1. A method of monitoring the health of plants in a field, the method comprising: acquiring an aerial thermal image of at least a portion of the field that is indicative of thermal energy emitted by the plants within the portion, the thermal image being acquired from an aircraft as the aircraft flies over the field, the thermal image having a resolution of at least 480×640 pixels; processing the thermal image to assess variations in temperatures among the plants from which disease is detected in the plants; mapping the location of the thermal image to a position on a geographical map; overlaying the thermal image on the geographical map; providing the thermal image on the geographical map to a user in the field; and locating and displaying the user's present location on the thermal image on the geographical map and thereby enabling the user to navigate to areas in the field.