GIS For Dummies

Chapter 6: Keeping Track of Data Descriptions 101

When you establish a relational join, all the information in the first table is shared with all the information in the second table, which is much easier than trying to make a huge, complex, comprehensive table from scratch. It’s much easier to design small, individual, focused tables (that can be related to other small ones) than to create large, unwieldy ones. Using smaller tables also gives you flexibility if you need to add new types of data to your database. You simply add a new small table that contains a column common to other existing tables.

Relating tables with joins sounds easy, doesn’t it? Well, it is, and it also adds flexibility and robust search capabilities to your GIS database. But you do need to be sure that the tables have at least one column in common. The common column enables the join and allows the tables to share information.

Managing data in Vector GIS

ESRI ArcInfo is a classic model of how Vector GIS employed the power of the database management system to store and manage all attribute information. Figure 6-5 is a simple graphical description of how the system looks. One set of programs (the Arc side) contains the graphics portion of the database (the entities) and the other side (the Info side) contains the descriptions (the attributes).

Graphics

Attributes

Database management systems may look like the tables of a spreadsheet, but they’re actually much more complex, and they operate quite differently. For a system such as ArcInfo (or other Vector GIS that uses database management systems), the points, lines, and polygons all have descriptive information

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Searching with SQL in any GIS

Modern GIS software relies heavily on the idea of storing descriptive information in a set of tables, but not just any tables — database management system tables. The database management system contains a powerful set of query tools because, like any RDBMS, it employs the Structured Query Language (SQL). The database contains data organized in tables (relations), and the SQL searches these data.

SQL is a fairly standard computer language designed specifically for accessing, querying, and manipulating databases. It allows you to search for very specific information, retrieve it, delete or add to it, update it, and combine queries.

SQL searches database tables by using commands such as Select, And, Or,

Insert, Delete, Order By, and many more.

The following steps show how to retrieve a subset of data by using the SQL Select command:

1.Determine the subset of information that you want to retrieve from your GIS data.

Suppose you have a table of land attributes like the following (Land_ Ownership), and you need only the information related to the land use and ownership categories.

2.Structure a SQL SELECT command to specify the attributes that define the information you want to retrieve.

Because you are interested in only the land use and ownership categories, you specify those attributes and issue the following command

SELECT Land_Use_Type, Public_Private FROM Land_Ownership

where Land_Use Type and Public/Private are the attribute names and Land Ownership is the table name.

That command returns the following subset of the original table:

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The OODBMS has the same descriptive items that you normally find in a GIS implementation of the RDBMS. What gives this system its power and makes it a major innovation in GIS isn’t in the way it stores traditional descriptive data, but in that it also has a set of tables that go beyond describing the qualities of geographic features — what they are, how big they are, and so on — by also describing how those qualities translate into real geography.

Using object orientation to enhance descriptive information

Every geographic feature, whether it’s manmade or natural, has properties that affect its functionality. Superhighways have the ability to move more cars than two-lane roads, so superhighways are the better choice for constructing an interstate highway system. And some types of vegetation grow faster than others, so the fast-growers are often chosen for shelterbelts and ornamental use. These and the millions of other functional properties make the simple descriptions of content, color, size, shape, and so on less useful for decision-making. Through experience, you know how a feature’s properties affect its use and implementation.

Now, imagine you have that practical experience stored explicitly in your GIS tables. Experience is translated into a set of rules and behaviors that exist for each object (for example, each road type). The biggest advantage of object-oriented systems such as the ESRI geodatabase is that they add these behaviors to the stored objects. So, if you try to put geographic features into a database that violate the known behaviors of the object class they belong to, the software rejects them. Also, the behaviors often translate into how you can and can’t link different objects in your GIS.

Knowing the packaging descriptions for different objects

An enormous number of industries, organizations, and settings use GIS, which makes it an enormously powerful tool. But it also has the potential to be a rather large, even ungainly, piece of software because it tries to meet too many different user needs. The geodatabase steps in and allows for the creation of large, comprehensive sets of rules and conditions that largely depend on the types of data, conditions of use, and methods of geographicfeature interaction specific to each set.

106 Part II: Geography Goes Digital

In RDBMS terminology, a general data model that’s modified to address the specific data and query needs for a given application is called a schema. So, for example, you might have a general schema for vegetation analysis that you can modify for desert vegetation. ESRI and its software users have created an ever-increasing array of industry-specific schemas that they refer to as geodatabase models. ESRI intended not only to share these schemas, but also to use them as examples for other industries when developing their own schemas.

Because ESRI wanted to share the geodatabase, the institute has developed best practices that act as a guideline for developing these geodatabase models, regardless of content. These best practices indicate that each model should include the following:

A case study that implements the model as an example for new users.

An import template into which users can import their own data.

A white paper that explains the model.

A data-model poster that provides a graphic idea of how all the data fit together.

A set of tips and tricks for new users based on the case study.

(Optional) Web-based collaboration with other users.

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These geodatabase data models can get quite complex because of the number of geographic features that users must consider, the properties for each of those features, and the potential number of rules for interaction. The posters that accompany many of these existing models provide an excellent graphical view of their operation and organization. Figure 6-7 shows a small portion of a poster for the water utility model from Table 6-1.

Chapter 7

Managing Multiple Maps

In This Chapter

Knowing the importance of managing multiple maps

Figuring out what map-handling capabilities you need

Choosing a system for your needs

If a picture is worth a thousand words, a map is worth a million. With GIS, you can work with more than one map at the same time. So, GIS gives

you many millions of words at your fingertips. Every portion of the Earth, whether it’s natural or man-made, is very complex. Even if you ignore for a minute that every portion of land has depth, you can map many possible features, including houses, people, insects, plants, diseases, pollen, sidewalks, gas lines, salaries, streets, temperature, and pressure. The list is endless.

When mapped, each of these features provides a portion of the geography of a piece of land, and each set of similar features represents a separate theme. Within each theme, the explicit locations of each object, combined with the thousands of possible implicit spatial relationships, tells volumes. But, like any story, your land needs many characters — many themes. To get a complete picture, you need to have many mapped layers.

ESRI provides software, lessons, and technical support for K–12 Education around the world as a means of encouraging spatial thinking. The K–12 educators at ESRI call the map-layering process “making an Earth sandwich.” And, like an Earth sandwich, a GIS works only when it has more than one layer. The individual layers become useful only when you can extract information from each and compare and contrast that different information. To make this comparison, the software must be capable of storing, retrieving, editing, analyzing, and producing new data from all layers. In this chapter, I show you how the various types of GIS systems do all these data management tasks.