Among the buzz words associated with precision agriculture technology is “remote sensing”.  Remote sensing has nothing to do with feeling for the channel surfer device between cushions of the Lazy Boy.  However, it could mean watching from your kitchen window for the UPS truck to deliver the replacement TV remote.  In precision agriculture remote sensing most often refers to viewing the earth’s surface from aircraft equipped with scanners or cameras, or from outer space via satellites.

Historical Remote Sensing
In 1858, just nineteen years after the camera was invented, a Frenchman flying his balloon over Paris recorded the first aerial photograph. In 1861 the U.S. Army used a hydrogen-filled balloon, called the “Intrepid” to take aerial pictures during the Civil War.  In 1909 Wilbur Wright used an airplane to photograph Centrocelli, Italy. Aerial photography then got a boost during World War I.

Jumping to 1972, the first Earth Resources Satellite known as Landsat 1 was launched containing multispectral scanners. Technology has been evolving creating special detectors and filters making it possible to view objects beyond what we see with the human eye.  The remote sensing data now available in the public market can be used as a tool for making land management decisions.  However it is important to understand the basic science of these products in order to make a good interpretation.

Why are Plants Green?
Objects that we perceive in the light are reflecting, absorbing, or transmitting energy from a source. The source we are mainly interested in is the sun, which propagates energy in a measurable form called electromagnetic energy.  The wavelength is a unit of measure for the electromagnetic field. This represents the distance from the bottom to the crest of the wave in graphical form.  The scale of wavelengths propagated by the sun varies in size proportional to their frequency and energy output for the entire spectrum. Ultra-violet rays that can be harmful to your skin are high energy and have short wavelengths with high frequency (0.3-0.4 um micrometers). Infrared rays have much lower energy and longer wavelengths with low frequency (0.7-14 um).   The spectrum of energy that we see, visible light, is between those two and is a very narrow range (0.4-0.7 um), compared to the entire scale of different wavelengths (gamma rays 0.0003um – radio waves >1,000,000um).  Within the visible (or VIS) range, wavelengths are perceived as three primary colors: blue (0.4-0.5 um), green (0.5-0.6 um) and red (0.6-0.7 um). 

A portion of the infrared from 0.7 to 0.9um, also known as near-infrared (or NIR) is important to us since we find that plants reflect light in this region.  The NIR region is much more pronounced than the VIS region of the spectrum.  The reason plants appear green to our eyes is that they absorb the red and blue VIS light to create energy.  Therefore plants are reflecting green.  However, when measuring the plant’s response to a much larger area of the spectrum, they are reflecting four times more NIR.  The amount of NIR reflected is directly related to the amount of water inside the leaf as well as the variety of plant. A healthy corn plant and one stressed by drought will both appear green to the eye, but will show up distinctly different in an infrared photo.

Wet and dry soils appear different because water absorbs NIR light in the soil. This difference also shows up in the visible range but is more prevalent in the NIR spectrum.

Seeing the Invisible in IR and NIR
In a black and white infrared image healthy plants will appear white or light gray, and wet spots will appear dark gray. In a color infrared image color is added to the shades of gray. These images are created when the visible red band is substituted for the infrared red band (the closest region where plants are reflecting light: 0.6-0.7um). The red visible band is then shifted to the green visible band and likewise the green visible band is shifted to the blue band.  The result is an image that has different shades of red from scarlet-crimson to pink representing plant health depending on species. 

What Can Remote Sensing Do for You?
You may already have some ideas of how you can use remote sensing imagery.  For example, aerial images of bare soil in VIS or IR can help in locating old tile lines by shooting a few days after a soaking rain. IR images may help you in variable rate application of herbicides, fertilizer, or irrigation water during the growing season. You may want to record drought damage for crop insurance purposes.

Where Can You Find Images?
If you want to see a picture of your farmstead, and you have access to the Internet, go to: http://terraserver.homeadvisor.msn.com/ and type in your address in the search menu. 

Visible and color infrared images are available through Landsat 7 satellite images available on the web at http://dmc.ohiolink.edu/GEO/LS7/. 

Call your county extension office to find out about aerial imaging services in your area. Some small companies provide aerial imaging and mapping to help with drainage and plant management decisions. Satellite imagery is available from several web sites including: http://www.earthscan.com/Corporate/Default.asp . 

If you want to learn more about remote sensing and view some interesting pictures, here are a few on-line tutorial sites:

http://asd-www.larc.nasa.gov/SCOOL/intro1.html

http://www.uswcl.ars.ag.gov/EPD/remsen/rsagintr.htm

http://www.geog.ouc.bc.ca/physgeog/contents/2e.html

http://rst.gsfc.nasa.gov/start.html

http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fundam/fundam_e.html

Most people who see their farm from an aerial view for the first time will notice something they haven’t seen before. It adds a new perspective. As remote sensing technology continues to improve and products become more affordable there will be more practical reasons to use this data as a land management tool. 

Phil Levison, research associate, can be reached at 614-688-5543, or levison.1@osu.edu. This column is provided by the OSU Department of Food, Agricultural and Biological Engineering.