GIS For Dummies

Defining Your Map’s Contents

Before you create a map, whether it’s analog (hard copy) or digital (electronic), you first need to be aware of the geography that you want to include. What elements you put into the maps, and what maps you put into the GIS, ultimately determine the effectiveness of your analysis.

Each particular subject has many kinds of geography associated with it. For example, you might be working with economic geography, medical geography, transportation geography, urban geography, and physical geography — and literally hundreds more. Each geography relates to the types of data you want to use in your own projects. So, for example, suppose you’re using GIS to improve the productivity of your stores — you rely on these geographies of your area:

The economic geography: Points out economic factors — such as average income of shoppers in your area — that apply to your stores

The transportation geography: Gives you an understanding of the transportation systems that people use to move to and from your stores

The physical geography: Helps you understand what impact climate and weather have on what you sell and when

Your mission, should you choose to accept it, is to pick the most appropriate geographies and the most useful parts of those geographies to build your GIS and its component maps.

Choosing a theme to map

I have a very simple trick that I use to decide what map or maps I’m going to include in a GIS. First, like with any project, I define the final product. If you plan to use GIS to define pieces of land that you can use for a housing development, for example, your final map should show you a list, preferably

prioritized, of possible locations for your land purchase. To outline the specifications for the content of the final map, you ask yourself these basic questions:

How big a piece of land do you need?

How much are you willing to pay for the land?

What are your locational constraints? Where would you prefer the land to be?

What are the legal constraints — such as zoning and ownership — for developing your land?

What physical constraints on the land are you willing to accept?

What are the infrastructure needs for this development?

Your answers to the preceding questions suggest what data you need to put into the computer so that you can produce the final map you need:

You need a map that includes all the known land parcels so that the software can measure the size of each for you and find parcels that fit within your size requirement.

A map that includes the current sale price of the land allows the GIS to find parcels of land that are in your price range.

If you need land near hospitals, schools, the mountains, or other features, maps for each of these features can help you measure proximity to them.

Maps of zoning help you determine whether the land is zoned for residential development or whether you need to get a zoning variance to build there. You also want a land ownership map so that you know from whom you need to purchase the land, or even whether you can buy it at all.

You can map topography, soils, vegetation, bedrock, and other physical factors, and each factor provides important information that you can use to decide how much land manipulation you need to perform to develop there.

Maps that show availability of power, water, curb and gutter, and telecommunications systems help you understand if you have to pay to have these infrastructures put in.

By starting at the final product, you can break the final map down into the individual data component that are required to construct it. In the preceding example, you specify the details by asking questions about what you need to know to buy a parcel of land for development. You then translate your questions into the individual mapped features that you need to answer those questions.

Applying the methodology to any GIS project

If you don’t want to purchase land, or build homes, or anything related to real estate, you may be wondering if the example in the preceding section even applies to you. What if you’re trying to locate a new store, find the best place to establish a wildlife preserve, determine where crime is most likely to occur, or predict the risk of exposure to a hazardous waste cargo spill? Each scenario, no matter what the subject matter, still asks some question about location — where features are, where they could be, and/or where they should be.

Most major GIS decision-making operations deal with the locations or distributions of features. Sometimes, you use GIS to describe the impact that existing locations or distributions have, and other times, you use it to make decisions about where you want or need to put things such as businesses, public facilities, services, research facilities, personnel, or almost anything you can think of that occurs on the Earth. So, whatever your application, the methodology applies.

No matter what topic you’re mapping, you can best begin by specifying what you want as the final product. When you know what your final map should consist of, you can separate and categorize its relevant parts.

Breaking down the data you want to include

Each of the maps you select to include in a GIS database is composed of preselected geographic features, and each of those features has various geographic data associated with it. In some cases, you need all the data about a particular feature, and in other cases, you need only a selection.

Consider the housing development example from the section “Choosing a theme to map,” earlier in this chapter. You might find a map of the soils in your development area useful, but you really don’t care about the name of the soil nor whether it conforms to the latest soil survey classification

methods. You do want to know about the engineering properties of the soil, however. Can the soil support housing, or do you have to bring in soil from somewhere else? If you have to build a septic system, does the soil support that activity?

At this point in the model-creation process, you need to outline details of your project and ask the specific questions that tell you exactly what data you need from the thematic maps you’ve already chosen. The housing development project involves more than just land, and you need to outline all the pieces to end up with a complete database for your GIS. By creating this outline, you break the problem into neat chunks that you can more easily work with.

Say that you want to find a place to locate a bird sanctuary as part of your housing development, so you need to find a chunk of land that meets your project requirements of availability, size, price, location, accessibility, and usability. You have the following map themes at your disposal to help you find the right location:

Verifying your data’s characteristics

The preceding sections show you how to select appropriate map themes and then divide those themes into the individual data components that you need to match project details and answer your questions. So, you’ve identified the data components, but you need to know a little more about how those components are represented in a GIS as data.

Each digital thematic map is part of a larger group of digital maps that you use to answer your GIS question. Each map has features represented by points, lines, areas, and volumes that contain useful information for you. If you’re just starting your current GIS decision-making process, you need to know what data you have in the GIS database in the first place. And you also need to know the characteristics of those data, including

Level of data measurement: The data measurement available in the GIS can be nominal, ordinal, interval, or ratio. (See Chapter 3 for an explanation of these data measurements.)

Think of raster, or grid cell, GIS like a checkerboard or chessboard. Each square is a single grid cell, and each grid cell represents a portion of the Earth. When mapping, you assume that portions of the Earth are square for simplicity, and the GIS software can adjust the Earth coordinates for this assumption.

Raster GIS represents all forms of geographic features by using square grid cells, each of which represents a portion of your map. You can conceptualize each of these grid cells as a part of a checkerboard. The following bullets take you through the thought process for each feature type:

Points: In a conceptual model of geography, points have no dimensions (length or width). Of course, you have to take liberties to use grid cells to represent dimensionless points because grid cells do have both length and width. If you make these grid cells really, really small, like the dots on a TV screen, this whole idea is a bit easier to accept.

So, if you think about raster GIS as a checkerboard, a single grid cell is a point object (see Figure 4-2), such as a church, mosque, or syna-

gogue. Each different type of place of worship, such as churches, has a unique grid cell color on the raster map that’s associated with a different number inside the computer. So, unlike a checkerboard, you can have more than two colors of squares. Say that you use the number 1 to represent churches, 2 to represent mosques, and 3 to represent synagogues. These different types of features all have something in common — each represents a place of worship associated with a common theme or map layer. This common characteristic limits the number of different codes and colors that you might need for your map of point features.

Lines: In grid cell–based GIS, a string of grid cells represents a linear object (such as a road, a railroad, or a walking path). This string of cells can be lined up orthogonally (edge to edge), diagonally (along the corners), or some combination, depending on how curvy the feature is. Line objects are considered to have only one dimension (length), even though they usually have some width in the real world. So, when you use grid cells to represent lines, you have a bit of the same problem you have with point features — you have to use a symbol that has area to represent an object that doesn’t.

Just like with point features, on a raster map you use different colors or patterns for each line category (see Figure 4-3), and the computer associates those colors or patterns with particular numbers. So, when you input these, you might have the computer program assign roads a value of 1, railroads a value of 2, and walking paths a value of 3. All three of these features share a common theme — in this case, transportation.

Figure 4-3:

The line on the left is represented as grid cells on the right.

Figure 4-4:

The area feature on the left is represented as grid cells on the right.

Figure 4-5:

Surface features on the left are represented as grid cells on the right.

Raster GIS represents points by using a single grid cell, lines by using a line of grid cells, areas by using a group of grid cells, and surfaces by using groups of grid cells that have additional unique values.

Choosing a vector-based model

In a vector-based conceptual model, representing point, line, and area features is pretty straightforward. The vector data structure represents each point object by a single X and Y coordinate pair, each line by a set of two or more X and Y coordinate pairs, and each area by at least three lines that create a closed figure (the lines’ start and end points coincide to create closure, as shown in Figure 4-6).