Tag: roof survey

01 Jun 2020
Aerial Map For Topographic Survey

Topographic Survey Using Drone Photogrammetry a Case Study

We were invited by a local council to demonstrate our aerial topographic survey capabilities for a small culvert project. This was challenging as the route passed through particularly dense tree coverage. Typically multi return LiDAR would offer better penetration to identify true surface through vegetation and is typically chosen in this situation (See our article comparing Photogrammetry and LiDAR). In this instance we wanted to show photogrammetry as a valid option during winter months which would have a much smaller cost to the client over LiDAR data collection (typically costing thousands a day). The survey manager was specifically interested in a topographic drone survey to see how this compared with existing measurements of the site. The culvert ran next to an existing tarmac walkway under some small tree’s densely packed together.

A processed orthomosaic of the 400m long site taken in February 2020.

The Survey

In February we returned to site to conduct the survey. This was to be a geo-referenced survey and we chose to use ground control points and manually collect their locations using a survey pole and base station for later post-processing.

Firstly, the base station was set up to collect satellite data for 2 hours. In this case we used the Emlid Reach RS GNSS that we use for our own RTK drone setup. However, typically we use which ever survey equipment clients prefer such as those from Leica and Trimble. In our test we knew the accuracy of the Emlid would still be several magnitudes better than the accuracy of positioning information available from the drone. We laid out 12 ground control points along the length of the route with square/diamond patterns indicating the center point. Tthe position was then taken from this point with the rover pole and later tagged in the drone imagery.

GCP’s along the route to georeference the point cloud with some used as validation points.

In this case since the mission was a demonstration and close to residential property so we chose to use the DJI Mavic 2 Pro. Typically we would use a drone platform with higher image quality such as the DJI Matrice or heavy lift drone with a full frame camera. For mission planning we chose Drone Deploy with a one pass Nadir grid with standard settings at a low altitude of 20m. The mission was flow collecting 199 images of 5472×3648 resolution. Some blurring was observed due to poor winter light. After the mission was flow we measured the position of the GCPs before clearing up.

Drone Survey Data Processing

Later the survey data was post processed using RTKLib and local RiNEX data to provide accurate positions of each GCPs. This provided an accuracy beter than 2cm X,Y and under 4cm in height. The images and GCP locations were input into Pix4D and the mission processed to obtain a classified point cloud, DEM, DTM, mesh model and contour map as show below. This had an ground sampling accuracy of 0.81cm/px over the area of 2.47ha. The absolute camera position uncertainties X,Y,Z(sigma) were 0.112(0.021), 0.317(0.028) ,0.71(0.051)m. The error of a couple of the GCP measurements was high as the rover had poor line of sight to the base station on the hill through vegetation.

Rendered DSM with contour map overlay derived from Pix4D.

From the results we observed holes in the point cloud due to the dense wood branches over the pathway. This led to a poor contour model as seen below. This was disappointing but highlighted the importance of planning in accordance with the task which Drone Deploy alone didn’t consider.

Improved 3D Survey Model

We returned and manually flew and acquired photos around the problematic area this time taking additional oblique images perpendicular to the walkway. Processing these images provided an improved point cloud with the walk way visible through the trees to measure the topography along the route.

Classified point cloud derived from oblique images with vegetation.

Extra processing was done to classify the point cloud this time so that high vegetation could be classified and removed from the point cloud to provide a more accurate DSM for measurement and topographical contour creation.

Classified point cloud derived from oblique images with high vegetation removed.

This example from the earlier batch of processing without point cloud classification and high vegetation removal shows peaks along the DSM profile. The profile was much more accurate after point cloud classification.

Profile derived from the DSM along the proposed culvert route used to determine if the run off is sufficient.

Our conclusion is that during winter months photogrammetry can provide actionable topographic survey data over woodlands, an area where LiDAR is typically required. With anticipated or non essential delivery schedules this can provide huge financial savings. Sky Tech are happy to work with local surveyors on projects such as this where drones can provide more rapid data collection over convential site walks. Please get in touch if you would like to work with us:

Quick video overview of the outputs from the topographical survey.
10 Jan 2020
Roof Inspection By Drone

Roof Inspection And Survey

Conducting roof inspections with drones is providing advantages in terms of speed, coverage, cost savings and more over traditional methods.

What Is a Roof Survey?

A roof survey is where a professional surveyor makes an assessment of the condition of a roof. The condition of tiles, chimney pots, flashing, guttering etc are noted from the ground mostly by visual inspection on the ground or using binoculars. During property transactions roof surveys are carried out and for landlords it is a legal requirement to do so routinely.

A roof inspection provides images alone in a methodological manner which can then be passed to a professional surveyor or repair company. Which really depends on the outcome if it is for an property transaction, warranty or insurance claim or general repair.

A written roof inspection report will highlight the severity and wear or damage that requires immediate action. A detailed roof inspection report is beneficial to property owners to pass to insurers and roof repair companies. It allows accurate cost estimates to be made for repairs and transparency to the client what has to be rectified. Something as simple as finding a slipped tile or blocked gutter could save a lot of money in further interior damage after heavy rain.

A roof survey may take place as part of a wider building survey. It may be neccessary for an professional surveyor to access the internal roof space to assess structural integrity, insulation and membrane material. At the very highest level a roof surveyor would assess a roof by hand using a cherry picker, ladder or scaffolding prising at different elements such as mortar, mastics, tiles and guttering. But this level of attention is very unusual and also considerably more expensive.

This is where conducting a roof inspection using a drone provides considerable advantages. A roof in bad condition could be dangerous to access with risk to the assessor and those below should anything be dislodged.

For the rest of this article we predominantly discuss external roof inspections with no contact to the roof.

Roof inspection utilising a camera drone.

Roof Inspection Using Drones

Advantages
  • Fast to deploy
  • Extensive and concise coverage
  • Access to any style and height of roof
  • Images show exact position of issues
  • Historical record of condition
  • No contact with dangerous buildings
Disadvantages
  • Drone must keep safe distance from public
  • Noise and privacy concerns
  • Visual assesment only, can’t pull or tap
  • Can’t fix simple faults at same time

Drone roof inspections give a reduction in cost, risk, and time to report delivery. On large and difficult to access roofs, drone imagery provides more extensive and concise coverage. Digital imagery can then be uploaded and processed in the cloud for further analysis and compilation of a final report. Sky Tech can advise on the most cost effective solution for your needs.

Example of online digital reporting from roof inspection.

Digital Reporting Tools

Once the roof inspection is completed images and video from the survey are uploaded to the cloud for processing. Digital survey images can be examined remotely by a professional surveyor and the extent of visual faults tagged by severity either manually or with an AI assistant. Images are also on hand as proof to insurers that repairs are necessary.

Is is also possible to create 3D models for special cases such as recording historic buildings. See some excelent examples here.

This is an excellent visualisation tool for virtual and augmented reality either for professional use by say architects or for public displays in exhibits.

3D models also allow the measurement of distance, area and volume when done correctly.

Example of tagged inspection report

Our Inspection Methodology

Our drone roof survey provides a detailed and extensive coverage of different roof aspects and elements as follows:

  • Overview from above building
  • Each roof aspect orthogonal to the slope
  • Ridges, hips, valleys and parapets
  • Chimneys, dormers, bell towers
  • Gutters, downpipes, vest pipes, vents
  • Gable ends mainsonary and pointing
  • Roof antenna, satellite dishes and cabling

How the inspection images are assessed are up to you. Either self assess, ask us for assistance or allocate your own roof survey professional to assess the imagery. The type of fault would can be reported in terms of severity, as well as other minor notes such as wear, moss growth, birds nests. Read more about our roof inspection service here.

References