Ambient light conditions change constantly throughout the day and growing season due to weather, angle of the sun, etc. Uncalibrated sensors are useful for taking snapshots in time of the conditions of a given field, but accurately comparing reflectance data from hour-to-hour, month-to-month, and season-to-season is impossible unless the changes in ambient illumniation have been quantified and accounted for.
The patented calibration techniques used by the SLANTRANGE system measure incident light via the AIS, and calibrate the data so that stress conditions can be accurately compared across datasets gathered in all lighting conditions.
The SLANTRANGE Stress measurement is an absolute measurement that is a ratio of the four spectral bands measured by the sensor. It is our proprietary formula for identifying stress conditions with higher sensitivity and accuracy than can be achieved from industry standard two-band NDVI measurements. Given the variety of crops that our customers measure with their sensors, the system has to be adjustable to display a broad range of reflectivity profiles and stress conditions in an intuitive map.
For example, say the sensor measures the reflectivity profile of healthy mature corn and computes stress values between .1 and .2. In another corn field, at an earlier growth stage, in a different part of the world with different soil, the sensor measures stress values between .4 and .5. In these extreme examples, the regions of corn with stress values of .1 and .4 are the healthiest areas from their respective fields. A particular field's conditions must be considered when comparing stress between different crops in different growth stages. At the moment, there is no library of multispectral reflectance signatures for stress conditions in every variant of every crop, in every type of soil, across the world, to reference when assessing the level of stress and causes of stress, in your field. The Stress map shown in SlantView shows the range of Stress values across a field and makes it easy to isolate areas that may require immediate attention or action, but determining what the Stress values measured by the sensor actually correspond to (such as a specific pest, nutrient deficiency, disease etc.) is a question best answered by the agronomist or grower. The SLANTRANGE system can be referred to as a "rapid scouting tool", and the agronomist and grower are still an integral part of interpreting the data.
We allow users to adjust the application of our Stress Color Scale to the actual Stress values measured by the sensor (this can cause confusion about the absolute nature of the stress measurement) to show the differences in the measured Stress values in an intuitive way. Typically, we adjust the color scale to show the least stressed areas in shades of green and the most stressed areas in shades of red. To easily compare one field to another, the user simply has to make sure that a particular shade of green corresponds to the same measured stress value in both SlantView Map Windows using the buttons next to the color scale to make incremental adjustments (e.g. setting the darkest shade of green to a stress value of .1).
The maps in the center, and right of Figure 1 show non-intuitively adjusted color scales. In center of Figure 1, the majority of the stress data (with measured stress values of .1 to .35) is mapped across shades of green. The areas of the field with the highest stress are shown in light green, and are difficult to distinguish from the healthy areas, also shown in green. Similarly, the example on the right has a color scale with the stressed and healthy areas mapped across shades of red. The well adjusted color scale of the map on the left intuitively shows the range of stress values measured in the field, with stressed areas clearly in yellow and red, and the healthy areas in green.
Figure 1: Stress color scale adjustments