These are words to remember for anyone considering the “dos” and “don’ts” of ordering and using commercial high-resolution satellite imagery products to map submerged water features. I learned this important lesson during the 1960s while working on Texas A&M University’s Spacecraft Oceanography Project. After analyzing hundreds of color photographs of ocean and coastal areas taken from NASA’s Gemini and Apollo spacecraft, I realized that astronauts focused too much on photographing Earth’s rim. But even the downward-looking images were often useless due to excessive “sun glitter”—reflected sunlight from rough water. All of these problems were the result of undesirable viewing directions. The best pictures came from unmanned spacecraft taken at nadir. This experience indicates that what really matters is the combination of target azimuth and off-nadir angles, as well as the sun’s azimuth and elevation angles, as they define the critical angle called the Forward Scattering Angle (FSA).

Imagery Sources Today
Today, there are several U.S-based commercial, high-resolution Earth imaging systems: DigitalGlobe’s QuickBird (www.digitalglobe.com), OrbImage’s OrbView 3 (www.orbimage.com) and Space Imaging’s IKONOS (www.spaceimaging.com). They all collect multispectral (MS) imagery—blue light (BL), green light (GL), red light (RL) and near-infrared (NIR) light—with higher-resolution panchromatic (PAN) imagery. And they all have azimuth and nadir-angle agility. Selecting imagery wisely is essential to successfully mapping submerged water features.

For ocean and coastal area mapping purposes, the  2-degree angular field of view of these commercial cameras minimizes disparate effects of variations in atmospheric reflectance and attenuation, refractions at the air-water interface, rough-surface reflections and attenuation within the water column. This greatly increases the chances of successfully imaging underwater features.

All of the commercial cameras capture precise image data over a large dynamic range of digital numbers (DNs). This also is important for mapping ocean and submerged features, which are often dark and become darker with water depth.

Using published coefficients and algorithms, DNs from these imagers can be converted to quantitative estimates of the spectral radiances at the top of the atmosphere (TOA) and, more importantly, to apparent reflectances at the TOA or at the surface (see Author’s Note). This allows for consistent, quantitative handling of the image data after collection.

 

Be Careful What You Order
Be careful when ordering an image either as a new collection or from an image library. Unfortunately, image libraries don’t list all four of the angles you need to place a proper order. And placing constraints on target azimuth isn’t normally part of a new collection order. I suggest you speak with a customer service representative to help resolve these matters. For example, send an e-mail message with catalog IDs to get data about sun elevation and azimuth angles. If you have purchased imagery, target and sun angles are documented in the metadata file (see Author’s Note).


Be careful about the kinds of standard image products you order. Most are optimized for land mapping. For example, popular pan-sharpening products use the high-resolution PAN image, without alteration, to enhance MS images. Unfortunately, this leads to products that have brightness attributes that best meet the needs of land mapping, but that perform poorly for ocean and coastal mapping. The problem lies with the PAN image, which is dominated by the RL and NIR regions in which water reflectance is nearly zero. But PAN images can be corrected to reduce the RL and NR influences so a more appropriate image is made that matches the spatial properties of submerged objects in the BL and GL bands.

Physical Considerations
Willebrord van Roijen Snell discovered the Law of Refraction in 1621; Augustin Fresnel described the Law of Reflection in 1822. From these quantitative physical optics laws we know that off-vertical angles change dramatically as radiant energy travels from air to water and from water to air. In the visible region, the refractive index of water is about 1.33. Sunlight travels through the atmosphere, across the air-water interface and into water. The chart below shows the results of Snell’s Law of Refraction and how the refraction angle (in the water) varies as a function of the sun elevation angle. A more vertical pathway—i.e., refraction angle near zero—allows sunlight to penetrate water better to illuminate submerged objects.

The chart also shows the results of Fresnel’s Law of Reflection and how the transmittance (100 percent minus percent reflectance) of the air-water interface changes with sun elevation angle. Technically, there are two reflectances—one for vertical polarization and another for horizontal polarization. But sunlight is mostly un-polarized; therefore, the chart includes average transmittance values.

The transmittance is better than 94 percent when the sun elevation angle is larger than 30 degrees. But sunlight has to cross this interface twice—from the sun to the water and again from the water to the imager. A worse case is when the imager’s off-nadir angle is 30 degrees and the sun elevation angle is 30 degrees. In this case, the two-way transmittance would still be better than 92 percent. When the sun elevation angle is 30 degrees, the refraction angle is only 40.6 degrees.

Consider the 2.2 percent to 5.9 percent reflectance that occurs at the air-water interface. This amount of reflection may seem insignificant. If the interface is flat, then the reflected radiant energy travels in a direction not likely to be seen by the imager. But if the interface is rough, which is typically the case, then the reflected radiant energy is dispersed over a range of upward traveling angles as a function of the FSA. If the camera looks too closely to the sun’s reflection center off the water’s surface, the apparent reflectance will be much higher than 2 percent to 6 percent.

To successfully map submerged water features, I’ve found that the FSA should be greater than 40 degrees. For mapping oil slicks and surface features, however, the FSA should be less than 40 degrees. In clear water, BL penetrates water better than GL. But in more turbid water, GL penetrates better. In either case, neither NIR nor RL imagery will penetrate water well enough to show submerged water features.


Preferred Image Products
Image providers need to perform well-chosen post-collection processes that maximize the likelihood of the successful observation and quantification of underwater features. The dynamic ranges of brightness in image DNs over water areas often are limited to only a few hundred integers—out of perhaps thousands for the dynamic range over land.

Here are some guidelines for imagery intended for mapping submerged features:
• Purchase either MS data or an MS/PAN bundle as 16-bit integers—8-bit products suffer from the scale reductions that force water features into a low range for the overall range of 1 to 255. The darkest and brightness features will be truncated to fit the DNs into the smaller 1 to 255 range.

• Don’t order Dynamic Range Adjustment (DRA) products.

• Don’t buy pan-sharpened products. The NIR and RL parts of the spectrum dominate the PAN band. In most pan-sharpening processes, the overall intensity of the image is controlled by the intensity of the PAN image. For water, PAN intensities tend to be very dark. Thus, the PAN band will suppress spectral information in the BL and GL bands during sharpening. It would be far better to modify the PAN band by subtracting NIR and RL components before using the modified PAN image for sharpening purposes—probably for sharpening a natural color image.