PfCO, CAP 722 & the 50m bubble

A new edition of CAP722. Following discussion with PfCO holders  I have been looking at the prohibited space restriction as published in the July 2019 edition of CAP722:


As published in CAP722, the diagram does not show 50m clear of the top of the building.

The hatched area are those zones in which small UAS flight is prohibited under standard permission for persons vessels vehicle or structures NOT under the control of the UAS operator. This diagram is published in CAP722 Ch2 para 3.3.1.

The legal, written description of the restriction in article 94  of the ANO  2016:

A permission from the CAA is required to be held if you wish to conduct a commercial operation with your aircraft (iaw ANO 2016 article 94[5]), or if you wish to fly your aircraft:

  • at a height of more than 400 ft above the surface (iaw ANO 2016 as amended article 94A),and/or

  • within 150m of either a congested area or an organised open-air assembly of more than 1000 persons (iaw ANO2016 article 95), and/or

  • within 50m of people or properties/objects that are not under your control (iaw ANO2016 article 95)

It is the last paragraph that pertains to proximity in the case of overflying neighboring properties for roof inspection. A literal interpretation (and simplified with un-mixed units) looks like this:

A 50m buffer looks more like this. To keep it simple I convert 400feet to 120m,  know it’s 121.9 really.

A 50m bubble. Working with the written description the above makes more sense. It is widely accepted the prohibited space conforms to a 50m radius sphere or ‘bubble’ around the drone rather than a zone around obstacles. It is this aspect of the restriction that allows proximity to structures depending on the height of clearance, the 50m zone shrinks as the drone rises and this can make tight sites with a narrow buffer zone possible by raising the drone to, say 50m.


The bubble concept moves the prohibited zone to the centre of the drone. As the drone rises the zone rises with it.

My objective is to fly for roof inspection without breach of the ANO for neighboring properties. A sphere at 20 and 40m heights  looks like this as a contact disc on the ground:

On 3D a 50m sphere cuts a 3m circle on the ground at 40m elevation. The drone is shown 50x larger than actual size for clarity.

The solution of the radius to height ratio is a Pythagorean triangle:

A 3,4,5 triangle is a useful way of working out the radius of the buffer on the ground.The drone is shown 50x larger than actual size for clarity.


The effect of the 50m bubble at 20m and 40m respectivly, Showing the drone at a mere 10x actual size.

The only way is up! So raising the drone 40m reduces the buffer to 30m for objects on the ground. Flying at 50m vertically clear of  persons vessel vehicle or structure not under the control of the operator might push the resolution of photography but it can mean horizontal offsets can be zero for nadir cover.


About billboyheritagesurvey

Heritage worker
This entry was posted in SUA photography, Survey Practice. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.