I need better imagery!
Frequently we are asked about “aerial” imagery and sources for high resolution imagery. Here we will examine some aerial imagery: facts, myths, and sources.
Aerial imagery is the process of taking photos of the ground from an elevated position. This imagery can be collected using various methods including fixed-winged manned aircraft, balloons, kites, and a whole host of other methods. Another form of remotely capturing images taken of the earth’s surface, known as satellite imagery, has become a strong contender to true aerial imagery. Recent advances in satellite imagery has led to some confusion about what types of imagery we are actually viewing. For example, let’s take a look at the great tool all of us GIS folks and most of the public use frequently, Google Earth.
Often people viewing Google Earth are under the impression that they are looking at aerial imagery. This is not always the truth. In fact many of the viewing scales of Google Earth are using satellite-based imagery. Most GIS folks know the difference and are at least somewhat familiar with the various image sources Google Earth uses. However, sometimes this can be confusing as different locations on the planet use different forms of imagery for display in Google Earth, from Landsat imagery at some scales to high-resolution, manned flight aerial imagery. The sources of this imagery do not vary based on scale alone, they also vary based on location. But I digress, this is not an article examining Google Earth, it is an article examining remotely-acquired terrestrial imagery. But for the record, Google Earth provides a great example of the various forms of “aerial” images.
Before I proceed further, I must mention that often the best source for GIS-ready aerial imagery is Esri’s World Imagery Map Service. This is really a great source for imagery, so before you go digging around for recent imagery, consider using this service. Sometimes this image service may not be exactly what a GIS professional needs. Below I have outlined a few different options we GIS professionals have.
Let us first examine true aerial imaging. As the name implies, these are images taken from the air, more specifically, from within our atmosphere. So what forms of aerial imagery are publicly available? There are many sources available to GIS professionals for consumption in a GIS. Perhaps the most comprehensive publicly available (free) source of aerial image products is the USGS, particularly through the USGS EROS website. This site is a clearinghouse for aerial images collected over decades. The variety and availability images on this site is truly staggering. From 1960s aerial images all the way to the most recent NAPP imagery, this USGS website is an excellent resource for obtaining high quality imagery. The EROS website also includes satellite imagery.
Fig 1. Screen shot of the USGS Aerial Image Portal.
If you haven’t explored the imagery available on the USGS website follow the link above and explore. You may be surprised by what you find. While all these USGS datasets are great, we are often left wanting different or higher accuracy data. For map production even the high resolution datasets from USGS may meet their limitations when making small-scale or highly localized maps. Even the relatively high-resolution NAIP imagery’s 1m (GSD) resolution can appear blurry or inadequate.
So what are our alternatives? As mentioned before, satellite imagery is available in increasingly higher resolutions. With advances in technology satellite imagery now achieves accuracy as high as 0.31 meter. These images and the nature of a satellite vehicle’s acquisition process allows for vast image availability. There are several high-resolution satellite vehicles capturing RGB and multispectral images of the surface of the earth 24 hours a day, 365 days a year (or very close to it). A list of the common and publicly available satellites producing these images are detailed in Fig2. Many of these satellites’ image products are available for purchase and commercial use. If you really want a recent image for your project, or a series of photos for a particular project; some of the satellites can even be tasked to capture your particular image(s). Tasking of Satellites is well beyond the scope of this article but some of the links below will help guide you to additional resources for this processes.
Now that we know what types of imagery are available, how do we acquire these data? There are several options for purchasing data, including directly from an operator or through a clearinghouse. My personal experience has been that using a commercial clearinghouse is the most effective and easiest way to acquire these datasets. Satellite Imaging Corporation, established in the 1990s, has a very user-friendly system, but they are far from the only player in the image delivery game. A simple Google search for “high-resolution” yields a vast array of results.
These conventional sources of aerial imaging are what we currently have available and provide a great archive for GIS-ready historical images needed for long-term projects. While amazing in their own right, these sources do have their limitations. From atmospheric distortion to image frequency, these sources may leave the discerning GIS professional wanting.
I know all you progressive GIS folks are screaming “what about UAS!?!?” Fortunately, unmanned aircraft systems (UAS) technology is developing rapidly. While current regulations are preventing widespread adoption of UAS in the United States, recent developments indicate we may see the permitted use of small UAS within the year. Read Chase Fly’s upcoming post about UAS technology: Elecdata Receives 333 Exemption for Commercial UAS Operations.
Do you have your own experience and insight on aerial imagery? Contribute to the GIS community and share it below!
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USGS High resolution OrthoImagery site: https://lta.cr.usgs.gov/high_res_ortho
List of State GIS clearing houses: http://www.gisuser.com/content/view/16966/
Satellite Imaging Corp: http://www.satimagingcorp.com/
Institute for Mapping’s YouTube tutorial for finding Hi-Res Imagery (USGS Site): https://www.youtube.com/watch?v=BOUv2CoIOgg&feature=youtu.be
Digital Globe (Commercial Provider): http://www.digitalglobe.com/
Air Bus Defense & Space: http://www.astrium-geo.com/
Esri World Imagery Description: http://www.Esri.com/Esri-news/releases/13-1qtr/Esri-and-digitalglobe-expand-world-imagery-map
Directions Mag Article on Aerial Imagery (2 Part article): http://www.directionsmag.com/channels/remote-sensing/articles/the-origins-of-high-resolution-civilian-satellite-imaging-part-1-an-ov/303374