Latitude and Longitude
It is important when using topographic maps to
have some way to express location. You may want to tell someone
where you are (i.e. help we are sinking at this location), or where
to go (meet me at this location), or even just what map to look at
(look at the map showing this location). In each
need to be able to express your location as precisely as possible.
There are many systems for expressing location. We will start by
looking at one you are already familiar with: latitude and
Latitude and longitude lines form a grid on the earths
surface. Latitude lines run east to west, longitude lines run north to
south. Latitude lines run parallel to the equator and measure the
distance north or south of the equator. Values for latitude range
from 0° at the equator to 90° N or 90°S at the poles. Longitude
lines run parallel to the Prime Meridian (arbitrarily set to run through
Greenwich, England) and measure distance east and west of this line.
Values of longitude range from zero degrees at the Prime Meridian to
180°E or 180°W.
The basic unit of latitude and longitude is the
degree (°), but degrees are a large unit so we often have to deal with
subdivisions of a degree. Sometimes we just use a decimal point,
such as 35.789°N . This format referred to as decimal degrees.
Decimal degrees are often found as an option on Global Position Systems
(GPS) or with online topographic maps, but decimal degrees are not used on
printed maps. On these topographic maps the latitude and longitude units are
expressed in degrees, minutes, and seconds. Each degree is subdivided
into 60 minutes('). Each minute is divided into 60 seconds('').
Note the similarity to units of time which makes these relationships easy to remember.
If we are interested in a general location we may just use
degrees. For more precision we specify minutes, or even
seconds. Note that we always need to specify the larger
unit. You can't specify your latitude or longitude with just
minutes or seconds. A coordinate such as 25' is meaningless unless
the degrees are also given, such as 45° 25'.
North-south running lines of longitude, and east
west running lines of latitude, combine to form a grid on the earth
surface. This grid is used to define the boarders of topographic
maps. Each rectangle in the grid (like the one shown in red) would
represent topographic map covering a certain area, referred to as a quadrangle. Each
quadrangle has a name based on a feature found within its
boarder. For example the Alliance quadrangle contains the city
of Alliance, Ohio.
The area covered by the quadrangle depends on
the spacing of the latitude and longitude lines used in the grid.
For maps of roughly the same size closer spaced lines produce maps that cover less area, but show more
detail. Lines that are spaced further apart produce maps that cover
much larger areas, but are not as detailed. Quadrangles are often
referred to by the spacing of these lines. For example we
distinguish 7½ minute quadrangles, that cover an area of 7½ minutes of
latitude by 7½ minutes longitude, from 15 minute quadrangles, which cover
an area of 15 minutes latitude by 15 minutes longitude. For standard
topographic maps each type of quadrangle is associated with a specific
map scale as shown in the table below.
|Coverage (Latitude x Longitude)
||7½' by 7½'
||15' x 15'
||30' x 30'
||60' x 60' (or 1° x 1°)
|1/2 degree by 1 degree*
||30' x 60' (or ½° x 1°)
|1 degree by 2 degree*
||1° x 2°
Maps commonly available in continental U.S. Other scales found
on older maps, though 15' maps are still used for Alaska.
Lets look at how we can determine location in terms of latitude and
longitude from a topographic map. The diagram below is a very
simplified version of a topographic map. While no features are
shown on the map, the marking for latitude and longitude found in the
margins of topographic maps are shown.
First lets determine what the numbers on the map
mean. The numbers on the left and right side of the map are
latitude. (As always we are assuming that north is to the top of the
screen). The numbers across the top and bottom of the map are
Lets determine what type of quadrangle this map
represents. Longitude on the left of the map is 118°, longitude on
the right side of the map is 117° 45'. The difference between these
two is 15'. Latitude of the top of the map is 40° 30' and the
latitude of the bottom of the map is 40° 15'. The difference
between these two is also 15'. This map, which covers an area that
is 15' x 15' would be referred to as a 15 minute map (see table above).
Notice that latitude and longitude is only fully written
in the corners of the map. Along the edges of the map only the
minutes are written. The map reader must realize that 20' latitude
on this map is actually 40° 20', because 20' lies in between 40° 15' and
We can also use latitude and longitude to give the
location of points on a map. Estimate the location of each of the
red letters on the map in terms of latitude and longitude. The
answers are below.
||40° 30' N
Point A is in the upper left corner of map so its
coordinates are the printed coordinates of this corner. The
one thing that needs to be added are the direction notations of each
coordinate. They are not printed on the map because it is
assumed you can tell what hemisphere you are in. When you are
asked for latitude and longitude you must add these letters.
It is easy to tell where you are by which direction the numbers for
latitude and longitude increase. Latitude increase going north
on this map so we are in the northern hemisphere. Longitude
increases going to the west, so this map is located west of the
||40° 25' N
||117° 55' W
||To determine the location of point B we
need to read across to the side of the map (to determine latitude)
and up to the top of the map to determine longitude. Point B lines
up with labeled tick marks labeled 25' and 55', but we know these
numbers are incomplete. looking at the corner of the map we see that
the latitude is 40° 25' N (north because of same argument for point
A) and the longitude is 117° 55' W.
||40° 20' N
||117° 50' W
Follow the same procedure as point B above.
||40° 27' 30" N
||117° 47' 30" W
||Point D does not line up directly with
tick marks. Instead we need to estimate its location. Point D looks
like it is half way between the 25' and 30' marks for latitude and
half way between the 45' and 50' marks for longitude. Half way
for each of these is 27'30" and 47'30". Remember one
half a minute is 30 seconds. Adding the remaining parts of the
coordinates as we did above give us the answer.
||40° 16' N
||117° 52' 30" W
||Solved the same as point D above.
The only difference is in estimating the minutes for latitude. Point
E seems to closer to 15' that to 30' so I have estimated it as
16'. This is only an estimate so the answer can vary, but it
should be greater than 15' and less than 17' 30"
Topographic Map Page
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