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Map Intrepretation IV
Topographic Map Interpretation
Contents
A.
Physical Features
B.
Cultural Features
C.
Conclusion
Everything up to now can be applied to topographic maps as well
as other kinds of maps and information. The rest of this discussion
focuses specifically on how you can use topographic maps to extract
information about the landscape. Both physical and cultural features
are often discernible from topographic maps. This section draws
many ideas from a course handbook titled "The Language of Maps,"
by my advisor at the University of Minnesota, Phil Gersmehl.
The US Geological Survey has published many different kinds of
maps, including several series of topographic maps. Only a few
series are in common use. The 7 1/2-minute quadrangles, which
have a scale of 1:24,000, are the most popular. These detailed
maps are supplemented by the 1:250,000-scale series. A third series,
the 15-minute quad at 1:62,500, was popular but discontinued by
the USGS for mysterious reasons (probably because the detailed
1:24,000 maps had to be done, and the 1:62,500s would duplicate
some of their uses). The 1:100,000-scale metric series is a recent
addition, and is now complete for the US. All of its maps show
features in metric, for example elevation is in meters.
Table : USGS Topographic Map Series
Series |
Scale |
Lat. x Long |
Area (mi2) |
Paper Size |
7-1/2 Minute |
1:24,000 |
7-1/2' x 7-1/2' |
49-70 |
22" x 27" |
15-Minute |
1:62,500 |
15' x 15' |
197-282 |
17" x 21" |
Metric |
1:100,000 |
30' x 1ª |
3,173-4,334 |
|
1:250,000 |
1:250,000 |
1ª x 2ª |
6,346-8,669 |
24" x 34" |
1:1,000,000 |
1:1,000,000 |
4ª x 6ª |
73,734 x 102,759 |
27" x 27" |
In addition to this section, see also the discussion of contour
lines and slopes above under Symbolization in Part I. You may
also want to read (or reread) a text @ecoatlas.GIS.physical geography
or geology to help in understanding the terms and concepts here.
1.Landforms
Contour lines and other map symbols can give you clues about
the geology and geomorphology that underlies the landscape. In
general, rocks differ in their resistance to weathering and erosion,
and these differences are reflected in the landscape. Some examples:
Fault lines.
Some faults can sometimes be seen on a map where streams have
been offset to the left or right along a line, then continue downstream.
These are strike-slip, or transform, faults, where one section
of the crust is sliding past another. California's San Andreas
and associated faults are examples of transform faults. In other
faults, one section of the crust slides under another. These normal
or reverse faults are usually less visible unless considerable
uplift has occurred. Death Valley is the result of a pair of normal
faults letting the valley floor downward several thousand meters.
Cuestas and hogbacks. These are the result of a block of terrain
being tilted upward at one end. The result is a gradual upward
slope with a cliff at the uplifted end. Good examples of these
can be found in the desert Southwest.
Volcanoes.
In contrast to most hills, volcanoes are usually much rounder,
so that contour lines around them are circular. This is especially
true for stratovolcanoes, the most violent and prominent volcanoes.
Examples are Mt. Shasta in California and Mt. Rainier in Washington.
Some volcanic eruptions are gentler and form gentler hills or
even broad plateaus. These flows are more difficult to detect
from the topographic map, unless the USGS adds a dotted pattern
that indicates a lava flow.
Wave action.
Waves are very efficient at cutting back into cliffs. The waves
crash against the base of the cliff and undercut the cliff. Eventually
a chunk of cliff topples into the sea. But waves don't cut down
much below sea level, especially where the rock along the shore
is relatively hard. The result is that wave action can create
a long wave-cut platform or terrace along the shore. These platforms
are often visible on coastal maps.
If the coast is uplifted, this flat wave-cut (or marine) terrace
can be high and dry above the ocean. The California coast has
been rising relative to the sea over the past several hundred
thousand years. In many areas a series of terraces are visible
on maps and in person. The Sea Ranch, along the coast in northern
Sonoma County, is built on a marine terrace.
Glaciation.
Glaciers can occupy either mountainous or flat terrain, and result
in corresponding alpine or continental glacial features. Many
kinds of features can result, so I'll mention only a couple of
examples. For alpine glaciation, the ice will occupy a valley
and hollow out its sides and head. This turns the valley from
V-shaped to U-shaped, with steep sides and flat bottom. Small
lakes may form in the valley, known as tarns. As it excavates
into the mountain at the head of the valley, another glacier may
be doing the same on the other side of the mountain. This results
in a sharp ridge, or arete. If three glaciers surround and erode
a mountain, they may form a three-sided peak, known as a horn
(the Matterhorn is the best-known example). California's Sierra
Nevada was covered by glaciers during the last glacial peak (about
15,000 years ago), and many of these features remain as evidence.
Continental glaciers, or ice sheets, produce different landforms.
The most striking is where the glacier's front has remained stationary
for a long period. The glacial ice will be moving forward toward
the front, carrying rock and debris in the ice. The ice melts
at the front and leaves the debris as a moraine. Moraines are
hilly ridges perhaps a mile wide and up to several hundred miles
long. They may have a hundred feet of local relief (vertical elevation
change). The ridges contain many depressions that fill with water.
If you see a map with irregular hills and many small lakes, glaciation
may be the cause. Ice sheets covered most of the upper Midwest,
down to about the Missouri and Ohio Rivers. Nearly all of Minnesota's
10,000 lakes were formed by glaciers. California didn't have any
continental glaciation to speak of--only alpine.
Karst.
Another type of area with irregular terrain, sometimes with many
lakes, is known as karst. Karst forms in limestone and other rock
that easily dissolves in water. The water dissolves both the surface
and the subsurface. Caves result below the surface--most caves
are formed in limestone rock (Carlsbad Caverns, Mammoth Cave,
etc.). But the surface may also show the result of being dissolved:
sinkholes (small depressions) and disappearing streams (where
a stream suddenly vanishes) are evidence. Karst is common in Indiana,
Kentucky, Arkansas, Missouri, and Florida (where houses have been
swallowed by sinkholes). California has a small area of karst
in the Marble Mountains, in the extreme northern part of the state.
Winds.
Winds rarely shape the landscape enough to show on topographic
maps. The exception is in areas with accumulations of loose sand,
which wind can move effectively. Sand may accumulate in deserts,
along shores, or where the bedrock is sandstone. The wind usually
shapes the sand into dunes. There are many types of dunes, mostly
due to different wind speeds and directions. Often the wind is
consistent enough from one direction to line up the dunes, so
a linear pattern shows up on the topographic map. Streams are
usually absent or rare, since water flows into and through the
dunes easily. Topographic maps usually show sand dunes with a
distinctive dotted pattern. Large sand seas are found in the Nebraska
Sand Hills, White Sands National Monument, and southeastern California.
Mass movement.
Gravity affects all events on Earth, but it is the defining force
in mass movement. Landslides, rockfalls, and slumps are all examples
of mass movement. When a slope becomes too steep for the cohesive
forces retaining it, it gives way. The reasons for gravity overcoming
the slope can include uplift from a fault, an earthquake, a stream
undercutting a slope, burning of vegetation that holds the slope,
or excavation by humans. Small mass movements may be too small
and temporary to be visible on a topographic map. Large landslides
and slumps may be evident, especially in certain areas where they
are frequent. Mountains are good candidates. Sonoma County is
also prime terrain, as winter rains saturate hills that have been
overgrazed, undercut or built upon.
2. Drainage Patterns
The terminology of drainage patterns is not particularly useful
in and of itself. But drainage patterns can suggest what is happening
with the underlying geology and landscape.
Dendritic.
The "normal" pattern of stream drainage is called dendritic,
since it looks like a tree's roots or branches. This pattern occurs
when no strong geological factor controls the drainage network.
Meandering.
A stream that has had a chance to erode for a long period tends
to widen its path across a large floodplain, and meander back
and forth across the floodplain. These streams usually flood periodically
and fill their floodplain. While unwise to build on the floodplain,
the land is usually flat and fertile, and tempting for short-sighted
settlers. These streams are often channelized into straight courses.
This rids the local area of water quickly but creates even more
flooding downstream.
Braided.
Some streams in floodplains have many small channels that merge
and separate downstream. Braided streams are most common where
the water flow is variable (e.g., deserts or mountains) and the
stream bed is mostly sand and gravel.
Trellis & Rectangular.
These patterns are where streams flow in straight segments and
turn at sharp angles. Usually this results from sedimentary bedrock
controlling the direction of streams along the surface. Good examples
of trellis drainage are found in eastern Pennsylvania, where folded
sedimentary rocks cause streams to flow at right angles to each
other.
Radial.
Radial drainage occurs around circular hills and mountains, with
streams radiating out in all directions. This would be typical
around a volcano.
Centripetal.
The opposite of radial, here streams drain inward toward a central
basin. The basin usually has a dry or saline lake and has no outlet--a
sign of arid conditions.
Deranged.
Streams have no organized pattern. Some drain externally toward
major rivers, others may drain internally. This is usually a sign
of continental glaciation, or possibly sand dunes or karst.
Artificial.
Human-caused modifications of streams include channelized streams
(straighter than any normal stream), canals, drainage ditches
(may be a sign of natural wetlands), reservoirs (which not only
have a dam, but usually a more indented shoreline than a natural
lake), and levees.
3. Vegetation
USGS topo maps often contain some indication of vegetative cover,
but not always. Older maps lack this information, and some kinds
of vegetation aren't indicated.
Forest and woodland.
USGS maps indicate forest or woodland (area with trees but not
continuous canopy cover) by a uniform green tint. The size, orientation
and shape of tree cover can tell us about climate and land use.
Types of cover patterns include:
- Treeline forests, in higher elevations where cold climate
prevents trees from growing any higher;
- Moisture-favorable forests, in canyons, ravines, on slopes
away from the Sun (especially north slopes), or up slopes in
arid lands;
- Dry-land forests, on drier islands in wetlands; good examples
may be seen in Florida's Everglades, in "mahogany hammocks";
- Landslides or avalanche chutes, where falling rock or snow
keeps areas clear of mature trees;
- Human-caused forest patterns, including clearcuts with sharp
edges in otherwise forested land; bottomland forests in wet
areas where the surrounding land has been cleared; and woodlots,
shelterbelts and windbreaks around farms and ranches.
Orchards and vineyards.
USGS topos give these distinctive patterns of regular dots. It
is interesting to examine where these have been placed on the
terrain--usually up above the valley floor slightly to avoid cold-air
drainage that occurs at night.
Scrub.
Scrub or shrub vegetation has an irregularly-dotted pattern.
Chaparral vegetation in California normally has this pattern,
although it is not always mapped. Chaparral is a sign of Mediterranean
climate, that is, mild, wet winters and warm, dry summers. Soil
is also a factor. For example, in our area, soils that dry out
thoroughly in summer are more likely to harbor chaparral.
Wetland vegetation.
USGS topographic maps have distinctive symbols for wooded wetland
(also called swamp), nonforested marsh, and mangroves. Mangroves
are shrub or small tree-sized plants that grow directly in warm
salt water along tropical and subtropical coasts. Florida and
other Gulf Coast states have many mangrove areas, although many
have been cleared for development.
1. Transportation Patterns
Besides the types of roads listed in the key for topographic
maps (primary, secondary, light-duty, unimproved), look also at
the placement of the road. It may follow along elevation contours,
even if the route is circuitous--an indication of steep terrain,
relative to the amount of traffic carried. Heavily-traveled routes
tend to get straightened.
Other areas may have roads that follow flat terrain, and cross
steep slopes only when connecting level areas. The flat areas
are either valley floors, with few roads going over mountains,
or mesas or interfluves, where the flat land is dissected by streams.
The Appalachians are good places to find transportation along
valleys. A glance at a road map of eastern Pennsylvania will indicate
that valleys and hills trend from southwest to northeast. Southern
California has good examples of mesas, most of which are marine
terraces formed long ago and since lifted above the sea.
Grid patterns are the rule where areas were surveyed before settlement,
as in most lands under the Public Land Survey system. Roads in
these areas often follow the square grid regardless of terrain.
Don't overlook other kinds of transportation, such as railroads,
canals, and trails. These often indicate something about the history
or land use of an area. A map with many railroads, for example,
likely says the area was important during the early 20th Century.
You also may notice corridors that used to be railroads--often
shown as "old railroad grade." Many of these are being converted
to foot trails or bicycle paths. Canals for transportation are
rarely built nowadays, but were popular in the 19th Century.
2. Rural Settlement
Rural housing.
Rural houses and buildings are usually shown on topographic maps
as small black squares. Rural settlement patterns usually reflect
the original land survey. Regions surveyed under the PLS started
out with a very dispersed settlement pattern. Homes were scattered
on farms and ranches, usually no more that four per square mile.
Areas surveyed under irregular survey systems (metes-and-bounds,
land grants, etc.) tend to have homes more clustered, or along
roads that went through or around land parcels. Land in long-lot
surveys typically has houses closely spaced along a few main roads
or rivers, with few houses elsewhere.
Service facilities.
Along major transportation routes in rural areas, small settlements
sprung up to service travelers and their vehicles. These settlements
concentrated at railroad stations in the 19th and early 20th Centuries.
As the automobile became the primary means of transporting people
in the mid-20th Century, service facilities became more dispersed
along highways. With travel more concentrated on interstate highways,
facilities and settlements are once again becoming more concentrated,
this time at major exits on interstates.
Urban invasion. Many rural areas have been "invaded" by urbanites
for summer or weekend homes, or even long-distance commute homes.
Lakes and rivers are popular places, especially in more arid regions
of the West. Campgrounds, resorts, and other tourist facilities
have expanded rapidly in the past few decades. A dense pattern
of these indicate either a nearby metropolitan area (such as Sierra
Nevada resort areas), an unusually popular location (such as Yosemite
or Redwood parks), or a wealthy clientele (such as Aspen or Vail).
3. Urban Development
The patterns of streets and facilities in urban areas can suggest
much about the function and history of the city. Often you must
investigate the particular groups that settled the area to fully
appreciate the patterns. The myriad groups who immigrated to the
US brought their own traditions of land-use. Of particular influence
were the English, French, German, and Spanish. Subtle influences
of others, such as Native Americans or Asians, can sometimes be
detected.
Irregular street patterns.
Early in our history, streets were laid out according to the
major trading routes and the shape of the land. This produced
an irregular pattern, especially in smaller towns.
Regular grid patterns.
Most cities adopted regular grids for streets to make surveying
and development easier. This is particularly true in PLS areas.
At first, street grids were oriented with respect to the major
transportation line, even if the line went northwest-southeast.
Later, most street grids were oriented north-south and east-west.
The changing pattern can be seen in many cities as they grew outward
from the center.
Irregular patterns, again.
In the past few decades, the regular grid of street became associated
with uniform houses and middle-class lifestyles. Wealthy people
(and some who want to look like they are) like to distinguish
themselves from ordinary folk, and have used winding, irregular
streets for their housing to do so. This also goes along with
the trend in the US for wealthy people to live in hills above
the city; regular grids would be difficult here anyway.
Industrial areas are often visible on topographic maps as clusters
of larger buildings, and may help discern the area's function.
Economic geographers often divide industries into raw materials-oriented,
such as gravel pits and food processing plants; those in manufacturing,
which often cluster together where power, labor or transportation
is available; and market- and service-oriented, which cluster
where customers are, for example bottling plant, shopping malls
and strip developments.
4. Place-names
Place-names often give clues about the area's history or occupants.
Often this can suggest a series of different groups in an area,
or sequent occupance in geographic jargon. In California, of course,
many names reveal the Spanish and Mexican presence--which is both
historical and continues today. Town names are not the only clue.
Other words were applied to places and have stuck, sometimes as
more reliable than city names. For example, I doubt many of these
communities were founded by Spanish-speakers: Del Mar, Tierrasanta,
San Anselmo, El Cerrito, San Carlos and Rancho Mirage. On the
other hand, words like verde, agua, mesa, arroyo, iglesia, and
oso occur on maps as part of other names and indicate early settlement
by the Spanish. Laguna lake is redundant--laguna means lake in
Spanish--and the name may reveal an early Spanish presence.
Other groups settled throughout the US, and even California.
Groups including French, German, Czech, Swedish, Norwegian, Danish,
Irish, Polish, Chinese, Korean, Japanese, and recently Vietnamese
and Hmong have established place-names. The Russians colonized
the northwestern coast of North America in the early 19th Century,
and some place-names remain even where few Russians do (Sitka,
Petersburg, Soldotna, Andreanov Islands, etc.). Sebastopol, however,
is a later revision having nothing to do with the Russian city
other than the name itself.
5. Mining
Mining operations disturb local areas, sometimes severely. Topographic
maps often show mining pits with a distinctive shading pattern
(see Topographic Map Symbols brochure). Gravel pits and quarries
also receive a point symbol. We could also include oil and gas
wells here, indicated on topo maps; presence of many such wells
suggest a major contribution to the local economy. Oftentimes
other clues also reveal mining, especially past mining: irregular
contours that do not match the general trend of the landscape,
unexpected depressions shown by contours, and rail lines or roads
dead-ending in the area. Large mines, such as strip mines in the
Appalachians and open-pit copper mines in Arizona show these features
prominently on topo maps.
Topographic maps can be a rich source of information about a
landscape, even absent other information. It is a good idea to
use collateral information, such as histories or geographies of
an area, before sharing your conclusions with friends and relatives.
Remember, maps are representations of reality, not reality itself!
  
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