Heritage Inspection Survey At Shropshire’s Historic Ludlow Castle

Crucial Services has had the privilege of conducting a drone heritage inspection survey at Ludlow Castle – one of Shropshire’s premier historic assets and tourist attractions. The principal purpose of the survey was to lay the foundation for future drone surveys, future asset management surveys and maintenance of the castle. There will be some regulatory hurdles for us to overcome going forward, but the task is interesting and will be a first for the UK when it happens.

We were asked to carry out a specific task for the Estate – more on this at a future date. However, we needed to examine the castle closely to scope for and plan the mission for that task. We were asked to gather extra images for the Stone Mason while there due to water penetration in two areas currently being repaired. The overall survey will also help other contractors carry out future work at Ludlow Castle. This includes their architect preparing their quinquennial maintenance report for the estate office.

The site has several challenges. Most notably, poor radio signals and enclosed areas with no GPS satellite coverage. Both of these are important in accurately maintaining drone control throughout the flights.

Without appropriate mitigations on site, the following hazards could happen:

• Any sudden loss of the data link to the pilot’s remote controller could cause the drone to fly away or crash;
• Any sudden loss of GPS satellite coverage in a confined area could result in the drone crashing into the structure;
• Loss of GPS RTK positioning corrections due to poor mobile signal within the thick walls of the castle;
• Collision with members of the public in and around the castle;
• Damage to the fabric of the castle and the castle grounds;

It is always important to assess the situation on the ground before ever turning up to the site. We completed the desktop study to evaluate the whole drone/survey operation as part of the quoting process. We evaluate the site risks, mitigations, and methodology required during the survey. Our experience as Land Surveyors has taught us that good preparation is key to a successful survey. Preparation with desktop reconnaissance is an essential step to successfully completing any survey, be it on the ground or in the air.

Specific ground and air risks can be identified. These include but aren’t limited to:

• The air space classification and NOTAM restrictions – essential to know what the risk of aerial collisions;
• Powerlines, sensitive areas like SSSIs, schools or prisons – that need to be avoided;

Skipping this step poses a real risk to operations, other airspace users, uninvolved people

Walking a site to spot extra hazards not picked up during the initial desktop reconnaissance is vital to a safe drone flight. This is especially true on a complex site like Ludlow Castle. A full appreciation of the site before commencing a drone survey is very important and not less important than a recce for a ground-based survey.

Any additional hazards spotted were noted, risks were reassessed, and appropriate mitigations were implemented to minimize those risks. These can be temporary or permanent features not visible in the desktop recce. These can include:

• Overhead cables;
• New structure;
• Masts or other radio equipment that can cause interference for the drone;
• Trip hazards for the drone pilot who will have eyes on the drone;

The first stage of the ongoing project was to model the whole site. This 3d model is the backbone of all future drone flights for:

• building inspections with a drone;
• the efficient mission planning of all future drone flights;
• pricing future drone operations;
• the convenient use of any 3rd party contractors, including the stonemason, architect, etc.

Whilst on site, we took advantage of a demonstration of the new Faro Orbis handheld laser scanner. This valuable data helped us to create a higher quality model of the site when processed with the photogrammetry data.

The 3d Laser Scan data, when combined with the photogrammetry data, increases the visual and positional accuracy of the model. The scan wouldn’t have the same fidelity in the 360 camera images and colourization of the points compared to the photogrammetry images. The photogrammetry can be very accurate but can struggle in nooks and crannies that can’t be seen from the air. A hybrid approach to processing in our preferred software yields greater results.

A second visit was flown for closer inspection images of the castle. The inspection is for future repair and maintenance, including quinquennial repairs and weed control. Large parts of the castle aren’t visible from the ground or public areas. Some higher areas, like the Garderobe Tower shown above, aren’t accessible, which makes inspections difficult or impossible. Due to narrow doorways and uneven ground conditions, cherry pickers and scissor lifts can’t access these areas. Scaffolding would also be prohibitively expensive. Drone inspections, however, are more feasible and cost-effective. Drone inspection can still be complex and tricky, particularly in this use case.

For the heritage building inspection at Ludlow Castle, the extensive images were analysed, and issues requiring attention were identified.

A PDF report was created for the client to conveniently plan remedial works with the architect and the stone mason. All of this information was also made available to the client in the 3d textured model.

Overall, this has already been a fascinating project for us. We are now in deep planning and preparation for the next stage – spot-spraying the extensive weeds on Ludlow Castle. The weed colonies are causing significant damage to the fabric of the castle. Spraying the weeds with a drone is the only practical solution on this site. Cherry pickers, scaffolding and other lifts or climbing equipment will be very expensive or impossible to use. Watch this space for further updates as this exciting project continues to develop into the spring, which will be a first for spraying with drones in the UK. The 3d modelling and the drone heritage inspection survey are key components to enable us to safely plan and execute the spraying missions.

If you have a heritage asset that could benefit from a drone heritage inspection survey or a building inspection with a drone, then please feel free to get in touch and find out how we can help you.