Using R to obtain slope raster from DEM GRID raster

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01-15-2022 12:29 AM
NoorMaria
New Contributor

After some extensive googling, I wasn't able to find my answer (first time I couldn't surmount the issue by looking at others questions/answers). I am new to asking questions, so forgive any missteps.

I am attempting to perform what ArcGIS or QGIS performs with the slope tool, just within R. To do so I have been importing a raster that I exported from ArcGIS in GRID format with the following characteristics:

class      : RasterLayer 
dimensions : 821, 581, 477001  (nrow, ncol, ncell)
resolution : 4.996121, 4.996121  (x, y)
extent     : 2832147, 2835049, 14234048, 14238150  (xmin, xmax, ymin, ymax)
crs        : +proj=tmerc +lat_0=34.75 +lon_0=-118.583333333333 +k=0.9999 +x_0=800000.000000001 +y_0=3999999.99999999 +datum=NAD83 +units=us-ft +no_defs 
source     : rr_2020_shell 
names      : rr_2020_shell 
values     : 5623.253, 6401.356  (min, max)

It is already projected in the correct coordinate system (EPSG: 3423) but when I go to find the slope using the following code:

RR_2020_Slope = terrain(RR_2020_St1_Raster,'slope', units = 'degrees', neighbors = 8, filename = 'RR_2020_Slope.grd', overwrite = T)

The result is a slope raster that ranges from 0 to 1.28°, which is very different from what I have calculated in ArcGIS using the slope tool. Using the same DEM raster in the same projection in ArcGIS I used the slope tool with an input of 'Degree' for the output measurement, 'Planar' for the method, and 1 for Z factor and my resulting slope raster ranges from 0.001 to 73.396°.

Overall I am wondering where my mistake in R originates from, is it an elevation resolution problem? Are there issues with my projection? Forgive me, I can't necessarily include the data as they are sensitive materials but perhaps there is a clear and obvious mistake in my approach or assumptions about the functions I have used?

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ManiaMulti
New Contributor

observers can help you. 

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