QUESTIONS ABOUT USE OF SATELLITE IMAGERY
Question: How does satellite imagery improve estimates of crop area?
Satellite imagery has two uses:

  • to stratify land into land use strata,
  • to improve estimates of land cover.
The first use of stratification has already been discussed. This question will deal with the use of digital satellite imagery to improve estimates of land cover and to reduce survey error. The AF is a perfect tool to provide representative ground data for statistical calibration for atmospheric and growth state.
In general, an AF makes estimates of populations based on small samples of segments. Satellite imagery covers an entire population (all sampling units) but the information is not perfect. That is, one has reflected energy at 704 km above sea level for all sampling units in the population. One must use the reflected energy in the satellite imagery in order to reduce the sampling error of the estimates. With AF methods, one has actual ground observations. One uses reflected energy from known fields in order to calibrate the reflected energy. We then classify the entire satellite image. The last step is to evaluate the misclassification of the satellite data using AF ground data and adjust the full frame classification based on misclassification identified in the AF segments where ground truth is available.
Question: What are the prerequisites of digital satellite imagery?
A few basic prerequisites must be met before a decision is made to use digital data to improve estimates of crop and land cover. If one prerequisite is lacking, the entire effort may be seriously hampered. The prerequisites are the following:
  • Satellite availability
Current digital satellite images must be available for the area of interest. In most areas of the world, obtaining satellite data is not a major problem. Moreover, the new satellite systems provide a wide variety of resolution size and spectral bands that are available to solve problems.
  • Acquisition Date
The acquisition of imagery must be timed to correspond with the critical periods in the growing cycles when the spectral values of the crops allow them to be differentiated.
  • Ground Data
Ground data from an AF must be available.
  • Sufficient Field Size
Fields of sufficient size of all crops to be classified must be available in the segments.
  • Sound Statistical Methodology
Using digital data requires sound statistical methodology. The AF method used is one AAIC staff helped develop and one that is used by experienced personnel. Currently it is used in the United States by the National Agricultural Statistics Service and in Europe by the Joint Research Centre of CEC.
  • General Resources
Resources and personnel are required to implement the technology by:
  • collecting data in the field and run survey operations
  • obtaining digital satellite and AF data.
  • performing digital analysis
If these requirements can be met, using digital satellite data to improve crop and land-cover estimates can be rewarding.
Question: When Does Digital Image Analysis Become Cost Effective?
If the area of interest is rather small (contained within three to five TM scenes) or the land cover of interest is very important, then use of satellite imagery can be cost effective.
Question: How can a group learn more about AF methods?
One possible approach would be to arrange for AAIC staff members to conduct a seminar at your local office or in DC regarding the methods to estimate: 1) crop production, 2) forecast yields, 3) livestock, 4) natural resources and environmental parameters, 5) social and economic status, and 7) land tenure.
AAIC's professional staff can present a two hour or two day seminar or discuss individual cases with project managers and program officers. A combination of both is also possible.