Geology Reference
In-Depth Information
Map Symbols
Conventional symbols, used to indicate natural and
manmade features, make it possible to put a vast
amount of detailed information on topographic maps.
In general, cultural (man-made) features are shown in
black and red, all water features are blue, and relief is
shown by contours in brown. Green overprint is used
on some maps to indicate forest areas. Maps that have
been photorevised use a purple or gray overprint to
show cultural features constructed since the map was
originally published.
Topographic map symbols used by the U.S. Geo-
logical Survey are inside the front cover. Information
on how to order maps, a description of the national
mapping program, and map scales are in Appendix IV.
the "Land Ordinance of 1785," uses selected principal
meridians and base lines as reference points
(Figure 2.1). Starting from these meridians and base
lines, the land was surveyed into congressional town-
ships, six miles square, making the problem of locating
smaller tracts of land simpler. Townships were further
subdivided into 36 sections, each one mile square
(Figure 2.1).
One section of land contains 640 acres, a half-
section 320 acres, and so on. As a matter of convenience,
sections are divided into halves, quarters, halves and
quarters of quarters, and so on, called fractions, which
are named or located by points of the compass; such
as NW 1/4, SE 1/4, S 1/2, or NE 1/4. In finding the
location of a described tract of land, the description is
analyzed by first determining the location of the
township by range and township number, then the
section by its number. The exact fraction is deter-
mined by beginning with the last (which is the
largest) and working back: T. 2N, R. 2E, Sec. 10, NW
1/4, NE 1/4 (Figure 2.1).
A third way for locating places is the
Universal
Transverse Mercator (UTM) grid.
The UTM system is
becoming more popular with increasing use of Global
Positioning Systems (GPS). This is in part due to the
fact that the UTM system locates features using
meters, rather than degrees, minutes, and seconds. It is
far easier to imagine what the distance between two
features is if the distance is reported in meters. It is dif-
ficult at best to imagine distances that are given in
minutes and seconds.
In the UTM system, the world is divided into 60
zones of 6 degrees longitude. As shown in Figure 2.2, the
conterminous United States is located in zones 10-19.
Locations in the US are reported in meters north
of the equator (called northings) and meters east of
an arbitrary zero line that lies 500,000 meters west of
Methods of Describing Locations
There are many ways to locate places or areas on maps.
One way that is commonly used is a description in
terms of
latitude and longitude.
Lines of latitude (paral-
lels) and lines of longitude (meridians), form a coordi-
nate system superimposed on the Earth's surface.
Parallels are east-west lines that parallel the equator.
Meridians are north-south lines that converge toward
the poles. By this method location is expressed in terms
of the number of degrees (°), minutes ('), and seconds
(") of latitude north or south of the equator and longi-
tude east or west of the prime meridian. The prime
meridian, which passes through Greenwich, England, is
the reference meridian
(O
0
OO'OO"
) from which longi-
tude is measured. The equator is the reference parallel
(0°00'00"). The most commonly used topographic
maps are either 7
1
/
2
' or 15' quadrangles.
A second method by which areas or places may
be located is by using the
township and range system
(TRS). This system, established by congress as part of
FIGURE 2.1
Subdivision of land by township, range, and section. A township consists of 36 square miles; a section, 1 square mile (640
acres). Point Z, shown on the maps above, is located in the NE1/4 of the NW1/4 of Sec. 10, T. 2N, R.2E.
