Definitions

• Airphoto
• Black and White Infrared Film
• Color Infrared (CIR) imagery
• Mosaic
• Stereoscopic Viewing
• Orthophotos

Airphoto

An aerial photograph is a bird's eye image of a section of land, often taken from an airplane. The area of land covered in each photo depends on the plane's flight lines, rather than following township lines or roads.

A question people frequently ask about airphotos is "what is the difference between an aerial photo and a map?" One easily identifiable difference is that airphotos are actual images, rather than representational drawings of the ground. People often think that an aerial photo can be used like a road map, though this is not the case. A road map is an exact replica of what you might see on the ground. It can be used to, for example, measure the distance between two points.

Aerial photos, on the other hand, are similar to a picture that you might take with your own camera. Suppose you are standing on top of the Sears Tower, and you aim your camera downward to take a picture. The developed picture may not look directly down on the street below. You might, for example, see the sides of some buildings, rather than seeing only the tops. Therefore, you cannot accurately measure the distance between any two points on the street.

The same can happen with an airphoto. Though the camera in the airplane (or other airborne device) is directed towards the ground, the pictures may be taken at a slight angle, or tilt. Variations in topography also distort distances in photographs. This means that you cannot place a ruler on the photo and accurately measure the distance between two points, even if you know the scale. The only way to find distances in airphotos would be to use mathematical equations and\or measurement devices such as stereoplotters, orthophotoscopes and engineer scales that take any angle distortion into account.

The airphotos provided by the DNR are taken to assist DNR foresters in land management. The foresters have determined that the most helpful photos are black-and-white infrared or color infrared (CIR), taken when the leaves are on the trees, at a scale of 1:15,840. These photos are also available to the general public for purchase.

Black-and-White Infrared Film

DNR Forestry's airphotos prior to 2006 were taken with black-and-white infrared film. This type of film is sensitive to UV and visible energy, along with near-infrared energy (unlike black and white panchromatic film). Black and white infrared film helps to distinguish between, for example, deciduous and coniferous trees, as the conifers are generally darker.

Color Infrared (CIR) Imagery

DNR Forestry's airphotos taken since 2006 are taken with color infrared (CIR) film. Vegetation reflects more infrared than visible light, and this helps subtle differences in physical characteristics of species to show up as large differences on CIR film. CIR imagery presents a "false color" picture that combines infrared reflectance with green and red visible bands, enabling the user to see differing features of plant species and vegetation in particular. For example, needle foliage of conifers creates internal shadows and the leaves themselves reflect less infrared radiation than hardwoods. This gives them a darker appearance in the CIR than hardwoods such as oak and aspen. Furthermore, the amount of reflectance may also be used to interpret the relative health of vegetation. Stressed vegetation has lower reflectance, resulting in a more cyan color. Healthy vegetation is red. Hence, CIR is a very useful tool in indentifying tree species and forest cover types. (See Hershey and Befort - 1995, "Aerial Photo Guide to New England Forest Cover Types" - for more information on CIR interpretation to forest type.)

Mosaic

A number of contiguous aerial photos lined up side-by-side to provide a continuous image of a large area of land.

Stereoscopic Viewing

Stereoscopic viewing provides the third dimension of height. Think of what you see when you're in an airplane. You can see differences in the heights of buildings, trees and topography. The airphotos were taken with about 60% overlap in the flight line. Adjoining flight lines overlap each other by about 30%. This allows an image to appear on at least two consecutive photographs. This overlapping feature provides for viewing stereoscopically (in three-dimension) by aligning two consecutive photos and using a stereoscope. Enlarged airphotos are generally too large for this purpose.

If you are interested in being able to view photos in three-dimension, at least two consecutive airphotos must be purchased and viewed with a stereoscope.

1. Obtain a lens stereoscope (usually $5-$45), which looks like two magnifying glasses mounted in a mask that rests on a table.
2. Obtain two overlapping photos of the area which you want to view three-dimensionally.
3. Align the photos side by side and place the stereoscope over them.
4. Look through the stereoscope to see the image become three-dimensional.

Orthophotos

Unlike airphotos, orthophotos do not contain scale, tilt or relief distortions. Despite differences in terrain, orthophotos have only one scale, so they can be used like a map with the additional benefit of having an image of the ground, rather than the symbols normally used on a map. Find out more about Orthophotos (exit DNR)