GIS For Dummies

Chapter 2: Recognizing How Maps Show Information 31

GIS

Comparing thematic maps

The more detail, the more complete the information, and the more precisely located the geographic features, the better the analytical results you can get. You might think that the poorest quality map determines the quality of your analysis. This weakest link hypothesis makes sense on the surface, doesn’t it? In cases where each map layer is of equal importance, this hypothesis might be true. However, in most GIS analysis that you do, the different map layers aren’t equally important. Maps that are poorer in quality may still add value to your GIS as long as their lower accuracy does not affect the overall quality of the analysis.

Working with Projections and Datums

Except for members of the Flat Earth Society, everyone accepts that the Earth is roughly spherical. This spherical shape has some major drawbacks for the mapmaker who’s faced with producing a flat map that correctly represents the shapes, angles, distances, and sizes of objects on the Earth. In simple terms, the conversion from a sphere to a flat map is always going to result in some distortion. Peel an orange and try to flatten the peels on a table; now you have a visual idea of the problem that the mapmaker faces.

Picking the right projections

Figure 2-4 shows some of the many ways a map can look. Map projections — the process of converting the spherical Earth to a flat surface — come in many different types, from contiguous to interrupted, from those that look like photographs of the Earth to those placed on cones or cylinders. In fact, cartographers often refer to the three families of map projections as planar, conic, and cylindrical, which describe the surfaces onto which the surface of the Earth would be projected (hence the term projection) if a light bulb could be put in the center of the Earth.

Figure 2-4:

A map can represent a place in many different ways, each of which produces some form of distortion.

The curved surfaces of the Earth just don’t cooperate when you try to make them flat. The mapmaker has to figure out which of the map’s properties he or she most wants to preserve. For maps designed to measure distance, the mapmaker might choose to preserve distance on the map. Another map might be used to determine directions, which requires that shapes be pre-

served. Anyone trying to measure areas or changes in areas wants a map that preserves area.

The good news is that you don’t have to know all the gory details about how the mapmaker preserves certain properties to work with GIS — but you do want to know which properties are mostly preserved.

When working with GIS, pick the map projection that best represents the properties you want preserved when you create output maps from your analysis. For example, if your analysis involves changes in area, a map projection that preserves area is the best type to use to display your results.

Most high-end GIS software has the capability to convert back and forth from one projection to another. Most have more map projections than you’ll ever use. But every time you change from one projection to another, the software

Chapter 2: Recognizing How Maps Show Information 33

rounds off numbers and makes tiny errors in position. If you convert projections repeatedly, these tiny errors in position start to add up. Eventually, you can add enough error to have a negative impact on your analytical calculations.

Good projections depend on accurate datums

The impact of converting projections, like with many other GIS-related transformations, depends on scale. Small scale maps will be affected more than large scale maps. Having an accurate representation of distance and area measurements in any projected map depends on having accurate measurements of the spherical Earth. The science of geodesy deals specifically with these measurements.

Like with any measurement, you need to have a place to start, a baseline. In geodesy, such baselines for measuring the Earth are called datums. In simple terms, the datum is a model of the Earth’s shape that allows geodetic scientists and surveyors to accurately measure the Earth and place the origins and orientation of the coordinate systems.

Each of the hundreds of datums has different measurement units and methods for measuring the Earth. Some datums start by measuring the Earth’s diameter in different directions, others focus on flatness ratios (ratio between the polar and equatorial diameters), and still others are based on measuring the Earth’s circumference at different places Because each datum uses different methods and different models of the Earth (which are called reference ellipsoids), the maps associated with a datum depend on these properties. Table 2-2 gives you an idea of the possible datums you might encounter in your work. You can find a quite exhaustive (or is that exhausting?) list of datums online at

www.colorado.edu/geography/gcraft/notes/datum/datum_f.html

(continued)

Your GIS software needs to know what datum you’re using for each set of map data that you put into your database. Attaching your coordinates to the wrong datum can result in location and measurement errors. You’ll commonly begin with maps that use different datums. To make your GIS data from maps of the same area correspond to each other, they all need to be converted to a common datum.

When loading up your GIS, be sure to use the correct datum for each map source as you add it. Also, convert all your map data to a common datum when you work with more than one source map at a time.

Working with Coordinate Systems and Land Subdivisions

No matter what the theme, projection, or scale, you need to find your way around on your maps — and that’s what coordinate systems help you do. On a globe, you use the latitude-longitude system. On flat maps, you have to

Measuring the land

Coordinate systems help you measure geographic features, as well as find your way around and locate things on a map. One major feature you’ll likely encounter in your GIS work is land ownership, and you’ll find many systems that measure and record this feature. I describe a common one used in the United States to help you understand the relationship between land measurement and the coordinate system.

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Maps as questions, as well as answers

This land ownership recording system is based on the acre, which is 1/640 of a square mile (a section). Thirty-six sections are grouped together into townships and numbered in serpentine fashion from upper-right (section 1) to lower-right (section 36). A land records specialist can subdivide these chunks of land into halves and quarters so that he or she can describe the land holdings by these fractions. For example, the PLSS system describes the plot of land you own as something like NE1/4 SE1/4 N1/2 Sect. 23, T144, R35E.

Making Sense of Symbols

Many types of maps are available, and each type has a unique set of symbols. Symbols are lines, objects, or pictures on the map that represent real objects on the ground. With so many map types, each with its own set of symbols, you can easily become confused when trying to interpret the maps. You can make the process easier on yourself by looking at the dimensions and the types of measurement scales available for mapping and GIS operations, as described in the following sections.

Categorizing the space on a map

Categorizing the space represented on maps helps make symbology easier to understand. The types of space that occur in maps are

Points: Considered zero-dimensional because they take up no space at all. Points show up as symbols representing the locations of objects such as towns, wells, or churches. These symbols might be dots for towns, a tiny caricature of a well for wells, or a cross for Christian churches.

Lines: One-dimensional (measured by length). Lines might be streets, roads, or railroad tracks symbolized by different types or colors of lines on the map.

Areas: Two-dimensional (measured by length and width). Areas might be color patterns on city street maps that indicate different neighborhoods or suburbs, or they might be patches of blue to show the location of lakes.

Volumes: Three-dimensional (measured by length, width, and depth or height).Volume spaces are important when you want to identify something such as a buried coal seam. Volumes show up as area symbols on a map.

When it comes to map symbols, volumes and surfaces are treated pretty much the same. Unlike volumes, however, surfaces are thought to “blanket” something, rather than include what’s inside. For example, if you’re interested in the vegetation covering a mountain, you’re really interested in only the elevation’s surface. But, if you’re interested in how much coal is located underground, you need to know not just the surface of the ore seam, but also its whole volume because you’ll be extracting that volume.

Four types of objects are symbolized on your map — points (zerodimensional), lines (one-dimensional), areas (two-dimensional), and volumes (three-dimensional).