FIELD TO FINISH AERIAL SURVEY

FIELD NOTES: TEMP. 67 Degrees

sunny

10-12 sw

Four band imagery is multispectral, which means that it is collected from several parts of the electromagnetic spectrum. The spectrum is the entire range of light radiation, from gamma rays to radio waves, including X-rays, microwaves, and visible light. Four band imagery, when delivered to a customer, typically contains red, green, blue, and near infrared bands. Only three bands can be viewed at one time in most software applications in use at present. A GIS user can choose to display an image as either natural color (red, green, and blue bands) or color infrared (infrared, red, and green bands).

What is color infrared imagery?

Color infrared (CIR) film is a three layer product, with individual layers being sensitive to red, green, and near infrared wavelengths. Near infrared (NIR) wavelengths are slightly longer than red, and they are outside of the range visible to the human eye. Blue wavelengths, a part of natural color film, are filtered out of CIR.

Digital imagery acquisition uses charge coupled devices (CCDs) which are sensitive to different wavelengths in the electromagnetic spectrum. All four bands are routinely collected in digital acquisition. A CIR product can be created from the raw digital data, or all four bands can be delivered to the customer.

The user must have software which will recognize all four bands. The software will usually have some type of interface where the band assignments can be changed.

If an image is created with the red (wavelength) band as band 1, green as band 2, blue as band 3, and near infrared as band 4, a natural color display on the computer screen would be set up with the red (display) channel as band 1 (red), green channel as band 2 (green), and blue channel as band 3 (blue). CIR would be set up with the red channel as band 4 (NIR), the green channel as band 1 (red) and the blue channel as band 2 (green). Band 3 (blue) is omitted.

CIR film was developed during World War II by Eastman Kodak to assist in detecting camouflage, because it can distinguish a plant cover from other materials. It has since been used for many other applications involving vegetation; the primary purpose being to monitor the health of crops or forests. Trained analysts can sometimes distinguish different plant species from their tone in the image, known as the “spectral signature.”

What is the advantage of CIR?

Color infrared is especially useful because the internal cell structure of healthy plants reflects near infrared wavelengths. Chlorophyll in plants reflects green wavelengths; this is why healthy plants appear green to humans. Consequently, the reflected infrared is more reliable in monitoring plant health than the reflected green wavelengths.

CIR tends to penetrate atmospheric haze better than natural color, and it provides sharper imagery. This makes CIR especially valuable in areas of the country which are hot and humid in the summer.