Introduction to Geographic Information Systems in Forest Resources
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Introduction to Geographic Information Systems in Forest Resources |
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Discussion
Every GIS uses certain types of data sources or file formats. Because most GIS software applications have been developed in competition with each other, many GIS software brands do not use other vendors' data types. Regardless of the brand name on your GIS, it will have limitations with using data of a particular types. In order to make effective use of your GIS, you will need to know what types of data are available for use within your software. Efforts are underway to make data sources that can be used across GIS platforms. See the Open GIS Consortium for details on this effort.
And regardless of the brand of GIS software, it will have specific methods of making the data useful. For some GIS packages, the interface is a command line, and data sources are accessed via commands and arguments. For other GIS software, such as ArcView, the data are accessible via a GUI.
This section describes what data sources are available to ArcView, and how to load them in for use within an ArcView project.
A theme is a set of geographic features representing a class of features that exists in the world. For example, a single theme may represent a group of sampling points, a transportation network, or a group of forest stands.
A theme should not simultaneously represent more than one class of features, although it may represent several subclasses (i.e., a theme should not represent roads and streams, but may represent roads, railroads, and trails as a transportation network).
A project can contain many views, and each view can contain many themes. Generally, the themes within a single view represent data for a common area of the earth. For example, you may have a single view containing themes that represent roads, streams, and forest stands for a single national forest, but it would be unlikely (or not very useful) to have a single view containing roads for the Mount Baker-Snoqualmie National Forest and streams for the Gifford Pinchot National Forest.
Theme features represent objects that exist on the ground. The representation of real-world features is made by using points, lines, polygons, or raster cells.
As was discussed in the Spatial Data Model, vector data represent real-world features as
Theme features are symbolized according to the feature class of which they are members.
Certain disciplines expect to use formal symbols and colors for depiction of ground conditions (e.g., urban planners use certain colors for different densities of residential housing areas). Colors are also often used to indicate intensity (e.g., blue is cool or safe, while red is hot or unsafe). Line thickness may be used to denote importance of features (e.g., Interstate highways vs. forest roads). Sizes of point symbols can also reflect importance (e.g., major cities vs. rural towns).
When themes are added to views, ArcView automatically assigns random colors and simple patterns and symbols. The user may alter the symbology of theme features at any time. The next topic, discussing Display of Themes, will focus closely on feature symbology.
Spatial data sources for themes
Spatial data for use in a GIS is composed of coordinate data (points, lines, polygons, and cells) coupled with associated attribute tables. Each coordinate feature is also represented by a record in an associated theme table. Several different data sources may be used in ArcView, which are listed below:
- ArcView Shapefiles
- Shapefiles are the preferred vector data source for use in ArcView.
- Shapefiles can be created within ArcView from any other supported vector data source.
- Shapefiles can also be created from scratch in ArcView by adding graphical primitives to newly created, empty shape files.
- Shapefiles are fully editable within ArcView (both the geometry and attribute data can be edited).
- Shapefiles can be only point or line or polygon.
- Shapefiles display rapidly within ArcView.
- ArcInfo Coverages
- Most of the data you will encounter will be ArcInfo coverages, because ArcInfo has been the leading GIS software for several years.
- Coverages can be composed of multiple feature types within a single data theme source (e.g., lines, polygons, label points, regions, routes, and annotations). However, to load multiple feature types from a single coverage data source, each feature type must be added as its own theme.
- Coverages must be converted to shape files before coordinate editing within ArcView.
- ArcInfo Map Librarian or ArcStorm Database Libraries
- Map libraries and ArcStorm databases are another ArcInfo data storage format available within ArcView. None of the data in this course will be obtained from these formats.
- Computer Aided Design (CAD) Data
- Before the development of GIS, CAD systems were often used to store geographic data. In engineering applications, CAD systems are still used in preference to GIS. A tremendous amount of data are available in CAD formats
- ArcView supports MicroStation DGN, AutoCAD DXF, and AutoCAD DWG formats.
- CAD data can be converted to shapefiles within ArcView.
- Spatial Database Engine (SDE) Data
- Another ESRI product, SDE, is beginning to come into wide and regular use. The SDE format allows GIS data to be stored completely within a relational database such as Oracle or SQL Server. SDE data are used for rapid access and client-server applications.
- SDE layers can be displayed and queried in ArcView and converted to shapefiles.
- We will have a small demonstration of SDE data sources.
Additional data sources for themes
In addition to the vector data sources named above, other data sources include raster data and tabular data. Many raster data sources can be used as spatial data sources within ArcView or other GIS software, but only if accompanied by a world file, which provides georeferencing information. Most raster data sets have minimal attribute data structures, one exception being ArcInfo grids, which we will deal with in detail later in the course. For more information on the structure of a world file, see the ArcInfo documentation for Georeferencing Images in ArcInfo.
Views are used to display, manipulate, and analyze the coordinate parts of spatial data. To create a view, open a new or existing project. If you open a new project, it will be called Untitled by default. Once you save a project, it will be called by the name you give it..
Then either double-click on the Views icon, or single-click the Views icon and click the New button. An empty view will open.
By default, the new view will be called ViewN (where N is the number of the view that has been created). The View name can be changed by altering Properties in the View menu.
Adding a feature theme to a view
Once a view is created, you can add spatial data themes to it. When themes are loaded into a view, the source data are not copied; instead, the project file contains references to the disk location of the theme source. In order to maintain the integrity of the project, it is important that you do not move files around the disk. If theme source files have moved, the ArcView project will not find them.
You can add feature (vector) themes and image themes to a view. If the CAD Reader extension is active, CAD data sources will be available as feature data sources. If the Spatial Analyst extension is loaded, you can also add grid themes to views. With the 3D Analyst extension, you can load TIN data sources. We will cover grid and TIN data sources later in the course.
Use the Add Theme button 
or choose View >
Add Theme from the menu.
Click on a data theme to select it. If you want to add more than 1 theme, hold the <Shift> key down as you click more than one source. Each separate data source that is added becomes its own theme in the view. You can add a single data theme multiple times to a view if you wish to display different subsets of the same theme with different symbology.
When the themes are added to the view, they are assigned a simple symbol in a random color. You can change the symbology of theme displays by using the legend editor. Themes are turned off when they are added to the view. In order to see the themes, you need to check the theme box on in the view Table of Contents.
Data sources with multiple feature types
Some data sources contain multiple feature types. A single ArcInfo data source, for example, may contain label polygon, arc, label point, annotation, node, region, and route features. Multi-feature data sources are indicated in the Add Theme dialog with a folder icon next to the data source name. To view which data types are in these, single-click on the folder icon. The folder will open, and a list of individual feature types will be listed. Any of the listed themes can be added to the project. To close the folder, click again on the open folder icon.
Each of the individual feature data sources within the folder has a particular feature type and meaning in relationship to the concept of the data model as well as the particular data set.
Adding an image theme to a view
Image themes are added to views in the same way that feature themes are added. The only difference is that you will need to select Image Data Source from the Data Source Types dropdown list.
Image data are generally continuous across the spatial extent, and should be drawn first (place them at the bottom of the Table of Contents).
If the Spatial Analyst extension is not active, grid data sources will be available as image data sources. When a grid is loaded as an image, it can only be used for display, and will not be useable for analysis.
Themes from image data sources do not have theme tables; they are simply added to a view for display purposes.
If you have an ASCII file containing a series of records which include x,y coordinates, the data file can be added to the project as a table, and an event theme can be created from the coordinates. If there are other fields of data in the ASCII file, these will automatically be added to the theme table for the new point theme.
The event theme is a point theme, whose points are placed based on their x,y coordinates. Additional data from the original ASCII file can be added to the event theme attribute table via a join or link.
Theme tables exist for every feature (i.e., point, line, or polygon) theme added to a view. The records in the theme table have a one-to-one correspondence to the spatial features in the coordinate part of the theme. The records describe characteristics of the spatial features.
Because the theme tables are so closely associated with the spatial features,
ArcView automatically loads the theme attribute tables to the project, whether
the theme table is open or not. If a theme is active in the view (its legend
appears raised or chiseled in the view Table of Contents), you can open the
theme table by clicking on the Open Theme Table button 
. Each active theme's
attribute table will be opened.
Theme tables will contain one or more fields, depending on their data source:
As with any computer application, you should get in the habit of saving your projects very frequently. ArcView is sometimes somewhat unstable, and recreating a project from scratch can be a time-consuming, tedious, frustrating task.
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Syllabus