Why do we need to map the coast?
After a powerful M7.8 earthquake shook North Canterbury, New Zealand in 2016, iXblue was contracted to perform detailed seafloor mapping to update nautical charts. An important part of making charts is accurately drawing the coastline. The complex tectonic environment in New Zealand makes the coastal zone rather tricky to map because of the rocky outcrops and uneven topography.
The earthquake caused extensive landslides and massive uplifting along the shoreline. The impacts of the Kaikōura earthquake were wide-ranging. Thousands of people were affected with significant damage to transportation networks and other lifeline utilities as well as disruption to the agriculture and tourism industries.
The earthquake and surface rupture involved more than 20 different faults (a world record), triggered the biggest local-source tsunami recorded in New Zealand since 1947, caused extensive coastal uplift, widespread landslides and landslide dams, as well as slow-slip 'silent' earthquakes.
Linking dynamics between land and sea
Land Information New Zealand (LINZ), funded several onshore and offshore mapping initiatives using the techniques described here:
LiDAR (Boat-based and Aerial)
Light Detection and Ranging (LiDAR) point cloud data was obtained using a Renishaw Merlin Mobile Marine Laser Scanner for shoreline mapping. LINZ also conducted an aerial survey to map the larger region’s coastline. The aerial survey mapped from an airplane covered more area, however the boat-based survey acquired higher resolution and more detail along the complex rocky areas. Gathering elevation data from different perspectives provides surveyors with necessary information to designate “significant features” that might be dangerous to marine traffic.
Unmanned Aerial Systems (UAS)
Tellus4d Geoimaging Pty Ltd used a fixed-wing Unmanned Aerial System (UAS, or “drone”) to map the “white zones”. These are the areas which are too shallow to map using conventional hydrographic surveying methods (sensors mounted on vessels). The white zones are usually left uncharted, or are charted approximately, which can have serious consequences for near-shore navigation. A bird’s eye view of the white zones can therefore be of great value.
Tellus4d’s system not only acquires imagery at very high resolution, higher than any satellite-derived product, but also geo-references the data using post-processing kinematics (PPK). Every detail on the map is within 3 cm (horizontally and vertically) of its real-life position.
Tellus4d mapped the entire Kaikoura Peninsula (over 1000 Ha) in less than 6 days. With over 70 Ha covered per flight, UAS-based photogrammetry is extremely productive, while the on-demand nature of UAS imagery means that the recorded data is contemporaneous - another advantage of drones over satellites.
The orthomosaics, created by automatically “stitching” thousands of high-resolution pictures, can be also be rendered as 3-dimensional models. These products greatly facilitate “coastlining”, the process of drawing subaerial features into nautical charts. Another advantage of using these clever unmanned systems is that no lives are put on the line, while pushing a survey boat ever so close to the limit of what it can map in shallow, rough waters can be very dangerous.
Benefits of mapping coastlines
- Mapping faults onshore and offshore
- Quantifying elevation changes from uplift/landslides
- Detecting groundwater flow patterns
- Filling in data gaps nearshore for safety and navigation