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I processed one country with TPI, and I got a certain Landforms classification of terrain. (Topographic Position Index (TPI) calculation as proposed by Guisan et al. (1999)). When I processed only one municipality around that state with TPI, I got a different result, in fact more detailed. The calculation parameters were the same. This worries me because I thought to analyze the whole country, and then I would use the zonal statistics tools for each municipality individually and make conclusions based on the obtained forms of the field. I am writing a scientific paper in this field, so I really care about real data.

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  • Are the large and small rasters at the same resolution? – jberrio Jan 31 at 7:10
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    @nagib You are talking about how TPI is calculated. My point is about how the tool put into classes based on TPI. Besides, a lot more than 8 neighboring cells are within 1000m outer radius in this calculation. – Kazuhito Jan 31 at 12:19
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    We can try to find sample places to be used as "ground truth". Compare calculated classifications against the real topography (as best as you know about the region). Especially ridges (> 1.0 SD) and valleys (< -1.0 SD) are good controls (i.e. high and low extremes). If you do not have ridges in your study area, try to expand the area slightly to cover such topography. It will be tedious trial and error but worth trying. – Kazuhito Jan 31 at 14:09
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    I found out what the problem was. I have square DEM. The state has curved boundaries. All pixels across the boundary have a value of 0 (was clipped). The TPI algorithm counted these pixels as well, and so I got such a result. I clipped again my country and removed pixels with a value of 0 and got a satisfactory result. ;-) – nagib Jan 31 at 20:07
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    I think you should write an answer based on your comment. Self-answering is both permitted and encouraged, – PolyGeo Feb 5 at 22:12
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I found out what the problem was. I have square DEM. The state has curved boundaries. All pixels across the boundary have a value of 0 (was clipped). The TPI algorithm counted these pixels as well, and so I got such a result. I clipped again my country and removed pixels with a value of 0 and got a satisfactory result.

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