Recently I was working to update my Te Reo Maori Map of New Zealand with a new background. I learnt a couple of things while making it, so I thought it might be interesting to quickly share the process.

The map uses the wonderful NZ wide 10m cloud-free Sentinel imagery provided by LINZ under CC-BY-4.0. Unfortunately, while the imagery looks great on land, it doesn’t extend very far off the coastline as you can see below, which doesn’t make it so useful for a nationwide scale map. Therefore we need to do some work to smooth/fill out out the look of the ocean.

To create a realistic look, we need to bring in LINZ’s coastline polygons and NIWA’s bathymetry layer alongside our Sentinel imagery in QGIS.

• Clip the bathymetry raster using the coastline as a mask to remove areas of land. This allows the bathymetry to cover the ocean edges of the satellite imagery but not the land.
• Create a custom continuous colour ramp for the bathymetry layer. I sampled a lighter blue and a dark blue colour from the sea in the imagery to approximate how water depth looks.
• Create a subtle gradient along the coastline, to emulate the shallow water along the coastline. To do this, symbolize the coastline layer using an inverted shapeburst two colour fill, I sampled the turquoise colour from the imagery close to the shore and faded it out to transparent using a set distance. It’s worth experimenting with transparencies and distances to get the effect that you want – I used a 3 mm distance.
• Since the Chatham Islands aren’t part of the Sentinel dataset, I used Landsat 8 imagery from the USGS instead and repeated the above steps.
• Lastly, I added some OpenStreetMap highways to add some context.

This is the end result (before labels):

If you’re interested, the new basemap is available as an option for the Te Reo Maori print on the store.