds of soil maps being made in Col-
orado at this time - the detailed and the general. The
detailed soils map is made on aerial photograph bases
ranging in scale from 2.64 to 8 inches per mile. The scale
of the base map used in surveying is controlled by the
detail of mapping required. The detail in turn is deter-
mined by projected use. General soil maps are pub-
lished at scales of 1/16 to 1/4 inch per mile. Examples of
detailed vs. general soil maps are shown in Figures 2, 3,
and 4. All are shown in the actual size that they appear
in final print.

The map illustrated in Figure 2 was taken from the
Colorado General Soil Map. The areas delineated on
the map are separated in order to group like soils as
much as possible. The scale ofthis map is 1/16 inch per
mile. In the hatched (lower one-third) area of Figure 2,
there are four different groups of soils delineated.
These soil groups are indicated by numbers 4, 11, 23,
and 30.

The soils description presented in the legend of the
Colorado Generalized Soil Map identifies these groups
as follows:

Soil Group 4 - Nearly level to rolling, well-
drained loam and clay loam soils of
the high plains. Deep and as-
sociated shallow soils.

Soil Group 11 - Gently sloping to rolling, well-
drained loam and clay loam soils of
the high plains.

Soil Group 23 - Gently sloping to rolling, well-
drained loam and clay loam soils of
the high plains in areas oflow pre-
cipitation.

Soil Group 30 - Shallow, well to excessively
drained loam and clay loam soils
and associated rock out-crops on
hilly, steep, and rough-broken
land.

As can be noted, the uuits described on the map in
Figure 2 reflect the characteristics of the land. There
are only a few delineations; this type of map is used
mainly for "nondetail" work such as broad land use
planning at a state or multi-state level.

Figure 3 shows another generalized soil map at a scale
of 1/4 inch per mile. This is the scale and detail as it
appears in the county soil survey report. Only that part
of the county which is hatched in Figure 2 is shown in
Figure 3.

As in the case of the generalized soil map ofthe state,
the soil association units on the map in Figure 3 are
defined in terms ofthe characteristics ofthe land. How-
ever, more detailed separations are made on the general
map shown in Figure 3 as compared to the general map
in Figure 2. This type of map is useful for planuing at
state, regional, and county levels.

Figure 4 shows a detailed soil survey map of the
hatched area shown in Figure 3. This map has 19 differ-
ent delineations. The same area has 2 dileneations in
Figure 3 and one soil delineation in Figure 2. The scale
of the map is 2 inches per mile which is the scale and
detail as it appears in the county soil survey report. This
type of map is useful for operational planning at the
local or community level. In summary, it can be noted
that the kinds of units delineated on general soil maps
are greatly reduced in number with the geographic area
greatly increased. General soil maps are constructed
from detailed soils maps or by reconnaissance field
work and photo interpretation. It can be noted from the
figures that the delineation lines in Figure 2 do not
correspond exactly with the lines of the map in Figure 3.
This demonstrates that very general soil maps as shown
in Figure 2 do not contain as much accuracy in delineat-
ing soils areas as general soils maps do at a scale shown
in Figure 3. Obviously, general soil maps are useful
only for general planning. Detailed soils maps can be
used for operational planning. Modern detailed soils
maps are made with a planned accuracy factor which is
weighed by the cost of investigations in making the
survey, considering the predicted use. Thus, because of
mapping scale and cost considerations, the delineations
are not 1000/0 "pure," and on.site investigations are
necessary for specific sites. The land use planner can,
however, use soil maps of different types and scales to
find areas where he has a high degree of predictability in
avoiding undesirable areas - such as soils that have a
high shrink-swell behavior and are unsuitable for foun-
dations - or he can find areas where he has a high
degree of accuracy in finding soils permeable enough
for septic tank filter fields.

SOIL SURVEY INTERPRETATIONS

As one might expect, the limitations of a given soil vary
for particular uses. For example, a very sandy soil may
have only slight limitations for use as a footing material
for buildings because of its high bearing capacity, but
may have a moderate to severe limitation for use as a
cropland soil because of its low water-holding capacity
or droughtiness. In order to simplify the use of basic
soils data for people who are not trained in interpreta-
tion or do not have the time to become familiar with
basic soils terminology, soils information is presented
by means of interpretive ratings.

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