Slope measures the inclination of the land surface from the horizontal. The percent slope and degrees slope products represent this inclination as the ratio of change in height to distance.

The slope (percentage) and slope (degrees) products were derived from the Smoothed Digital Elevation Model (DEM-S; ANZCW0703014016), which was derived from the 1 second resolution SRTM data acquired by NASA in February 2000. The calculation of slope from DEM-S accounted for the varying spacing between grid points in the geographic projection.

The 3 second resolution slope products were generated from the 1 second percent slope/degrees slope products and masked by the 3” water and ocean mask datasets.

Lineage: Source data 1. 1 arc-second percent slope product or 1 second degrees slope 2. 1 arc-second SRTM-derived Smoothed Digital Elevation Model (DEM-S; ANZCW0703014016). 3. 3 second resolution SRTM water body and ocean mask datasets

Slope calculation Slope was calculated from DEM-S using the finite difference method (Gallant and Wilson, 2000). The different spacing in the E-W and N-S directions due to the geographic projection of the data was accounted for by using the actual spacing in metres of the grid points calculated from the latitude.

The slope calculation was performed on 1° x 1° tiles, with overlaps to ensure correct values at tile edges.

The 3 second resolution versions were generated from the 1 second percent or degrees slope products. This was done by aggregating the 1” data over a 3 x 3 grid cell window and taking the mean of the nine values that contributed to each 3” output grid cell. The 3” data were then masked using the SRTM 3” ocean and water body datasets.

References Gallant, J.C. and Wilson, J.P. (2000) Primary topographic attributes, chapter 3 in Wilson, J.P. and Gallant, J.C. Terrain Analysis: Principles and Applications, John Wiley and Sons, New York.