Unmanned Aerial Vehicles (UAVs) equipped with lightweight spectral sensors facilitate non-destructive, near real time vegetation analysis. In order to guarantee quality scientific analysis, data acquisition protocols and processing methodologies need to be developed and new sensors must be trialed against state of the art instruments. In the following study, four different types of optical UAV based sensors (RGB camera, near infrared camera, six band multispectral camera, and a high resolution spectrometer) were compared and validated in order to evaluate their applicability for vegetation monitoring with a focus on precision agricultural applications. Data was collected in New Zealand over ryegrass pastures of various conditions. The UAV sensor data was validated with ground spectral measurements. It was found that large scale imaging of pasture variability can be achieved by either using a true color or a modified near infrared camera. A six band multispectral camera was used as an imaging spectrometer capable of identifying in field variations of vegetation status that correlate with ground spectral measurements. The high resolution spectrometer was validated and found to deliver spectral data that can match the quality of ground spectral measurements.