r.slope.direction - GRASS GIS manual

NAME

r.slope.direction - Calculates slope following a direction raster.

KEYWORDS

raster, slope, direction, neighborhood, stream, trail, trac, path, road, street

SYNOPSIS

r.slope.direction

r.slope.direction --help

r.slope.direction [-a] elevation=name direction=name dir_type=string steps=string slope_measure=string output=name[,name,...] [--overwrite] [--help] [--verbose] [--quiet] [--ui]

Flags:

-a: Compute slope as absolute values; Compute slope as absolute values (default allows negative slopes)
--overwrite: Allow output files to overwrite existing files
--help: Print usage summary
--verbose: Verbose module output
--quiet: Quiet module output
--ui: Force launching GUI dialog

Parameters:

elevation=name [required]: Name of input elevation raster map
direction=name [required]: Input Direction raster map; Name of input raster map
dir_type=string [required]: Direction type; Type of diretion encoding in diections input raster map (default: auto); Options: 45degree, degree, bitmask, bitmask_k, auto; Default: auto
steps=string [required]: Number of steps; Comma separated list of steps in direction for which slope is computed; Default: 1
slope_measure=string [required]: Slope measure; Format for reporting the slope (default: degree); Options: difference, percent, percent_int, degree, degree_int; Default: degree
output=name[,name,...] [required]: Name for output raster map(s)

DESCRIPTION:
EXAMPLES
- Slope following a flow direction raster at different scales
- Slope along a street network at different scales
SEE ALSO
AUTHOR

DESCRIPTION:

r.slope.direction computes slope as elevation difference divided by distance along a user given number of steps following a direction map.

Difference in altitude to the neighboring cell in the given direction is measured and divided by the distance (determined by north south and east west resolution of the computational region). With the steps paramter the user can define how many steps along the direction map the algorithm should perform. For each step a temporary raster map is created. Thus, processing time is - in addition to the computational region - mainly determined by the maximum steps value. Multiple neighboorhoods can be given in order to produce slope measures at different spatial scales.

The method defines the algorithm to be used for measuring slope

mean measures slope as the average elevation difference over the user given number of steps along the direction map
absolute_mean measure like in the mean option but builds the average only over the absolute values of elevation difference in each step
total_gradient measures slope as the total elevation difference devided by the total distance over the user given number of steps along the direction map

The a-flag allows to compute slope as absolute elevation differences.

EXAMPLES

The following examples are based on the North Carolina dataset!

Slope following a flow direction raster at different scales

# Set the computational region
g.region -p raster=elevation

# Convert street network to raster and assign pixels direction value
r.watershed elevation=elevation accumulation=faccum drainage=fdir

r.slope.direction --o --v elevation=elevation direction=fdir \
steps=1,5,13 output=fdir_slope_1,fdir_slope_5,fdir_slope_13 \
method=total_gradient format=percent scale=3 type=CELL


Slope following flow direction for 1 pixel.	Slope following flow direction for 5 pixels.	Slope following flow direction for 13 pixels.

Slope along a street network at different scales

# Set the computational region
g.region -p raster=elevation

# Convert street network to raster and assign pixels direction value
v.to.rast input=streets_wake type=line output=streets_wake use=dir

# Directions output from v.to.rast needs to be adjusted so that:
# - direction information is coded a steps to neighboring cells
# - direction information always points to next pixel on the line
#   (only end pixels of a line should point to NULL cells)

# Deinfe variables
in_dir=streets_wake
tmp_dir=streets_wake_45
out_dir=newdir

# Recode direction information
r.mapcalc --o expression="${tmp_dir}=if(int(round(${in_dir}/45.0))==0,8, \
int(round(${in_dir}/45.0)))"

# Make sure that direction points to next non-NULL cell in network
r.mapcalc --o expression="
${out_dir}=if(${in_dir}==8,if(isnull(${in_dir}[0,1]), \
if(isnull(${in_dir}[-1,1]), \
if(isnull(${in_dir}[1,1]), \
if(isnull(${in_dir}[-1,0]), \
if(isnull(${in_dir}[1,0]),8,6),2),7),1),8) \
,if(${in_dir}==7,if(isnull(${in_dir}[1,1]), \
if(isnull(${in_dir}[0,1]), \
if(isnull(${in_dir}[1,0]), \
if(isnull(${in_dir}[-1,1]), \
if(isnull(${in_dir}[1,-1]),7,5),1),6),8),7) \
,if(${in_dir}==6,if(isnull(${in_dir}[1,0]), \
if(isnull(${in_dir}[1,1]), \
if(isnull(${in_dir}[1,-1]), \
if(isnull(${in_dir}[0,1]), \
if(isnull(${in_dir}[0,-1]),6,4),8),5),7),6) \
,if(${in_dir}==5,if(isnull(${in_dir}[1,-1]), \
if(isnull(${in_dir}[1,0]), \
if(isnull(${in_dir}[0,-1]), \
if(isnull(${in_dir}[1,1]), \
if(isnull(${in_dir}[-1,-1]),5,4),7),4),6),5) \
,if(${in_dir}==4,if(isnull(${in_dir}[0,-1]), \
if(isnull(${in_dir}[1,-1]), \
if(isnull(${in_dir}[-1,-1]), \
if(isnull(${in_dir}[1,0]), \
if(isnull(${in_dir}[-1,0]),4,3),6),3),5),4) \
,if(${in_dir}==3,if(isnull(${in_dir}[-1,-1]), \
if(isnull(${in_dir}[0,-1]), \
if(isnull(${in_dir}[-1,0]), \
if(isnull(${in_dir}[1,-1]), \
if(isnull(${in_dir}[-1,-1]),1,3),5),2),4),3) \
,if(${in_dir}==2,if(isnull(${in_dir}[-1,0]), \
if(isnull(${in_dir}[-1,-1]), \
if(isnull(${in_dir}[-1,1]), \
if(isnull(${in_dir}[0,-1]), \
if(isnull(${in_dir}[0,1]),2,8),4),1),3),2) \
,if(${in_dir}==1,if(isnull(${in_dir}[-1,1]), \
if(isnull(${in_dir}[-1,0]), \
if(isnull(${in_dir}[0,1]), \
if(isnull(${in_dir}[-1,-1]), \
if(isnull(${in_dir}[1,1]),1,7),3),8),2),1) \
,null()))))))))"

# Compute slope of the streets for three
# different step-sizes (step)
r.slope.direction -a elevation=elevation \
direction=streets_wake_dir45 steps=1,5,13 \
outputs=streets_wake_slope_1,streets_wake_slope_5,streets_wake_slope_13