lvslope
Input
1. img1 - A pointer to the input viff structure.
2. slope_flag - When set to 1, this flag causes the
slope image to be created. 0 means no slope image is
created.
3. slope_option - Determines the form of the slope
Output
0=degrees, 1=radians, 2=percent.
4. aspect_flag - When set to 1, this flag causes the
aspect image to be created. 0 means no aspect image is
created.
5. aspect_option - Determines the form of the aspect
output: 0=degrees, 1=radians, 2=quadrants.
6. no_aspect - Value given for the aspect of a flat
surface. The aspect gives the direction the slope is
facing. When there is no slope the, aspect is given
this value to denote that it is undefined.
OUTPUT
1. img2 - A double pointer to the slope viff struc-
ture. Only used if slope_flag is set.
2. img3 - A double pointer to the aspect viff struc-
ture. Only used if aspect flag is set.
Modifications
Description
_l_v_s_l_o_p_e computes the slope and aspect images from an input
elevation data.
The slope is always calculated as a positive number
and represents the slope in the direction of the gradient at
that point. The direction in which the slope is calculated
depends only on the direction of the steepest gradient at
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the point. The slope may be computed in three forms:
degrees, radians, and percent rise. Degrees and radians are
used to measure the angle between a tangent to the surface
at the point the slope is being calculated and a horizontal
Reference
plane. Percent rise is a measure of how much the
tangent rises with respect to the horizontal distance. For
instance, if the tangent at a point rises 50 meters for
every 100 meters horizontally, the slope is 50 percent. The
form used for the output slopes is selected using the com-
mand line argument '-os'.
The aspect of the elevation image tells which direc-
tion the slope is facing. The aspect is reported in the
direction of the increasing slope. The aspect may also be
calculated in three forms: degrees, radians, and quadrants.
Degrees and radians give the clockwise angle between the
positive x-axis (right to left on the image and east in geo-
graphical terms) and the direction the slope is facing. For
instance, if the slope is facing the top of the image
(north) then the aspect will be 90 degrees or pi/2 radians.
When reported in quadrants, the aspect is given a value that
depends on the range the direction it faces. The circle is
divided into 24 regions of 15 degrees each, and the region
that the aspect falls in determines the value that will be
assigned to it. North is always assumed to be at the top of
the image, and East is always to the right. A table of the
quadrant values is given below:
Aspect Value Range in Degrees Description
1 353 - 7 east facing
2 8 - 22 15 degrees north of east
3 23 - 37 degrees north of east
4 38 - 52 northeast facing
5 53 - 67 30 degrees east of north
6 68 - 82 15 degrees east of north
7 83 - 97 north facing
8 98 - 112 15 degrees west of north
9 113 - 127 30 degrees west of north
10 128 - 142 northwest facing
11 143 - 157 30 degrees north of west
12 158 - 172 15 degrees north of west
13 173 - 187 west facing
14 188 - 202 15 degrees south of west
15 203 - 217 30 degrees south of west
16 218 - 232 southwest facing
17 233 - 247 30 degrees west of south
18 248 - 262 15 degrees west of south
19 263 - 277 south facing
20 278 - 292 15 degrees east of south
21 293 - 307 30 degrees east of south
22 308 - 322 southeast facing
23 323 - 337 30 degrees south of east
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lvslope(3) University of New Mexico lvslope(3)
24 338 - 352 15 degrees south of east
25 no aspect (flat)
The form of the output aspect is selected using the '-to'
command line argument.
The input elevation file must be in viff format, and
may be of type byte (VFF_TYP_1_BYTE), short
(VFF_TYP_2_BYTE), int (VFF_TYP_4_BYTE), or float
(VFF_TYP_FLOAT). The input data will automatically be con-
verted to float, and both the slope and aspect output files
will always be type float regardless of the input data type.
Any type of map that is not forced (VFF_MAP_FORCE) is
allowed on the input file, and the map will be transfered as
is to the output. Explicit location data is not allowed in
the input file, and will result in an error. The pixels in
the input file are assumed to be evenly sampled where the
sampling interval is given in the 'pixsizx' and 'pixsizy'
fields in the viff header. The units used for the sampling
interval are arbitrary but must be the same units that the
elevations in the file use. The sampling interval is not
allowed to be zero. Finally, the input file is restricted
to a single image.
See also
vslope(1), intro(3), vipl(3), verror(3), vutils(3)
lvsurf(3), lvconvert(3)
RESTRICTIONS
_l_v_s_l_o_p_e works on input images with data types byte
(VFF_TYP_1_BYTE), short (VFF_TYP_2_BYTE), int
(VFF_TYP_4_BYTE), and float (VFF_TYP_FLOAT), but the type of
the output image will always be float regardless of the
input type. Maps on the input image will be transfered
directly to the output image, but forced maps
(VFF_MAP_FORCE) are not accepted. Explicit location data is
not accepted. The input image is restricted to one image
per file.
Author
Per Lysne
COPYRIGHT
Copyright 1991, University of New Mexico. All rights
reserved.
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