High resolution Normalized Difference Vegetation Index (NDVI) estimated by optical sensors on board unmanned aerial vehicles (UAV) has been used effectively to predict nitrogen variability and yield of wheat crop. It has been noticed that the NDVI measurements changes with altitude due to widen swath and camera exposure. In this study, an effort is made to determine the effect of height, sun angle and exposure on NDVI measurements for wheat crop. Multi-spectral camera on-board a quad copter, fixed wing UAV as well as handheld sensor were used to measure NDVI at different heights. The height varied from 0.5 m (in case of hand held sensor) to 120 m for quad copter and fixed wing UAV. Similarly, sun angle and camera exposure were tested. The initial results showed a variability of 13% and 19% in NDVI with a change of height from 5 m to 120 m using quad copter and fixed wing UAV. The effect of height is sensor and platform specific and has to be taken into consideration while conducting yield estimation  studies based on NDVI.