maps and globes
A MAP IS AN abstract representation of a selected set of features on or related to the surface of the Earth. The map reduces these selected Earth features to points, lines, and areas, using a number of visual resources such as size, shape, value, texture or pattern, color, orientation, and shape. Whereas aerial photographs and satellite images are realistic representations of the Earth, maps are an abstraction designed to focus and communicate specific information. The map is often drawn to scale, has a coordinate reference system to locate features, and is constructed on a flat medium such as paper or plastic film.
A globe, on the other hand, is a scaled representation of features on or related to the surface of the Earth and constructed on a three-dimensional surface such as a sphere. Globes are both objects of decoration and scientific value. Cartographers refer to a hypothetical “reference globe” as a scaled representation of the Earth that is then transformed point by point on a flat medium in an important process known as map projection. But globes are expensive to make, difficult to reproduce, inconvenient to store, and difficult to make measurements on. Maps created on a flat surface are not affected by these challenges.
The oldest known maps are preserved on clay tablets from the Babylonian period (2300 B.C.E.). The Greeks also possessed advanced mapping knowledge and the concept of the spherical Earth was well known to Greek philosophers such as Aristotle (350 B.C.E.). During the 12th century onward, maps became influenced by religious views, exploration endeavors, and political ambitions and were highly valued for their economic and military uses. A significant benefit of historic maps is that they provide important clues about the social and cartographic traditions of past societies and civilizations.
Modern maps became increasingly accurate and factual from the 17th century onward. Developments in astronomy and cartography had provided a scientific basis for preparing maps. An appreciation of maps and their uses begins with an understanding about how maps work. All maps are about spaces and places that are represented by shape, area, distance, direction, and location in a graphical medium. The surface of the Earth is not a flat plane, so a modification is required to transform the positions of places on the curved Earth to the flat sheet of the map so that distortions in shape, area, distance, direction, and location are minimized. This process is called map projection, and the transformation is governed by rigorous mathematical rules. The projection process takes the lines of latitude and longitude of the spherical Earth and arranges them on a flat plane as a uniform grid. These grids together with a scale that links the relative linear proportions of the spherical Earth and its representation on the flat plane allow the map space to be structured such that map properties can be determined to a high level of accuracy. Some examples of projections include the Mercator and Robinson projections.
The map SCALE determines the level of detail the map can show. Maps of a large scale show more detail with greater accuracy. As the map scale becomes progressively smaller, larger swaths of geographic areas are shown and so features on the map must be generalized to avoid congestion. The generalization procedure involves stages of simplification, selection, enlargement, displacement, and merging. Simplification involves the progressive collapsing of map features from area to line to point representation as scale decreases. As an example, a lake may be represented as an area at one scale but as a point at a smaller scale. Selection attempts to retain features that are important given the goals and uses of the map. However, some of these important features might not be clearly visible at the desired scale and so enlargement artificially distorts their dimensions to enhance visibility. Displacement shifts overlapping features so that they become separate elements and more clearly identifiable. Merging aggregates multiple features into simpler ones to correct map overload that can arise from too much detail.
The mapping process also includes the symbolization of the real world using a standardized graphical language. The symbols used have dimensions—point, lines, areas, volumes, and duration—and can be distributed in a discrete, continuous or sequential manner to communicate feature patterns. Lettering and text labels also form an important part of the feature encoding process. Information about each feature, such as type of road or population, is encoded using variations in nine graphic variables. The graphic variables are size, shape, orientation, color value, color intensity, level of grayness, texture, focus, and arrangement. It is these graphic variables together with strict rules for their use that allow the cartographer to encode and distinguish the diversity of features from the spherical real world into the flat map.
Generally, there are two categories of maps: topographic and thematic maps. The topographic map shows the outlines of selected natural and anthropogenic features, and is used mostly as a reference tool or as a base for integrating other types of existing or new information. The thematic map is used to communicate geographical notions such as population densities and land use. Thematic maps are especially important because they are used extensively in geographical information systems (GIS) as digital mapping outputs. Choropleth thematic maps use units such as counties or census tracts to display aggregate data about income and population for example. The area class thematic map shows units of constant attributes, such as vegetation. The isopleth thematic map shows an imaginary surface constructed by using lines to join points of equal value such as in a temperature and contour map.
Maps provide useful and concise spatial information in a meaningful manner and we use them directly or indirectly in many aspects of our daily lives. Navigating the roadways, finding new places, or simply reflecting on the aesthetics of the map are typical examples of how we use maps. Maps are also used for storing data, as a spatial index for labeling features or integrating multiple map sheets, and as a spatial data analysis tool for planning and decision making. A simple but powerful analysis tool is the map overlay process developed by Ian McHarg in which multiple map layers on transparent film are overlaid to identify regions of interest.
The globe is a three-dimensional representation of the Earth and has an entirely different but familiar construction than a flat map. The globe is composed of longitude lines (meridians), which run north-south, and latitude lines (parallels), which run east-west. The longitude lines converge at the North Pole in the Northern Hemisphere, and at the South Pole in the Southern Hemisphere. The equator divides the Earth into two hemispheres. All meridians and the equator are GREAT CIRCLES since they can form planes that cut the surface and pass through the center of the Earth. Small circles such as latitude lines form a plane that cuts the surface but does not pass through the center of the Earth. In this system of reference, geographic coordinates are measured in units of angular degrees. There are 360 degrees of longitude around the equator, with each meridian numbered from 0 to 180 degrees east and west such that the 180 degree meridian is on the opposite side of the Earth from Greenwich, England. There are 180 degrees of latitude from pole to pole, with the equator being 0 degrees and the north and south poles being 90 degrees. Each degree can be divided into 60 minutes and each minute is divided into 60 seconds. The north-south line is called the prime meridian, which has been set to pass through the Royal Observatory in Greenwich.
The longitude is measured as the angle between the point, the center of the earth and the prime meridian at the same latitude. West is positive and east is negative, meeting at 180 degrees at the international dateline. The east-west line follows the equator and is midway between the north and south poles. Degrees of latitude are measured as the angle between the point on the surface, the center of the earth, and a point on the equator at the same longitude. Here, the size of the cells is largest at the equator and the zones are square. At the poles the zones are smallest and mostly triangular.