This work presents our initial approach to remote sensing and precision agriculture for monitoring crop fields in Colombia. Using an autonomous quadcopter UAV (unmanned aerial vehicle) equipped with a multispectral camera onboard, our goal is to provide farmers with an integrated tool for measuring and assessing live green vegetation by assembling a terrain image mosaic based on capturing multispectral images of the terrain. This paper presents how to integrate an UAV-based solution for capturing geo-tagged multispectral imagery and the methods to compute multispectral image mosaics. A field report shows preliminary results of the experiments carried out in crop farms located nearby Bogota, Colombia.