Environmental Analysis 2000
Map Lab 1 (two pages), Scale, Latitude and Longitude
J. M. Stroh
Map review.
Topographic maps from government agencies serve as the base map for many purposes.
These maps represent both topography (landforms represented by contour line elevations)
and other themes like roads, public land survey boundaries, water features, urban areas,
distinctive landmarks like schools, towers, wells, cemeteries, and many other features
depending on scale.
To use a map correctly you must know its scale.
Verbal scale equals (distance on the map)/(distance on the ground) and usually stated as
something like “scale = 1:250,000.” Note that the map must be reproduced exactly as
originally scaled or the verbal scale will be wrong.
The bar scale has a line or bar drawn, scaled, and labeled in appropriate units. The bar
scale will shrink or expand with the map. All maps should have a bar scale.
Large scale maps show more detail than small scale maps because of the fractional
relationship given in verbal scale above.
Government maps usually come in scales of 1:250,000, 1:100,000 and 1:24,000. The
1:62,500 scale maps will not be reprinted or updated.
The geographic graticule, latitude and longitude.
Everyone should have familiarity with the geographic coordinate system. By convention
the earth has an angular “graticule” of N-S meridians of longitude which go through the
poles and parallels of latitude in the same plane as the equator. For a spherical earth the
meridians and the equator are great circles, arcs of a plane that goes through the center of
the earth. By convention the zero of longitude goes through the observatory at Greenwich
UK and the zero of latitude coincides with the equator. Latitude is measured north or
south of the equator degrees, and latitude east or west of Greenwich in degrees. For
numerical calculation south latitude and west longitude have negative values.
The geographic graticule has many advantages for maps. It applies everywhere on the
planet (universal). It has obvious applications for navigation. The Global Positioning
System uses latitude and longitude as its base mode. Many databases used in Geographic
Information Systems use latitude and longitude as the base geographic reference system.
It has some disadvantages too. The meridians converge, so length E-W is not a linear
proportion of angular change. This can be very severe on very small scale maps (whole
states, continents and world maps), especially with some map projections. Also, the
angular measure has several means of expression, most commonly:
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Degrees, minutes seconds with 360 degrees for a full circle, 60 minutes in each
degree, and 60 seconds in each minute. For angular measure less than one second use
either a fraction of a second (like ¼), or a decimal fraction of a second.
Whole and decimal degrees (usually referred to as decimal degrees).
Whole degrees with minutes and decimal fractions of a minute (uncommon).
Radian measure where the circumference of a circle = 2 radians. So 2 radians =
360 degrees. Spreadsheets and math software use radians in the “native mode” of
formulas.
Back to topographic and other base maps. The area represented on a map varies with
scale. For the topographic maps commonly used in the USA the small scale 1:250,000
maps usually cover 1 by 2 degrees; the 1:100,000 scale maps cover ½ by 1 degree; and
the 1:24,000 scale maps cover 7.5 minutes by 7.5 minutes of latitude and longitude.
In today’s exercise take each of the three maps of different scale (three scales), do the
following, or answer the questions. (Hint look around the edge of the map as well as
looking in the interior for information.) Show Dr. Stroh your results before leaving, and
keep these responses in your portfolio.
1. Convert –123 degrees, 34 minutes and 17 seconds to decimal degrees (yes a negative
number).
2. Convert –123.145567 degrees to degrees, minutes and the nearest whole second.
3. Find the maximum dimensions of the map in degrees or fractions of a degree
(example 15 minutes latitude by 15 minutes longitude).
4. Find who published the map.
5. What map projection was used for preparation of the map?
6. Does the map have special marks dividing the geographic graticule into smaller units
than those given on the corners? If yes what do they look like and what are the
coordinates (latitude-longitude) of these marks?
7. Find a point-like feature on the map and describe it in degrees, minutes seconds and
in decimal degrees. This might present a problem for you and points out the limitation
of the geographic graticule as a quick and easy way to define or locate points. Use
proportional distance for this.
What the heck is proportional distance? Example. The Bee Spring 7.5 minute quadrangle
in California has the following extents in latitude and longitude and in centimeters:
East boarder of map
Latitude extent = 7. 5 minutes Latitude dimension in cm =
57.6
from 36 45' N
South boarder of map Longitude extent = 7. 5
Longitude dimension in cm =
46.5
minutes from 118 00' W
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Bee Springs is 28.0 cm from the south boarder and 22.0 cm from the east boarder of the
map. If 57.6 cm is equivalent to 7.5 minutes of latitude then the measured length to Bee
Springs (28.0 cm is equal to 3.646 minutes of latitude (7.5 min/57.6 cm*28.0 cm = 3.646
min). Similarly 7.5 min/46.5 cm*22.0 cm = 3.548 min gives the minutes of longitude west
from the east boarder. The location of Bee Springs is 36.811 N and 118.0608 W (or 118.0608). A sketch will help you figure this handy method out.
8. Use the Xerox copy of a map provided in class to find the features, also given in
class, by latitude and longitude value. Work in pairs and do not mark up the maps.
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