For safety reasons, aircraft are not normally permitted to fly below a certain height, perhaps 500 feet, although the limit varies with location. This limitation reveals itself most acutely when detailed aerial photographs of a structure or property are required, perhaps from a particular angle. A cherry pickers or telescopic masts may suffice, but can be used only up to a maximum height of about 50 feet. Above this limit, and below the minimum height for aircraft, is the range of heights within which tethered helium balloons may be useful.
Tethered helium balloons, typically with a diameter of about six feet and tethered by two lines, can be used with local air traffic permission up to a height of about 200 feet. The balloons are equipped with a self-levelling radio-controlled pan, tilt and zoom camera mount and a live video feed is available to the operator. The height advantage, when compared with platforms such as telescopic masts, facilitates photography of larger areas and subjects and provides a wider field of view and better perspective for tall subjects. Helium balloons Tethered helium balloon can also be flown using two separate tethers to allow unobstructed overhead survey shots. In suitable weather conditions a tethered balloon can also be walked around a site to obtain a range of different viewpoints.
The number of litres of helium required to lift a certain weight in grams, is calculated by multiplying the weight by 0.9743640 . however this figure should be increased by a factor of at least two or three to enable sure a balloon to fly satisfactorily in light breezes. Helium is relatively safe to handle given the observance of a few simple precautions and can be purchased locally wherever compressed gas services are offered. "Helium" supplied for filling party balloons may be mixed with other gases and hence decrease lifting capacity.