Topographic Position Index derived from 1" SRTM DEM-S

Brief description

Topographic Position Index (TPI) is a topographic position classification identifying upper, middle and lower parts of the landscape. This dataset includes a mask that identifies where topographic position cannot be reliably derived in low relief areas.

The TPI product was derived from Smoothed Digital Elevation Model (DEM-S; ANZCW0703014016), which was derived from the 1 arc-second resolution SRTM data acquired by NASA in February 2000. A masked version of the TPI product was derived using the slope relief classification product.

The TPI data are available at 1 arc-second and 3 arc-second resolution.

The 3 arc-second resolution dataset was generated from the 1 arc-second TPI product and masked by the 3” water and ocean mask datasets.

Lineage

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

Topographic position index calculation
TPI is a measure of topographic position, classified into three classes corresponding to upper slopes, mid-slopes and lower slopes. The method follows that of the "Drainage Channels Class" section of Warner, Cress and Sayre (2008) which is based on the TPI method of Jenness (2006) and Weiss (2001).

The TPI classification uses relative elevation as a fraction of local relief; where the relative elevation is high compared to the local relief the class is upper slope, and where the relative elevation is low compared to local relief the class is lower slope. Intermediate values are classified as mid-slopes. This use of residuals compared to a smoothed elevation model to produce relative elevations is similar to the method described by McRae (1992).

Relative elevation is the difference between local (cell) elevation and the mean elevation over a 300 m radius circle (approximately: the calculation actually uses 10 grid cells at 1 arc-second resolution). Local relief is calculated as the standard deviation of elevation over the same circular region. The classification is:

TPI = 1 if relative_elevation < -0.5 * local relief (lower slopes)
3 if relative_elevation > 0.5 * local relief (upper slopes)
2 otherwise (mid slopes)

In relatively flat areas the finite accuracy of a DEM limits its ability to discriminate topographic position. The mask included with the TPI layer identifies areas that are too flat to reliably identify upper, middle and lower landscape positions. It is based on the 'Slope-Relief' classification and the TPI mask has values of 1 where there is sufficient relief for TPI to be meaningful and 0 where TPI should not be used.

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

The 3” arc-resolution version was generated from the 1” TPI class and mask 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 result was then converted to integer format, avoiding truncation errors and ensuring that (for example) values between 1.5 and 2 were assigned to class 2, and values between 2.5 and 3 were assigned to class 3. The 3” TPI and TPI mask data were then masked using the SRTM 3” ocean and water body datasets.