Satellite imagery

Digital imagery can greatly enhance a GIS. Part of the raster data types, imagery is a powerful visual aid and serves as a source of derivative information such as planimetrics and classification schemes to derive such information as land use or vegetation. If your GIS covers a particularly large area, aerial imagery may not be a practical or economical choice. Satellite imagery is often the preferred choice of imagery for larger regions. More and more choices of satellite imagery are becoming available and the cost associated with its purchase is dropping.

Many of the satellite imagery can be purchased directly from the associated agencies. In addition, there are online sites that also specialize in satellite imagery sales. Plangraphics is a reseller of Space Imaging’s IKONOS imagery. EOMOnline sells several imagery such as Landsat 5, IRS Imagery and Landsat 7. Terraserver is also a well-known source of satellite imagery.

Satellite Imagery Terms

Hyperspectral imagery

Useful for classifying material types on the Earth’s surface – beneficial in agriculture and forestry management, mineral exploration, environmental monitoring and national security activities.

Interval

Amount of time that passes before the satellite scans the same point of the globe.

Multispectral

Two or more images taken simultaneously, but each image taken in a different part of the electromagnetic spectrum.

Panchromatic

Imagery taken of all wavelengths within the visible spectrum, though not uniformly.

Orbit

The act of circling the earth. The type of orbit describes the path the satellite takes as it circles the earth.

Resolution

The amount of ground covered in one pixel of the image. For example an image with one meter resolution means that each pixel in the image represents one square meter on the ground. Click here for examples of imagery at different resolutions.

Sun-synchronous

An orbit that always passes over the earth at the same local sun time.

Swath

Amount of ground covered lengthwise in the passing of the satellite.

Flash Earth

Flash Earth is an experimental application that loads in various imagery from some of the major online mapping applications.  Created by Paul Neave, Flash Earth (as the name suggests) is a Flash-based viewing application for exploring the different satellite imagery available from Google Maps, Microsoft Virtual Earth, Yahoo! Maps, Ask.com, Open Layers and NASA Terra.  While the functionality of this application is limited to a basic address search and the typical pan and zoom, it is useful for comparing the different aerial resolutions between these online mapping services.

Moderate-Resolution Satellite Imagery: USGS Open Report

The USGS has issued an Open Report that looks at the uses and users of Moderate-resolution imagery (MRI), particularly Landsat.  The report is the result of a survey of U.S. based MRI by social scientists at the USGS Fort Collins Science Center.  The study aimed to understand who the users are and what the benefits are of the use of MRI with a focus on Landsat use.

As defined by the report, MRI is imagery that meets four main criteria: covers a relatively large area of landmass (more than 60km),  has a spatial resolution between 5 and 120 meters per image pixel, has repetitive coverage, and  has measurements from several portions of the electromagnetic spectrum.  The USGS supplies a significant amount of MRI through the operation of Landsat 5 and Landsat 7 satellites.  Landsat imagery dating back to 1972 is made available through the Earth Resources Observation and Science (EROS) Center.  Imagery is available with no user restriction or costs and, as a result, is a popular choice for MRI among users.

Over 2,500 users responded to the survey.  Interestingly, more than half were not members of “any type of remote sensing or GIS organization and, during the snowball sampling, almost 35 percent indicated they did not know any other users of MRI.”  The survey found that the average Landsat user was “male, white, 47 years old, and highly educated.”  Additionally, “[t]hree-quarters of the users were male, more than 90 percent were white, more than 80 percent were between 30 and 59 years old, and two-thirds had masters degrees or above.”

The survey results further dissect what types of organizations use MRI and for what types of applications.  There are several sections that break down the how the imagery is used, how that use has changed over time, and a look at the impact of having a no cost data policy.

Read the report:

The Users, Uses, and Value of Landsat and Other Moderate-Resolution Satellite Imagery in the United States—Executive Report (PDF) – U.S. Geological Survey Open-File Report 2011–1031, 42 p.
Authors: Holly M. Miller, Natalie R. Sexton, Lynne Koontz, John Loomis, Stephen R. Koontz, and Caroline Herman

2001 Inventory of Satellite imagery

Listed here are some of the most prominent sources of currently available satellite imagery and future satellite launches based on an inventory performed in 2001. This guide is meant to be a launching point for searching for and acquiring information about available satellite imagery. Individual choice of imagery will depends on many factors such as resolution, image type and cost.

*one-meter fused panchromatic and multispectral resolution.