Surveying a confined space is always a challenge. Whilst LiDAR will almost always be the preferred method for gathering data, the size of the space, its regularity and what the space contains are all issues that effect the choice of approach. So, when Commendium were asked to survey a sewage outflow near Fort William, we decided to arm ourselves with a variety of approaches. The aim was to measure the drop in a treated sewage outflow pipe to ensure that planned improvements would maintain or improve the gravity-based flow of treated sewage all the way to the sea. This was being performed alongside visual inspection of the pipes with ROVs equipped with sonar and video.
After visual inspection, we established a GNSS fixed ground control network of survey stations between the two access portals and the high tide level on the beach. We LiDAR surveyed between these and the two outflow pipes to the sea, so as to gain a highly accurate, geolocated surface model. To survey the levels in the pipes we lowered a REB-Horizon portable LiDAR unit into the access portals, This negated the need and accompanying risk, of having to enter the portals ourselves; definitely a bonus. The base of the portals where still covered in about 0.8m of water, despite being pumped clean. So, to measure the base level we placed a precision GNSS unit on top of an 8m pole so as to gain an accurate GNSS fix of the base level.
The results showed a healthy gradient for the pipe all the way to the sea and provided engineering drawings on which to base further plans.