[GRASS-user] r.horizon result depends on mode

[Jared Oyler]

All,

I am currently trying to calculate east (direction=0) and west
(direction=180) horizons for a single location (lon,lat=-105.007,40.05).  I
am using a 30m DEM (USGS NED) in geographic coordinates:

projection: 3 (Latitude-Longitude)
zone: 0
datum: nad83
ellipsoid: grs80
north: 42:00:06N
south: 38:59:54N
west: 107:00:06W
east: 103:59:54W
nsres: 0:00:01
ewres: 0:00:01
rows: 10812
cols: 10812
cells: 116899344

When I run r.horizon in single point mode and degree output:
r.horizon -d elevin=elev horizonstep=180 coord=-105.007,40.05 dist=1.0

I get the following result:
0.000000, 0.574081
180.000000, 0.645220

However, when I run r.horizon in raster map mode for the w horizon and
extract the corresponding grid cell, I get a different result:

r.horizon -d elevin=elev direction=180 horizon=horizonW
r.what input=horizonW_0 east_north=-105.007,40.05

produces:
-105.007|40.05|2.247817

Is there any reason why single point mode and raster map mode would produce
different horizon results for the same point (0.645220 vs. 2.247817)?

Thanks,

Jared Oyler
Numerical Terradynamic Simulation Group (NTSG)
College of Forestry & Conservation
The University of Montana
Email: jared.oyler at ntsg.umt.edu
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