Many researchers who are getting into agricultural remote sensing are interested in a simple, inexpensive way to calibrate the aircraft remote sensing imagery acquired at their research site from digital counts to reflectance. One of the best ways is to construct folding calibration targets. These are superior to the typical reflectance tarps in that they are semi-permanent, and retain a constant reflectance over long periods.

These instructions describe the basic method for building large folding calibration targets from standard construction materials. You should feel free to experiment with other materials and procedures besides those described in these instructions.

Fig. 1

You will need 32 4'x8'x0.75" OSB or plywood panels. Get the cheapest, since surface roughness isn't a problem. Don't use panels with a very smooth surface (like interior wall paneling), as they tend to produce specular reflectance after they're painted. Don't use panels thinner than 0.75", as they are not as stiff and require more reinforcing beams. Remember that it will be necessary to walk on the target when painting it. Eight of the panels that will form the folding panels ("petals") along one side of the target will have to have their length shortened by around 2". This is necessary to allow the target to close completely without petals from one side of the target lying on top of the petals from the other side of the target (this is a consequence of the way the petal panels are attached with the hinges, as described below).

You will need 20 2"x6"x8' boards to form the main supports under the base (non-folding) part of the target. Try to get relatively straight ones. As shown in Fig. 1, 5 of these boards will lie under and hold together 4 panels that constitute a quarter of the target base. The panels can be attached with 1.5" wood screws, or nails (if you don't want to take it apart again). The two outer boards should be centered on the edges of the panels, as these boards will also provide support for the petal panels. Once assembled, adjoining quarter-sections of the base can be held together with 2' pieces of 2"x6".

Each petal is attached to the base using 2 hinges. Get the heavy-duty barn-door type of hinges, since they will have to endure a lot of stress during opening and closing of the petals. Hinges should be attached on the upper surface of the panels, so the panels of the base and petals will lie in the same plane when opened, with no gaps between the base and petals. This will cause there to be a space (the thickness of the hinge) between the base and petal when the petal is closed, but this will be taken care of later. On the base side, the hinges can be attached with screws driven down through the panel into the supporting board. On the petal side, the hinges can be attached with bolts through the panel, secured with nuts on the underside of the panel. It will probably be necessary to hold the petal in a vertical position to do this, with one person holding the bolt while another person tightens the nut.

You will need 16 2"x2"x8' boards to acts as supports for the petals. Each of these boards can be cut in half and attached to the panel as shown in the figure with screws or nails. This will allow each petal panel to be opened and closed separately. Joining adjacent petal panels results in a petal that is too heavy for most people to open and close easily.

Fig. 2

Once these main elements have been assembled, you can proceed to the finishing touches prior to painting. As stated above, when the petals are closed, there will be a space the thickness of the hinges between the base and petal panels. This space might allow dust to be blown in to contaminate the painted surface. To prevent this, a cloth gasket can be made to keep out the dust when the target is closed (see Fig. 2). You will need about 32' of cheap cloth from the fabric shop. Color is not important, since it will be on the underside of the panels when the target is open. Get the 3'-wide kind, since it can be split down the middle to give you the 64' that you'll need for gaskets. Cut the cloth in 4' lengths. With the target closed, attach a piece of cloth between the base and petal panel with staples as shown in the figure.

The target should now be ready to paint. The easiest way to paint them is to use a commercial paint sprayer, like those you can rent for painting houses. A roller also works, but does not produce as uniform a coat and takes much longer. As for paint, any good commercial flat exterior latex house paint will work. The most important characteristic of the paint is that it produces a flat, not glossy, finish. Depending on the porosity of the paneling, you might want to apply a primer first. Using a primer, you'll probably only need one good heavy coat of paint. Without a primer, you might need 2-3 coats of paint to get a uniform finish.

The choice of "colors" (shades of gray) depends on the remote sensing system and the application. Of course, you'll need one complete target per color (NOTE- because of atmospheric scattering optics, it will not work if you paint part of a target one color and the other part another color. See Moran et al., 1997). If your remote sensing system has a linear response between digital counts and observed radiance, then you only need 2 targets. If the response is non-linear, you'll need at least three targets. The "colors" should bracket the range of reflectances that you'll be measuring with your remote sensing system. For vegetation studies, you'll probably want one target with around 5-10 percent reflectance (looks almost black), and one with around 50 percent reflectance. You could have a white one (around 90 percent), but for most studies with natural surfaces (plants, soil) it's not necessary. Figure 3 shows three targets at the USDA-ARS research laboratory at Shafter, CA. These have reflectances of around 8, 30, and 90 percent.

Fig. 3

Once the targets have been painted and are dry, you will have to measure their reflectance using a portable spectroradiometer. You may find that the reflectance varies as a function of wavelength, but that's okay, because what is important is that you've measured what it is.

Finishing touches on the target are the addition of side skirts and rope handles. Cloth side skirts, like those on the targets in Fig. 3, keep dust from blowing into the targets from the sides. They consist of extra cloth tacked along the outside edge of the petals on the corners of the target. As shown in Fig. 1, a hole should be drilled along the outside edge of each petal for attaching a rope to assist opening and closing the petal. About 12' of 0.5" braided nylon rope should be used for each petal. In tying the rope to the petal panel, it should go around the 2"x2" board supporting the edge of the petal panel.

Care and maintenance of the target should be minimal. As in Fig. 3, you should have a plastic sheet (crumpled up in the photo), larger than the target, to cover the target when not in use. This will keep out additional dust and rain. Care should be taken in opening and closing the target petals, i.e., don't let them fall when closing them (let them down with the rope). This will prevent strain on the hinges and keep from stirring up dust. At Shafter, we have found that if the targets are closed except during remote sensing observations, their reflectance will not change significantly over several months of use. If in doubt, re-measure their reflectance periodically with the spectroradiometer. If a panel gets too dirty (maybe a dust storm came up during the observations), simply sweep it off and re-paint it.

REFERENCES:

Moran, M.S., T.R. Clarke, J. Qi, E.M. Barnes, and P.J. Pinter, Jr. 1997. Practical techniques for conversion of airborne imagery to reflectances. Proc., 16th Biennial Workshop on Videography and Color Photography in Resource Assessment, ASPRS, 29 April-1 May 1997, Weslaco, TX. pp. 82-94.

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