Two-stage area frame sampling on square segments for farm surveys

In the MARS Project (Monitoring Agriculture with Remote Sensing) of the E.C. (European Community), area frames based on a square grid are used for area estimation through ground surveys and high resolution satellite images. These satellite images are useful, though expensive, for area estimation: their use for yield estimation is not yet operational. To fill this gap the sample elements (segments) of the area survey are used as well for sampling farms with a template of points overlaid on the segment. Most often we use a fixed number of points per segment. Farmers are asked to provide global data for the farm, and estimates are computed with a Horvitz-Thompson approach. Major problems include locating farmers and checking for misunderstanding of instructions. Good results are obtained for area and for production of the main crops. Area frames need to be complemented with list frames (multiple frames) to give reliable estimates for livestock.