I am currently working on trend analysis over globe among hydrological variable. how I can count increasing and decreasing pixel for specific classes trend maps using ENVI, ArcGIS or IDL Codes? I wanna show increasing decreasing by bar graph?
This is just an idea in ArcGis. Since you are discussing trend, it has positive or negative slope. Assign or calculate to each pixel its positive or negative slope percentage for the variable(s) of interest. Then use color coding such as red = high negative %, yellow = neutral slope, and green = high positive % -- whatever color range that suites you for intensity. You might let the colors be transparent and put topography (elevation) in as shaded relief in background. You can make any bar graphs you want such as composite trend for each country or each physiographic area as a mean change based on all pixels. If the values you generate suggest no significance, then you may be stuck with a great looking map with little meaning. Since I cannot imagine the trends you are thinking about on a global scale, it is hard to visualize what you may find. Sometimes we would not know without trying. Your actual data calculation points you could also use the colors described, but change symbol for level of significance, such as star is highly significant, square is moderately significant and dot is low significance.
Hi,
Significance of spatial data is difficult to estimate as values are often spatially autocorrelated (nearby values are more likely to have similar values). This leads to lack of independence and means that traditional estimates of statistical significance are biased. There are a number of fairly complicated ways to get around this, but it might be easier to present the trends without significance estimates and let users of the data draw their own conclusions.
Sincerely,
Martyn
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