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USGS
Topographic Maps
The
distinctive characteristic of a topographic map is that the shape of the
Earth's surface is shown by contour lines. Contours are imaginary lines that
join points of equal elevation on the surface of the land above or below a
reference surface such as mean sea level. Contours make it possible to measure
the height of mountains, depths of the ocean bottom, and steepness of slopes.
A topographic map shows more than contours. The
map includes symbols that represent such features as streets, buildings,
streams, and woods.
USGS
Map Scales
The
following description is on the USGS Map Scale site:
Simply defined, scale is the relationship
between distance on the map and distance on the ground. A map scale might be
given as a drawing (a graphic scale), but usually it is given as a fraction or
a ratio—1/10,000 or 1:10,000.
These "representative fraction"
scales mean that 1 unit of measurement on the map—1 inch or 1
centimeter—represents 10,000 of the same units on the ground. If the scale
were 1:63,360, for instance, then 1 inch on the map would represent 63,360
inches or 1 mile on the ground (63,360 inches divided by 12 inches equals
5,280 feet or 1 mile). The first number (map distance) is always 1. The second
number (ground distance) is different for each scale; the larger the second
number is, the smaller the scale of the map.
"The larger the number, the smaller the
scale" sounds confusing, but it is easy to understand. A map of an area
100 miles long by 100 miles wide drawn at a scale of 1:63,360 would be more
than 8 feet square. To make the map a more convenient size, either the scale
used or the area covered must be reduced.
If the scale is reduced to 1:316,800, then 1
inch on the map represents 5 miles on the ground, and an area 100 miles square
can be mapped on a sheet less than 2 feet square (100 miles at 5 miles to the
inch equals 20 inches, or 1.66 feet). On the other hand, if the original
1:63,360 scale is used but the mapped area is reduced to 20 miles square, the
resulting map will also be less than 2 feet square.
| USGS Maps |
| Series |
Scale |
1 inch represents
approximately |
1 centimeter
represents |
Standard
quadrangle size
(latitude by longitude) |
Quadrangle
area
(square miles) |
| Puerto Rico 7.5 minute |
1:20,000 |
1,667 feet |
200 meters |
7.5 by 7.5 minute |
71 |
| 7.5 minute |
1:24,000 |
2,000 feet (exact) |
240 meters |
7.5 by 7.5 minute |
49 to 70 |
| 7.5 minute |
1:25,000 |
2,083 feet |
250 meters |
7.5 by 7.5 minute |
49 to 70 |
| 7.5 by 15 minute |
1:25,000 |
2,083 feet |
250 meters |
7.5 by 15 minute |
98 to 140 |
| USGS-DMA 15 minute |
1:50,000 |
4,166 feet |
500 meters |
15 by 15 minute |
197 to 282 |
| 15 minute* |
1:62,500 |
1 mile |
625 meters |
15 by 15 minute |
197 to 282 |
| Alaska Maps |
1:63,360 |
1 mile (exact) |
633.6 meters |
15 by 20 to 36 minute |
207 to 281 |
| County Maps |
1:50,000 |
4,166 feet |
500 meters |
County area |
Varies |
| County Maps |
1:100,000 |
1.6 miles |
1 kilometer |
County area |
Varies |
| 30 by 60 minute |
1:100,000 |
1.6 miles |
1 kilometer |
30 by 60 minute |
1,568 to 2,240 |
| 30 minute* |
1:125,000 |
2 miles |
1.25 kilometers |
30 by 30 minute |
786 to 1,124 |
| 1 degree by 2 degree or 3 degree |
1:250,000 |
4 miles |
2.5 kilometers |
1° by 2° or 3° |
4,580 to 8,669 |
| State Maps |
1:500,000 |
8 miles |
5 kilometers |
State area |
Varies |
| State Maps |
1:1,000,000 |
16 miles |
10 kilometers |
State area |
Varies |
| U.S. sectional Maps |
1:2,000,000 |
32 miles |
20 kilometers |
State groups |
Varies |
| Antarctica Maps |
1:250,000 |
4 miles |
2.5 kilometers |
1° by 3° to 15° |
4,089 to 8,336 |
| Antarctica Maps |
1:500,000 |
8 miles |
5 kilometers |
2° by 7.5° |
28,174 to 30,462 |
Hydrologic
Regions
The United States is divided into hydrologic units which includes 21 major
geographic areas, or regions. 
These are the major river drainage basins in the United States. These
geographic areas contain either the drainage area of a major river, such as
the Missouri region, or the combined drainage areas of a series of rivers,
such as the Texas-Gulf region, which includes a number of rivers draining into
the Gulf of Mexico. Eighteen of the regions occupy the land areaof the
conterminous United States. Alaska is region 19, the Hawaii Islands constitute
region 20, and Puerto Rico and other outlying Caribbean areas are region 21.
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