Whitchurch Silk Mill roof inspection
Objective and outcome of the task
Lynn Parnell of Logistics Partners has previously volunteered with the Whitchurch Silk Mill so at the request of the Mill Director Airborne Insights agreed to fly a mapping mission over their site in Whitchurch, Hampshire. The primary objective was to gather photographs which could be used to understand the condition of the roof after storms, and secondly for marketing & promotions and creation of a 2D map & a 3D model of the site.
This annotated image shows the detail that can be observed using a drone. It is a close-up of part of the Silk Mill roof showing the level of damage that can exist without initially exhibitng signs of water internally. Three slates are completely missing and had probably been the cause further damage to the slates on a lower roof. Also four slates are out-of-position with the potential to slip onto the lower roof and compound any problems caused earlier. This part of the roof had recently been refurbished but the scaffolding/staging removed before completion/acceptance inspection could be performed. This part of the roof was also not visible from the ground and only partially visible from an adjacent property.
(There are other images showing the conditions of the slates, flashing, chimneys, roof lights, earthing connections, guttering, downpipes, etc.)
How the data was gathered and processed
Previous Airborne Insights mapping missions were undertaken using the DJI Smart Oblique waypoint mission type, but in the surrounding environment of this property it was difficult to arrange the two perpendicular flight legs to avoid overflying a fire station, a fuel station and roads, while also avoiding several tall trees. A second mission planning app was used to combine a 2D oblique mission with a 3D helix mission which were able to be flown entirely within the boundary of the Silk Mill property avoiding all the identified obstacles. The two sets of missions were created to evaluate the functionality of the 3rd party mission planning app and its effect on photographic and map/model quality vs. the standard DJI mission planner.
Three manual missions were flown: 1) along the line of the gutter at the same elevation, 2) along the line of the gutter pointing vertical down into the gutter and 3) along the line of the gutter at the mid height of the slate tiled roof at a distance from the building for the roof height to fill the vertical field of view with the camera pointing horizontally. The photo shown above was gathered during these three manual flights. A fourth manual mission was flown to gather general views of the Silk Mill building and site in still and video for marketing purposes.
Three automatic missions were created flown: 1) a DJI Smart Oblique mission with two perpendicular flight legs, 2) a non-DJI 2D mission in a grid pattern over the Silk Mill building and 3) a non-DJI 3D helix mission orbiting the Silk Mill increasing in height to a zenith over the top of the building and then descending again in a helix to the start point. The following images show screenshots gathered during these missions.
The height and length of the Silk Mill building and the obstacles surrounding it presented a challenge to ensuring the drone remained within sight. To achieve this a remote pilot and an observer were deployed, constantly moving around the site and always staying in visual and voice contact.
Standard DJI smart oblique mission
Non-DJI 2D mapping ortho plus 3D helix modeling oblique missions
The mission took approx. 15 minutes elapsed flying time, and 537 photos were captured during four passes over the site. The photos were processed using the DJI Terra app, taking just over 60 minutes of processing time on a Windows 11 PC with a 32-core Intel Core i9-14900 processor, 64GB of RAM and an Asus Nvidia 4060 GPU.
2D Map
The 2D map was output by DJI Terra app as a GeoTIFF file. The GeoTIFF was used to create PDF and JPG files of the 2D ortho-mosaic and uploaded into Google Earth Pro to create a geo-referenced tile overlay on the Google Earth globe. This can help visualise the subject in its surroundings.
The GeoTIFF is also a very detailed ortho-mosaic image and can be examined to understand the condition of the subject, e.g. damage to woodwork, masonry, brickwork, roof tiles/panels, etc.
The JPG map images are firstly shown as the result of being processed separately.
Standard DJI smart oblique mission
Non-DJI 2D mapping ortho mission
Non-DJI 3D helix modeling oblique mission
And then in combinations to improve fidelity of the mapping.
Non-DJI 2D ortho mapping plus 3D helix oblique modelling missions
Standard DJI smart oblique mission combined with non-DJI 2D ortho mapping plus 3D helix oblique modelling missions
3D model & animation
The 3D model can be examined to enable measurements of distance, area, volume and slope to be taken; these have been verified to be within millimetres of the actual values.
The 3D model was shared as a link from the DJI Modify app, to be viewed in a web browser. Animation of the model in the viewer was screen recorded. (The viewer can navigate around the model using the mouse. The 3D model can also be output to a number formats compatbile with various CAD, GIS, etc. apps.)
Orbit video
A video was taken live during a manually orbit flown as part of the fourth manual mission around the building and site. This can be compared to the animation shown above to appreciate the fidelity of the data gathered and the photogrammetric processing used to create the 2D map and 3D model.
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