2.4 Representing Geographic Features

Maps are a representation of the earth. Central to this representation is reducing the earth’s features of interest to a manageable size (i.e., map scale) and its transformation into a functional two-dimensional form (i.e., map projection). The choice of both map scale and, to a lesser extent, map projection will influence the content and shape of the map.

In addition to the objective decisions made by cartographers and GIS users regarding map scale and map, projections concern what to include and omit from the map. The purpose of a map will certainly guide some of these decisions, but other choices may be based on space limitations, map complexity, and desired accuracy. Furthermore, decisions about how to classify, simplify, or exaggerate features and how to symbolize objects of interest simultaneously fall under the realms of art and science. Moving from the real world to the world of maps is map abstraction. This process involves making choices about how to represent features. Regarding geographic information systems (GIS), we must be explicit, consistent, and precise in defining and describing geographical features of interest. Failure to be explicit, consistent, and precise will return incorrect, inconsistent, error-prone maps, analyses, and decisions based on such maps and GIS.

One of the most pressing environmental issues facing the world is deforestation. Deforestation refers to the reduction of forest area. This is an essential issue because it has implications for climate change, global warming, biodiversity, and the earth’s water balance, among other things. Unfortunately, in the last century, deforestation has increased at an alarming rate and is attributed to human activity. Therefore, mapping forests regularly with a GIS is a logical way to monitor deforestation and has the potential to inform policies regarding forest conservation efforts. Easy enough, so let us get started.

So, what exactly is a forest? How do we know where a forest begins and where it ends? How can natural wildfires be differentiated from those started by humans? Can a forest exist in a swamp or wetland? For that matter, what is the difference between a swamp and a wetland? Such questions are not trivial in the context of mapping and GIS. Consistent and precise definitions of features like forests or swamps increase the reliability and efficiency of maps, mapping, and analysis with GIS.

Discrete and Continuous Features

The world comprises various features or entities within maps, cartography, and GIS. Such entities include but are not restricted to fire hydrants, caves, roads, rivers, lakes, hills, valleys, oceans, and the occasional barn. Moreover, such features have a form, more precisely, a geometric form. For instance, fire hydrants and geysers are considered point-like features; rivers and streams are linear features; lakes, countries, and forests are areal features. Features can also be categorized as either discrete or continuous. Discrete features are well defined, easy to locate, measure, and count, and their edges or boundaries are readily defined. Examples of discrete features in a city include buildings, roads, traffic signals, and parks. Continuous features, on the other hand, are less well defined and exist across space. The most cited examples of continuous features are temperature and elevation. Changes in temperature and elevation tend to be gradual over relatively large areas.

Projected Changes in U.S. Temperatures by Mid-Century” by NOAA is licensed under the Creative Commons Attribution Public Domain.

Geographical features also have several characteristics, traits, or attributes that may or may not be attractive. For instance, to continue the deforestation example, determining whether a forest is a rainforest or whether a forest is in a protected park may be necessary. More general attributes may include measurements such as tree density per acre, average canopy height in meters, or proportions like percent palm trees or invasive species per hectare in the forest.

Notwithstanding the map’s or GIS project’s purpose, definitions of features must be clear and consistent. Similarly, it is essential that the attributes of features are also consistently defined, measured, and reported to generate accurate and valuable maps efficiently. Defining features and attributes of interest is often an iterative process of trial and error. Accommodating a feature with a particular geometric form and determining the feature type is central to map abstraction, facilitating mapping, and GIS application.

Map Content and Generalization

The shape and content of maps vary according to purpose, need, and resources, among other factors. What is familiar to most maps and those within a GIS is that they are graphical representations of reality. Put another way, various graphical symbols represent geographical features. Annotation or text is also commonly used on maps and facilitates map interpretation. Learning about map content and generalization is essential because they serve as the building blocks for spatial data used within a GIS.

Building upon the previous discussion about the geometric form of geographic features, maps typically rely on three geometric objects: point, line, or a polygon.

  • Point – the x and y coordinate of a feature. Geographic examples could include the location of capitals on a small-scale map, fire hydrants, or mountain peaks.
  • Line – two geographic points connected together. Geographic examples could include rivers or streets.
  • Polygon – a series of geographic points connected and enclosed. Geographic examples could include parcels, political boundaries, large bodies of water, or wildlife refuges.
GIS Data Layers Visualization” by USGS is licensed under the Creative Commons Attribution Public Domain.

Simple and complex maps can be made using these three simple geometric objects. Additionally, by changing the graphical characteristics of each object, an infinite number of mapping possibilities emerge. Such changes can be made to the respective size, shape, color, and patterns of points, lines, and polygons. For instance, different sized points can reflect variations in population size. Line color or line size (i.e., thickness) can denote volume or the amount of interaction between locations. Furthermore, assorted colors and shapes can be used to reflect different values of interest.

Complementing the graphical elements described previously is annotation or text. Annotation is used to identify geographic features, such as cities, states, bodies of water, or other points of interest. Like the graphical elements, text can be varied according to size, orientation, or color. There are also numerous text fonts and styles that are incorporated into maps. For example, bodies of water are often labeled in italics.

Another map element that deserves to be mentioned and combines graphics and text is the map legend or map key. A map legend provides users with information about how geographic information is represented graphically. Legends usually consist of a title that describes the map and the various symbols, colors, and patterns used on the map. Such information is often vital to the proper interpretation of a map.

As more features and graphical elements are put on a given map, the need to generalize such features arises. Map generalization involves resolving conflicts associated with too much detail, too many features, or too much information and data to map. Generalization can take several forms:

  • Simplification or symbolization of features for emphasis
  • Masking or displacement of detail to increase clarity or legibility
  • Selection of detail for inclusion or omission from the map
  • Exaggeration of features for emphasis

Determining which aspects of generalization to use is mostly a matter of personal preference, experience, map purpose, and trial and error. Though there are general guidelines about map generalization, there are no universal standards or requirements concerning the generalization of maps and mapping. At this point that cartographic and artistic license, prejudices and biases, and creativity and design sense—or lack thereof—emerge to shape the map.

Making a map and, more generally, mapping involves various decisions and choices. From selecting the appropriate map scale and projection to deciding which features to map and omit, mapping is a complex blend of art and science. Many historical maps are indeed viewed as works of art, and rightly so. Learning about maps’ scale, shape, and content increases our understanding of maps and deepens our appreciation of maps and map-making. This increased geographical awareness and appreciation of maps promote the sound and effective use and application of a GIS.

Image Maps

Image maps, in large part derived from satellites, are ubiquitous. Such maps can be found on the news, on the Internet, in your car, and on your mobile phone. Moreover, such images are in living color and exceedingly high resolution. Not long ago, such satellite image maps were the sole domain of meteorologists, local weather forecasters, and various government agencies. Public access to such images was limited to the evening news.

Making Perals in the Persian Gulf” by NASA Landsat Science is licensed under the Creative Commons Attribution Public Domain.

In conjunction with the commercialization of space flight, technological advances in imaging technology opened the door for companies like Mexar (which used to be called Digital Globe) to provide satellite imagery and maps to the masses at the turn of the twenty-first century. With online mapping services such as Google Earth providing free and user-friendly access to such images, a revolution in maps and mapping was born.

Image maps now provide a geographic context for nightly news stories worldwide, serve as a backdrop to local real estate searches and driving directions, and are also used for research purposes. The popularity and widespread use of such images speak not only to recent technological advances and innovations but also, more important, to the geographer in us all.

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Geographic Information Systems and Cartography Copyright © 2022 by adamdastrup is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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