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Topographic Position Index (TPI) v. 1.3a
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Sep 15 2006
Status of work
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DESCRIPTION: Andrew Weiss presented a very interesting and useful poster at the 2001 ESRI International User Conference describing the concept of Topographic Position Index (TPI) and how it could be calculated. Using this TPI at different scales, plus slope, users can classify the landscape into both slope position (i.e. ridge top, valley bottom, mid-slope, etc.) and landform category (i.e. steep narrow canyons, gentle valleys, plains, open slopes, mesas, etc.).
The algorithms are clever and fairly simple. The TPI is the basis of the classification system and is simply the difference between a cell elevation value and the average elevation of the neighborhood around that cell. Positive values mean the cell is higher than its surroundings while negative values mean it is lower.
The degree to which it is higher or lower, plus the slope of the cell, can be used to classify the cell into slope position. If it is significantly higher than the surrounding neighborhood, then it is likely to be at or near the top of a hill or ridge. Significantly low values suggest the cell is at or near the bottom of a valley. TPI values near zero could mean either a flat area or a mid-slope area, so the cell slope can be used to distinguish the two.
This extension calculates Topographic Position Index (TPI) grids from elevation grids, and provides a simple and repeatable method to classify the landscape into slope position and landform category using the TPI values. This extension also provides additional functions to standardize and classify grids, and to generate grid and neighborhood statistics.
REVISIONS: Version 1.3a (September 15, 2006) fixes a bug in which Slope Position and Landform class grids were not sorted correctly, so that classes that referenced the same region on the landscape might not receive precedence according to class number.
Detailed operating instructions may be found in the "TPI_Documentation.pdf" document or on-line at:
For more ArcView Extensions and other software by the author, please visit the author's software download page at:
Enjoy! Please contact the author if you have problems with this extension.