Dear INVEST experts:
Im puzzled in the data of slope tif due to the value achieving 0 to 440 in the past vision(3.12) and now(3.14). I used the GIS function of slope and your definded Si, which value is 0 to 15. Now, why has the differences with the slope tif .
Hello @Bochen , welcome to the forums!
Is it possible you are referring to the LS factor from SDR? If so, we made a major change to how LS was being calculated in the InVEST SDR model, which brings these outputs to be more in line with other LS calculation methods, such as those in SAGA-GIS. If so, a discussion of the differences between the LS factor calculations in various SDR versions can be viewed here, as well as the actions taken.
If this does not address your question, please let us know!
James
Thanks for explaining INVEST application, but I still have doubts about the slope.tif output from invest.SDR model. Previous versions outputting LS factor values in the range of 0 to 5000 were unacceptable, while the LS values are reasonable in the range of 0-47 in 3.14 version. The output slope raster file outputs a slope raster as shown in your equation (70). Notably, my slope file outputs values from 0 to 400, which does not agree with the calculated values of slope (Si) in GIS according to your defined formula (70), and converting percentages to degrees results in an excess of 90 degrees. Please allow me to upload the uploaded slope tif file for your reference.
slope.tif (3.1 MB)
Again, the slope factor ,S_i, is not the same as slope. The slope factor is used as a component of the LS calculation, and slope is an input.
Having said that, I agree with you that these pixel values for slope in the raster you attached do not make sense. Would you please provide your DEM so I can take a closer look?
Thanks,
James
Hello @Bochen ,
Thank you for providing your DEM! After a careful review of your DEM and a review of the source code of our slope function, I conclude that slope is, in fact, being calculated correctly. However, the definition of S_i for which you provided a screenshot, has been slightly updated.
The reason I think the slope value is being calculated correctly is that the range of allowable slope values is (0, \infty). The slope values represented here are percentage slope. A 45-degree incline represents 100% slope. A 71.6 degree incline would be 300% slope. A slope value of 440% is allowed, and it represents that there are places where there is an approximately 75-degree incline. There are several places on your landscape where the difference between two neighboring pixels is very, very large: >50m elevation gain over only 12.5m change in distance! 50m change over 12.5m would represent a percentage slope of 400%.
So, I do not think there is an issue with the slope calculation.
The updated S_i definition in the user’s guide is:
Hope this helps, and please let us know if you have any further questions.
James
Thanks for the detailed explanation about the slope factor.
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