Penrhydeudraeth, near Porthmadog, Snowdonia

In November 2014 the National Grid identified 12 sections of high voltage power lines in 8 Areas of Outstanding Natural Beauty (AONBs) and National Parks having the most significant landscape and visual impact. The aim was to replace existing overhead lines with underground cables. Gwalia Surveyors was appointed to survey the section Penrhydeudraeth, near Porthmadog in Snowdonia from the final shortlist, producing both topographical, bathymetric, and underground utility surveys. Given the sensitive nature of this protected area this would prove to be a challenge. The survey area measured approximately 3.6km in length and 600m in width spanning and including the river Afon Dwyryd.

Due to the size and nature of the site a combination of technologies was used to produce the topographical survey. Aerial photography (UAV/drone data) data was fixed by ground control points by GPS systems which produced a geo-referenced point cloud. Additional information was recorded by GPS and total stations in areas where the UAV data was obscured. All this data was collated into a single 3D model from which the topographical survey could be processed. Once the topographical survey was complete we deployed multiple teams of surveyors on the underground utility survey.

The bathymetric survey was performed by a remote controlled HyDrone-RCV operating a Sonarmite echo sounder and on-board GPS system. Both cross sections and longitudinal sections of data was recorded enabling the plan production in both Chart and OS Datums.


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