Frequently asked questions
How is the growth vigour map generated?
The growth vigour map is generated using a vegetation index (VI). VIs such as the normalised difference vegetation index (NDVI), have traditionally been employed to monitor vegetation vigour using satellite imagery. The NDVI makes use of the relationship between the low reflectance of red light and high reflectance of NIR radiation by plants duding photosynthesis. Healthy crops generally have high NDVI values, whereas stressed crops have noticeably lower values. The NDVI is thus an effective index for monitoring crop condition. Smaller plants of the same species also have lower NDVI responses compared to larger plants, which means it can also be used to monitor growth.
How is the moisture map generated?
Satellite images can detect moisture by comparing the difference in reflection of near infrared and short-wave infrared radiation. The normalized difference moisture index (NDMI) exploits the knowledge that wet soil and vegetation reflects shorter infrared wavelengths more than longer wavelengths. NDMI values range from -1 to 1 and have been shown to be very effective in detecting small changes in water content in fields. It can also be used to support irrigation scheduling.
How should are the growth vigour and moisture "change" maps be interpreted?
The change maps show the differences that occurred between two images acquired at different dates. In most cases the change maps represent changes between consecutive images, but cloud coverage sometimes forces the intervals to be longer. The legend below should be used to interpret the change images. Red, orange and yellow hues indicate reductions in vigour/moisture, whereas green hues represent increases. Ideally, actively growing crops should always have green hues.
What is the difference between "raw" and "classified" maps?
The "classified" maps are exactly the same as the "raw" maps, except that the raw pixels are grouped (classified) into ten homogeneous classes. Each class has an equal number of pixels. Essentially, this exaggerates variance and helps to delineate zones in a field that require attention, even if the differences within the field are small. For instance, the maps below are of a rainfed wheat field produced from the same satellite image. On the raw map (left) there is little variance (everything looks pretty much the same), but on the right, the raw values were classified and now one can clearly see the zones in the field where growth vigour is relatively high (green) and low (brown).
Why am I receiving maps of some production units or farms, but not others?
The most likely cause is cloud cover. Clouds and cloud shadows can have a significant effect on growth vigour and moisture maps, which might lead to incorrect actions. Cropsmonitor will not create a map of a production unit or farm if clouds are detected in the vicinity.
Please note that automatic cloud detection is not a trivial problem. We make use of the European Space Agency’s cloud detection algorithm, which is accurate most of the time, but not always. Sometimes the algorithm misses some clouds and in other cases it is almost too sensitive (will classify hazy conditions as clouds). So please keep in mind that our cloud detection is not perfect and some of the maps that we produce could be affected by clouds and cloud shadows. These effects are easy to see when the maps of different dates are compared.
Another way to see if a particular production unit or farm was affected by clouds, is to have a look at the “colour” maps available in the ROOT\REGION\OWNER-BUSINESS\PDF\FARM_MAPS\ folder. These maps are always generated (even if clouds are detected). Clouds appear as white on these maps (see examples below).
To view cloud cover at regional scales, go to the ROOT\REPORT\PDF folder. There you will find maps of the full satellite image tile. Clouds are easily identified on these maps (see example below).
What happens after I subscribe?
Once you have selected a plan and have subscribed you will receive an email with instructions on how to send your information so that we can start generating the reports for your fields. The email should reach you within 24 hours.
Can I have multiple subscriptions?
What happens after my LITE (free) subscription runs out?
Once your LITE subscription runs out, the service will be suspended. To continue receiving the maps you will have to subscribe to the PRO, EXPERT or ENTERPRISE options by following the instructions on the Subscibe page.