Chapter 8: Accessing Data With ArcView

Summary of principles

The "Voodoo principle": Constantly be aware of the distinction between data and how data are viewed.

	Interfaces to complex software are context-dependent: A GIS typically manages different views of data; Arcview's "documents" are an example. Expect the graphical user interface (GUI) to change according to how you are viewing the data.
	Maps are made of layers: Every GIS will support a map-like view in which you manage separate geographic data sources as "layers," "themes," or "coverages." The restrictions on what kind of information can appear in any such data source will vary from one GIS to another.
	Know what to expect of software: Expect any GIS to support basic operations for managing geographic data sources, such as adding and deleting them, making them visible or not, activating them as targets of future operations, reordering them, and changing their appearance.
	The principle of "radical empiricism": learn by experimenting. Systematic experimentation (using data you can afford to lose or corrupt!) will teach you much about how the software behaves. This, supplemented with visits to the help pages, can usually get you up to speed quickly.
	Pay attention to the clues: Software usually provides a lot of information. You just have to read it. Pay attention to your screen, especially to anything in message boxes and on any status lines.
	Know your formats: You need to know whether your data are in vector or raster format. Choosing a format (and, for vector data, choosing a geometric type: point, polyline, polygon) or, at least, knowing what formats are available, is an important part of designing a GIS project.
	Exploit the metadata facilities: Naming the elements of a complex software project, such as a GIS project, is important. If your software provides facilities to insert comments, notes, or other documentation, then exploit them.

Exercise 8a—adding themes to a view

The data for this exercise are in GTKAV/Data/Ch08.

Documents

A document is a way to view data. It does not “contain” data; it displays them. The distinction between data (as organized collections of numbers, strings, graphics, and so forth) and how the data are viewed is a fundamental one in computing. Many examples of this distinction exist in ArcView and will be pointed out during this course.

(The software you use may or may not reflect this distinction clearly. For example, the document you are reading was originally formatted in HTML, "hypertext markup language." HTML begins with the text and pictures--the data--and inserts additional characters (the "markup") into the text to describe how the text and pictures will be presented. Although the text and pictures reside in different files, and therefore are clearly distinguished, HTML interweaves the text and the markup, thereby potentially confusing the data and their presentation.

Many databases will more clearly segregate data from their formatting. For instance, a dBase file (you will learn about these in a future section) contains only the data. You can open a dBase file with any spreadsheet software and add formatting, such as italics, color, background color, and so on. Indeed, you can apply the formatting to a blank spreadsheet and paste a dBase file's contents into the cells, thereby applying the formatting to the data. However, you cannot save this formatting information back into the dBase file itself: there is no provision for that in the dBase file structure.)

ArcView manages five (or more) document types: Views, Tables, Charts, Layouts, and Scripts.

	To see a list of each document type, click once on its icon.
	To open a document, double-click on its name in the projects window.
	To create a new document, double-click on its icon.

Documents are displayed in child windows. These are windows confined within the main ArcView window. They will disappear whenever the ArcView window disappears.

Views

A view displays geographic data on a map-like display. The Table of Contents (TOC) to the left of every view lists the “themes” or “layers” available for display, shows the method of display (the “legend”), and provides mechanisms for organizing the display and interacting with the themes. The display itself occurs in the remainder of the view.

The Add Theme button displays a new theme in a view. It opens the “Add Theme” dialog:

There are several subtle techniques to learn:

ArcView distinguishes data source “types.” If the type (indicated at the lower left of the dialog) does not match your source data file(s), you simply will not see your data files listed in the dialog.

Every GIS can present geographic data sources in a map view. Different software may place different restrictions on what can appear in a data source. ArcView imposes perhaps the most severe restrictions: each source must be of a homogeneous geometric type, whether points, polylines (representing curves), polygons (representing features with spatial extent), or images. Moreover, the same kind of information must be associated with each feature in a theme. Other GISes may allow you to mix the geometric types of features within one theme, perhaps including points along with polygons in the same theme. An object-oriented GIS may also allow you to attach varying types of information to features within a single theme.

Some forms of geographic data contain multiple sources. For example, CAD maps contain sources of point, line, polygon, and text (annotation) data. The icon next to a source type shows you whether it contains one or multiple sources:

Single source; this contains material for one theme.

Multiple sources; this contains material for one or more themes. Single-click on this icon to view and select the separate sources.

	You can simultaneously select multiple sources in this dialog (and multiple objects in many other dialogs) by holding the shift key down while you select with the mouse. Unlike most Windows software, this does not select a range of sources—it merely adds each newly selected item to the collection of all selected items.
	Themes draw in the View from bottom to top, as shown in the TOC. Re-order themes by dragging their TOC entries up or down. Practice this—it’s a little tricky to drop a theme in the correct place.
	Many ArcView operations act only on a designated theme or set of themes in a view. These are called the “active” themes. They are distinguished by a slightly raised look in their TOC entries. Activate or de-activate themes with single mouse clicks in the TOC.

By the way, the things shown in a theme--points, polylines, or polygons, usually--are called "features." An image or grid theme shows "cells." Features are vector data whereas images and grids are raster data. The differences between vector and raster data impose limitations on how you can manipulate and analyze these data, and converting between the two formats can reduce precision, so it is usually important to know what form of data you have.

Ignore the “Libraries” radio button. It is used to access certain kinds of proprietary commercial data (ArcInfo map libraries).

Laboratory Exercises

Find the answers by guessing and experimenting.

How do you activate more than one theme at once? How do you make no themes active?

When you select a multiple source in the Add Theme dialog without first clicking on the folder icon to open it, what happens?

What rule does ArcView use to select colors for the theme symbols?

How do you make all of a theme’s features visible (“turn them on”)? Invisible (“turn them off”)?

Why do aerial photographs usually appear at the bottom of the TOC?

What determines when a button or menu item in the View graphical user interface (GUI) will be disabled or enabled?

How many themes will fit into a view?

What kinds of files appear as “feature sources” in the Add Theme dialog? As “image sources”?

Find out how to remove a theme from a view. What happens to the theme’s data source when you do this?

Find a simple and fast way to create a copy of a View (that is, a second version with exactly the same themes shown in exactly the same way). Does this copy all the sources for the themes?

How can you attach free-form text documentation to a theme? To a document? What good is that?

For more information, see the “Add Theme” topic in the ArcView help. Go to the “Related Topics” link at the bottom of the page.

Exercise 8b—Understanding Theme Tables

A GIS maintains information about the geographic data it displays. Usually, the “feature” sources have additional information for each feature. Table documents display this kind of additional information, or feature “attributes”.

You create and open the feature tables for all active themes by pressing the Open Theme Table button.

You select features in a view using the Select Feature tool.

You select feature records in the feature table using the Select tool.

Here are some things to watch out for:

	Soon there will be many themes and many feature tables (attribute tables) open in ArcView. It is possible to have several tables, all with the same name, and all apparently associated with the same theme (but only one will be). You will become confused about which tables correspond to which themes. Therefore, always access a theme’s feature table by activating the theme and pressing the Open Theme Table button.
	It is simple and easy to activate a window by clicking within it. However, that will cause any active tool to operate, which could ruin your work. Get in the habit of clicking only on window title bars or borders in order to activate them.
	Notice that only features in active themes are selected or acted on with any tool or button.
	Let the cursor stop over any button (on the top row of icons) or tool (on the second row of icons). The name of the tool should appear over the object. A longer description may appear on the status line. Software usually provides a lot of information—you simply have to read it on the screen. Simple, obvious advice—but rarely followed.

Laboratory Exercises

Find the answers by guessing and experimenting.

Suppose you have selected some features in a view with the Select Feature tool . How do you add features to that selection? Remove them from the selection?

Same question for selecting rows in a table with the Select tool .

What does the Promote button do to a table? Why is that at all useful?

Describe what these buttons do to a table. Write down their ArcView names:

What happens when you press the Open Theme Table button when more than one theme is active?

What does the theme table look like for an aerial photograph?

Where on your computer system do a theme’s attribute data reside? How can you find out?

What happens when you type data directly into a table? How does this behavior differ from a spreadsheet? Why?

Explore the Window menu. Experiment with everything except the “Show Symbol Window” item (that comes later).

Create several views and several tables. Look at the Window menu again. What recommendations would you make about how to name ArcView documents?

This page was last modified on March 11, 2004. Its formatting was simplified, a "Summary of Principles" section was added, additional comments (indented, in small type) were inserted, and minor changes were made in the text.