Image data

An image is a graphic representation or description of an object that is typically produced by an optical or electronic device. Some common examples of image data include remotely sensed data, such as satellite data, scanned data, and photographs.

• Image data is a form of raster data where each grid-cell, or pixel, has a certain value depending on how the image was captured and what it represents. For example, if the image is a remotely sensed satellite image, each pixel represents light energy reflected from a portion of the Earth's surface. If, however, the image is a scanned document, each pixel represents a brightness value associated with a particular point on the document.
• With ArcView you can display image data and feature-based spatial data together in views. ArcView can display and print black and white, grayscale, pseudocolor and true color images. You can also include them in a layout.
• To add an image to a view, see Adding a theme to a view.
• To change the way an image looks and to derive additional information from an image you can edit an image's legend.
• Images are often used as the background to views, with spatial data being drawn on top of them. Themes representing images are therefore normally moved to the bottom of the view's Table of Contents, so that they are drawn on the view first.
• The main difference between image data and feature-based spatial data is that images do not contain attribute data about the features they show.

Feature Data

In ArcView, features are stored as vector data (entities -- points, lines, or polygons) and their attributes are stored in tables known as attribute tables. Each class of features (entity) is stored in a shapefile and has its own attribute table. Attribute tables contain one record for each feature of that class in the coverage.

• Points: Points represent features found at discrete locations, such as telephone poles, wells and mountain peaks.
• Arcs (lines): Arcs represent linear features such as streams, streets and contours.
• Polygons: Polygons represent areas enclosed by specific boundaries, such as countries, states, land parcels and soil types.

The Table of Contents

Each view has its own Table of Contents that lists the themes in the view. Like the table of contents of a book, you look at a view's Table of Contents to see what's in it. You can change how the table of contents looks by choosing TOC Style from the View menu. You also use the Table of Contents to control how the view is drawn.

The Table of Contents shows:

• The name of each theme in the view: Themes can be given any name. By default, a theme is named after the data source it represents, such as "COV143" or "Landuse". You can give themes longer, more descriptive names. See Changing a theme's name.

• The legend for each theme: A theme's legend shows the symbols and colors used to draw the theme. A theme may be drawn using one symbol, or a range of different symbols and colors may be used in order to classify the features in the theme. To learn more about how legends are used in thematic mapping, see Types of thematic maps.

• Whether a theme is on or off: Each theme has a check box to its left that indicates whether the theme is currently drawn in the view. You control which themes are drawn in your view by simply checking these boxes. See Turning themes on and off .

• The order the themes are drawn in: The theme at the top of the Table of Contents is drawn on top of those below it. Themes that form the background to your view are therefore at the bottom of the list. Simply drag themes up and down in the Table of Contents to change the order they are drawn in. See Changing the order themes are drawn in.

• Which themes are active: When you make themes active you choose which themes you wish to work with. When a theme is active it is highlighted in the Table of Contents. Simply click a theme's name or legend to make it active. To make more than one theme active, hold down SHIFT when you click on the themes. In this example, the Major highways theme is active:

• Most of the operations you can perform on a view work on the active theme(s). For example, when you select features on a view, features are only selected from the active theme(s).

• Which theme is editable: A dashed line around the theme's check box indicates that you are currently editing the features in the theme. Only themes based on a shapefile can be edited. See Editing a theme.

• Hiding the Table of Contents: To hide the Table of Contents, drag its right border all the way to the left. The view will redraw to fill the whole window. To show the Table of Contents again, drag its border back to the right again.

• Hiding a theme's legend: The Table of Contents normally shows the legend of each theme in the view. However, you can save space in the Table of Contents by hiding legends. This is especially useful when a view contains many themes. To hide a theme's legend, make the theme active and then choose Hide/Show Legend from the Theme menu. When a legend is hidden, the name of the theme and its check box remain visible.

Legend Editor

•  
  • Legend Type
  • Values Field or Classification Field &Classify
  • Color Scheme or Color Ramps
  • Advanced
  • Apply & Buttons

Palette Manager

•  
  • Fill Palette
  • Pen Palette
  • Marker Palette
  • Font Palette
  • Color Palette
  • Palette Manager

Shapefile

Shapefiles store feature data and are a simple, non-topological format for storing the geometric location (points, lines, or polygons with spatial address or geocode) and attribute (or descriptive) information of geographic features.

The shapefile format defines the geometry and attributes of geographically-referenced features in as many with specific file extensions that must be stored in the same project workspace. They are:

Required files

.shp - the file that stores the feature geometry.(points, lines, or polygons and geocodes)

.shx - the file that stores the index of the feature geometry.

.dbf - the dBASE file that stores the attribute information of features. (descriptors)

Optional files

• .sbn and .sbx - the files that store the spatial index of the features. These two files can be created in ArcView. See ArcView's on-line help for more information.
• .fbn and .fbx - the files that store the spatial index of the features for shapefiles that are read-only. Optional. These two files can be created in ArcView. See ArcView's on-line help for more information.
• .ain and .aih - the files that store the attribute index of the active fields in a table or a theme's attribute table. These two files can be created in ArcView. See ArcView's on-line help for more information.
• .prj - the file that stores the coordinate system information. This file can be created with and is used by the ArcView Projection Utility. It is not used by ArcView GIS Version 3.x or older versions, or by ARC/INFO versions previous to version 8.0.
• .xml - metadata for ArcInfo 8.0, for using shapefiles on the Internet.

Imagefile

Image types

Imagefiles store raster data and can be organized in a number of ways depending upon the particular image format. Typically, the image data file contains a header record that stores information about the image such as the number of rows and columns in the image, the number of bits per pixel (or raster), the color requirements and the georeferencing information. Following the image header is the actual pixel (or raster) data for the image. The internal organization of the image data is dependent upon the image format. Some formats contain only a single band of data, while others contain multiple bands.

The range of values the pixels in an image can assume depends upon the number of bits (binary digit) used to store pixel data. The more bits, the greater the range of possible values. For example, if one bit is used to store pixel values, the number of unique values that can be expressed is 2¹, or two. With eight bits, 2 or 256 unique values can be expressed.

There are four types of images:

• monochrome (one color usually black on white)
• pseudocolor
• grayscale (shades of black on white)
• true color/multiband