Mapping Data of Polar Bear Maternal Den Habitat, National Petroleum Reserve–Alaska (NPR-A)

We created a provisional polar bear denning habitat map from the DTM by extracting each 5 × 5 m cell with an elevation difference of 1.3 m or more between it and at least one of the eight adjacent cells. We used a moving-window analysis implemented through commands from the GRID module of ARC/INFO (ver. 9.2, ESRI, Redlands, California) to interrogate DTM tiles for cells that met our criterion of maternal denning habitat. When this initial map was compared to GPS locations of eight previously identified maternal den sites in the NPR-A, we found that the denning habitat at one den site was not detected. To ensure that our provisional map included known polar bear den sites, we performed a second extraction of the DTM by identifying each cell with an elevation difference of 1.0 m or more between it and at least one of the eight adjacent cells. This 1.0 m selection criterion ensured that our provisional map identified denning habitat at all known den sites. We used this provisional map to field test against actual landscape features so that we could further refine the polar bear denning habitat map. DTM cells identified as potential denning habitat were converted to an ARC/INFO vector coverage of polygons. To smooth the border across adjoining cells and to connect cells that were separated from the main group of identified cells, we applied a 5 m buffer to the outside margin of all clusters of adjoining cells and non-adjacent cells. This buffer allowed us to recognize landscape features composed of cells that were near but not directly adjacent to each other and to group them into a single polygon of denning habitat.