Introduction to Geographic Information Systems in Forest Resources
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Getting Data into ArcGIS


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 older GIS packages, the interface is a command line, and data sources are accessed via commands and arguments. For other GIS software, such as ArcGIS, the data are accessible via a GUI.

This section describes what data sources are available to ArcGIS, and how to load them in for use within an ArcGIS project.

Layer feature types
Spatial data sources for layers

Additional data sources for layers

Creating a data frame

Adding a feature layer to a data frame

Adding a layer from x,y coordinates

Layer tables

Saving a project

Layer feature types

A layer is a set of similar features representing a class of features that exists in the world. For example, a single layer may represent a group of surface water sampling points, a transportation network (lines), a group of forest stands (polygons), or a digital elevation model (raster). A layer is not actually a data source, but is an object within the GIS that represents a data source that may be present on a local or networked drive, or the layer data source may exist on an internet mapping server.

A layer should not simultaneously represent more than one class of features, although it may represent several subclasses (i.e., a layer should not represent roads and streams, but may represent roads, railroads, and trails as a transportation network).

A map document can contain many data frames, and each data frame can contain many layers. Generally, the layers within a single data frame represent data for a common area of the earth. For example, you may have a single data frame containing layers 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 data frame containing roads for the Mount Baker-Snoqualmie National Forest and streams for the Gifford Pinchot National Forest. If you have layers that do not represent the same place on the earth, the layers will not overlap.

Layer features represent objects that exist in the real world. 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

Raster data are usually used to represent phenomena that change continuously across space, such as pH, wind speed, or elevation.

Layer 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. local city streets). Sizes of point symbols can also reflect importance (e.g., major cities vs. rural towns).

When layers are added to data frames, ArcGIS automatically assigns random colors and simple patterns and symbols. The user may alter the symbology of layer features at any time. The next topic, discussing Display of Layers, will focus closely on feature symbology.

Spatial data sources for layers

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 attribute table. Several different data sources may be used in ArcGIS, which are listed below:

Additional data sources for layers

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 ArcGIS or other GIS software, but only if accompanied by a world file, which provides georeferencing information. Most raster datasets 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.

Creating a map document & data frame

ArcMap documents are used mainly to display, manipulate, and analyze the coordinate parts of spatial data (though attribute analysis and manipulations are possible as well). To create a map document, open a new or ArcMap session. If you open a new document, it will be called Untitled by default. Once you save a project, it will be called by the name you give it.

A map document always contains at least one data frame.

By default, the new data frame will be called Layers. The data frame name can be changed by altering Properties for the layer.

Adding a feature layer to a data frame

Once a data frame is created, you can add spatial data layers to it. When layers are loaded into a data frame, the source data are not copied; instead, the map document file contains references to the disk location of the layer source. In order to maintain the integrity of the project, it is important that you do not move files around the disk. If layer source files have moved, the ArcGIS project will not find them.

You can add feature (vector) layers and image layers to a data frame. 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 layers to data frames. 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 Layer button or choose File > Add Data from the menu. This opens the Add Data dialog. Any data sources that can be added to the ArcMap document will be listed. Each different data source has an icon indicating its data type (point, line, polygon, multi-layer, raster, table, etc.). As you become more familiar with ArcGIS you will be able to tell at a glance the data type for each data source.

Click on a data layer to select it. If you want to add more than 1 layer, hold the <CTRL> key down as you click more than one data source. Each separate data source that is added becomes its own layer in the data frame. You can add a single data layer multiple times to a data frame if you wish to display different instances or subsets of the same layer with different symbology.

When the layers are added to the data frame, they are usually assigned a simple symbol in a random color. You can change the symbology of layer displays by using the legend editor. Layers are turned off when they are added to the data frame. In order to see the layers, you need to check the layer box on in the data frame 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, or a single geodatabase may contain several different data sources. Multi-feature data sources are indicated in the Add Layer dialog with a special icon indicating the presence of multiple layers within the data source next to the data source name. To view which data types are in these, double-click the icon. The data source will open, and a list of individual feature classes contained within the data source will be listed. Any of the listed layers can be added to the project.

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 dataset. You can see that each feature class has an icon indicating a single layer.

Adding a layer from x,y coordinates

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 layer can be created from the coordinates. If there are other fields of data in the ASCII file, these will automatically be added to the attribute table for the new point layer.

The event layer is a point layer, whose points are placed based on their x,y coordinates. Additional data from the original ASCII file can be added to the event layer attribute table via a join or link.

Attribute tables

Layer tables exist for every feature class (i.e., point, line, or polygon) layer added to a data frame. The records in the attribute table have a one-to-one correspondence to the spatial features in the coordinate part of the layer. The records describe characteristics of each of the spatial features.

Because the attribute tables are so closely associated with the spatial features, ArcGIS automatically loads the layer attribute tables to the project, whether the attribute table is open or not. You can open the attribute table for a layer by right-clicking the layer name and selecting Open Attribute Table.

Layer tables will contain one or more fields, depending on their data source:

Saving a map document

When you have completed an ArcGIS session, or are going to take a break, you should save your project. When a project is saved, all the documents in the project are saved. This updates the project.mxd file (where project is the name of your map document file). The map document file specifies source data file location references, window positions, feature and record selections, joins, links, colors, and symbols. In essence, saving a project captures the current state of the map document when the save is made. When you reopen a map document, it will be in the same state as when you saved it

As with any computer application, you should get in the habit of saving your projects very frequently. ArcGIS is sometimes unstable, and recreating a project from scratch can be a time-consuming, tedious, frustrating task.


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