Imagery is a type of data that is useful for many GIS applications and is defined as any type of photograph. Normally photographs used for GIS projects consist of images gathered from a satellite or an aircraft. These can be photos that are printed on film or they can be taken with a digital camera and stored as digital images (NCSU Libraries). In addition, printed topographic maps that are scanned as digital images also constitute imagery.
In terms of the specific GIS data type, imagery is considered raster data. As such, all GIS images are made up of a grid of numbers that are arranged into rows and columns. Each grid is called a pixel and the pixels are assigned different numeric values. The different values assigned to the pixels typically represent quantities that identify things like elevation, slope gradient, or spectral brightness of an area (Verbyla, 1995). When all of the pixels are combined an image is formed.
Imagery is a data type that is extremely useful for GIS. It comes in many different types and it is able to show both large and small areas in varying levels of detail. This makes it versatile for a wide variety of different GIS projects and as such it is a common type of data for GIS projects.
Types of GIS Imagery
Imagery that is used in GIS can include aerial photos, satellite images, thermal images, digital elevation models (DEMs), scanned maps, land classification maps and surfaces created from a previous analysis that are saved as an image (Managing Imagery with ArcGIS 10). Hyperspectral, multispectral and panchromatic are general terms that describe imagery types. Hyperspectral imagery is imagery that is used for classifying different land types on the Earth (Dempsey, 2011). It is mostly used for agriculture, forestry management and other projects that examine the Earth’s physical landscape. Multispectral imagery is imagery that is made up of two or more images that are taken at the same type but in different portions of the electromagnetic spectrum.
Images gathered via satellite are some of the most commonly used images in GIS. There are two main types of scanning systems that obtain the various types of imagery used in GIS today (Verbyla, 1995). These are whiskbroom and pushbroom scanning. Whiskbroom scanning uses a scanning mirror that quickly sweeps back and forth over an area while taking an image. This type of scanning is used in the Landsat Thematic Mapper and Multispectral scanners to obtain satellite imagery. Pushbroom scanning is currently in use in the SPOT satellites and uses a row of silicon detectors to take images as the satellite flies over an area (Verbyla, 1995). Aerial photographs taken from planes are another method of obtaining imagery for use in GIS.
Accessing and Storing GIS Imagery
As with any type of GIS data being able to access the correct type imagery data needed is important to conducting a successful analysis. To access the plethora of imagery data that is available for use in GIS one has to understand three different techniques. These include direct access, access from a dynamic service and access from a static service (Visualizing and Analyzing Imagery with ArcGIS 10). Each method of access can provide a multitude of images.
Direct access involves using images that are stored on a personal computer, a local disk or server. Most often images obtained via direct access are stored on the hard drive of a personal computer. When images are shared among multiple users they are sometimes stored on a server or removable storage device that can be accessed on a computer. Access from a dynamic service involves specific data needed from an imagery provider. Images that are accessed from a dynamic service are often created when it is requested and as a result this method is time consuming and expensive. Image access from a static service involves the use of online via databases and web services. Government agencies such as the United States Geological Survey and websites produced by private companies such as ArcGIS.com and Google Maps are static image services. Images from such websites are predefined for specific areas and they are often digitally enhanced and georeferenced for use in GIS projects (Land Trust GIS, 2011).
Once the desired image is accessed via direct, dynamic or static access it is important to correctly store it so that it can be used effectively in a GIS application. The most common ways to store data that consists of many images is in a raster dataset or mosaic dataset. Images stored as a raster are images that are stored individually, while those stored as a mosaic dataset are images that are stored as a collection.
Next Up: Managing and Using GIS Imagery and Project Examples Using Imagery