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How to Calculate Topographic Position Index?

Published in Topographic Index Calculation 2 mins read

The Topographic Position Index (TPI) is calculated by comparing the elevation of a specific location to the average elevation of its surrounding area.

How TPI is Calculated

According to the provided information, the Topographic Position Index (TPI) is calculated by comparing the elevation of each pixel to its surrounding neighbours. This comparison quantifies whether a location is higher or lower than the average elevation of the neighbourhood around it.

The Influence of Neighbourhood Size

A critical factor in calculating TPI, as highlighted in the reference, is the size of the neighbourhood used for comparison. This refers to the extent of the area surrounding the central pixel that is included in the calculation. The number of neighbours you compare it to makes a significant difference in the resulting TPI value. Examples of neighbourhood sizes that can drastically affect the outcome include:

  • Small Neighbourhoods: E.g., 5x5 pixels
  • Medium Neighbourhoods: E.g., 19x19 pixels
  • Large Neighbourhoods: E.g., 61x61 pixels

Choosing a smaller neighbourhood highlights local variations in elevation (like bumps or dips), while a larger neighbourhood reveals broader landscape features (like ridges or valleys).

Interpreting TPI Results

The resulting TPI value provides insight into the topographic position of a pixel relative to its surroundings. Based on the reference:

  • A TPI value of zero or near-zero indicates an area that is flat or has a near continuous slope relative to the specified neighbourhood size.

Positive TPI values generally indicate areas higher than their surroundings (like ridges or peaks), while negative values typically represent areas lower than their surroundings (like valleys or depressions).