We have mapped historic coastal change around Aotearoa New Zealand using both historic vertical aerial photographs (from late 1930’s to present) and high-resolution satellite imagery (from early 2000s). The historic photos were sourced from New Zealand’s national archive, while the satellite images were obtained from commercial providers.

Georeferencing historical aerial images of coastal areas can be challenging due to the lack of clear ground control points, such as buildings or roads, especially in rural and sandy coastal regions. To overcome this, we used a mix of natural and man-made features as reference points. Where possible, we relied on roads, bridges, buildings, and fences. In some cases, stable natural features like rocky outcrops were the best options. For each image, we typically used 12 to 20 ground control points, focusing on areas along the coast to ensure accurate positioning.

New Zealand’s coastline is highly diverse, featuring sandy and gravel beaches, various cliff formations, and active coastal dunes. To account for this variability, we used different shoreline proxies that were easily identifiable in both aerial and satellite images. For sandy beaches, which accounted for the majority of the coastline, we usually mapped the edge of vegetation. For gravel beaches we mapped the storm ridge and for cliffed coasts we mapped either the cliff top or the cliff toe. In some rare instances we mapped the waterline, and where there were sea walls or rock revetments, we mapped the edge of these structures. Importantly, we mapped a single proxy at each location, so that change through time can be derived.

New Zealand was divided into over 400 Areas of Interest (AOIs), each ranging from 5 to 10 km in length and covering most open coast beaches and soft cliffed coasts. This detailed mapping effort produced a high-quality dataset of coastal change across New Zealand. By analysing this data, we calculated the rates of coastal change at 10-metre intervals, identifying patterns and hotspots of coastal change around Aotearoa. A relatively small number of areas remain unmapped, which we are hoping to complete in future work.