slpmap

This program creates a new cube containing slope (radians, degree,s or percent) or aspect (radians, degrees) data from an input cube. This program computes the slope and aspect of an input cube and outputs the results. An individual slope, aspec, or percent slope cube can be rendered using this program.

Data requirements to run slpmap:

• A digital terrain model (DTM) with DN values representing the radius or elevation of a target body, or
• Any ISIS cube that is a level 1 or level 2 image (can be multiband).

Horne's algorithm is used to compute the slope or aspect. A 3x3 kernel:

      A  B  C
      D  E  F
      G  H  I
    

Each output pixel is computed as follows:

      SLOPE EQUATION
      [dz/dx] = ((C + 2F + I) - (A + 2D + G)) / (8 * X_PIXEL_RESOLUTION)
      [dz/dy] = ((G + 2H + I) - (A + 2B + C)) / (8 * Y_PIXEL_RESOLUTION)

      slope = ATAN ( SQRT ( [dz/dx]^2 + [dz/dy]^2 ) )
      percentslope = slope / 90
    

The slope equation above assumes the pixels are not square: hence X_PIXEL_RESOLUTION and Y_PIXEL_RESOLUTION. That is, the x distance across the pixel is not equal to the y distance form top to bottom of the pixel.

      ASPECT EQUATION
      [dz/dx] = ((C + 2F + I) - (A + 2D + G)) / 8
      [dz/dy] = ((G + 2H + I) - (A + 2B + C)) / 8

      aspect = 90 - ATAN2 ([dz/dy], -[dz/dx])
      if (aspect < 0) then aspect = aspect + 360
    

Slope: Slope is typically between 0 and 90 degrees, were 0 is flat and 90 is vertical.Slope may be output in radians, degrees, or percent slope.

Aspect: The aspect represents the direction or 0 to 360 degrees of the slope in pixel space (see figure below). From the center pixel (E), 0 degrees is straight towards B (generally north), 45 is towards C, 90 is towards F, 135 is towards I, 180 is towards H, and so on.

      A  B  C
      D  E  F
      G  H  I
    

Scale Construction: By default, the program will attempt to remove any scaling differences by using the map projection information (PIXRES=AUTOMATIC). This computation is done at every pixel so the correct x-to-y ratio is computed. This is important, for example, for global maps where the x/y ratio deviates with distance from the latitude and/or longitude of true scale in a map projection. This only works for radius DTMs.

What if the input cube Z units are in elevation values instead of radius values?

The default setting PIXRES=AUTOMATIC will not work. You will need to set PIXRES=FILE (recommended for a map projected file) or PIXRES=USER. Note: unlike PIXRES=AUTOMATIC, these later two methods do not correct for distortions int he map projection. They will return good results for smaller regious when the map projection is defined to minimixe distortions (e.g. LROC NAC or HIRISE sereo DTMs).

What if the input cube is not map projected?

If the image lacks a map projection,you must provide the pixel resolution via PIXRES=USER (for this option the pixels are assumed to be square). You must also provide a single value using the RESOLUTION parameter that will be applied to all pixels in the image and in both directions.

What if the xy units are not the same as the z units?

The program assumes the xy units are the same as the z (pixel) units. By default the program assumes the units are the same, but allows you to scale the z units to the xy units using the CONVERSIONFACTOR parameter if the PIXRES=USER option is selected.

Description

Use this parameter to select the input cube.The algorithm is applied to all bands of a multiband cube.

Type	cube
File Mode	input
Filter	*.cub

Description

Use this parameter to define the filename of the resultant slope, aspect, or percent slope cube.

Type	cube
File Mode	output
Pixel Type	real

Description

This specifies the type of output pixels that will be created. The output file will contain either slope, aspect, or, percent slope.

Type	string
Default	SLOPE
Option List:	Option Brief Description SLOPE Create slope cube This creates a slope cube. ASPECT Create aspect cube This creates an aspect cube. That is, the direction/azimuth of the slope. Exclusions PIXRES RESOLUTION CONVERSION PERCENTSLOPE Create percent slope cube This creates a slope cube whose pixel values are percentages (slope/90) instead of angles. Exclusions UNITS

Description

Units of the output pixels

Type	string
Default	DEGREES
Option List:	Option Brief Description RADIANS Output pixels in radians The output pixels will be in radians, between 0 and PI/2 for slope and 0 and 2PI for aspect. DEGREES Output pixels in degrees The output pixels will be in degrees, between 0 and 90 for slope and 0 and 360 for aspect.

Description

This determines how the resolution of the input pixels will be defined.

Type	string
Default	AUTOMATIC
Option List:	Option Brief Description AUTOMATIC Automatically compute the resolution per pixel using the map projection The program will use the map projection to compute the resolution (X and Y) of each pixel to ensure the slope is computed properly. This option requires that the input cube be a radius file, a cube in which the DN values represent the radius of the target at that point (no negative DN values). Exclusions RESOLUTION CONVERSION FILE Pixel Resolution of FROM cube's projection. The input cube must be projected if this option is selected. The pixel resolution will come from the mapping group of the input cube. Exclusions RESOLUTION USER User defined pixel resolution If the input cube does not have a map projection this option can be used to define the resolution which is assumed to be square. A single resolution is used for the entire image in both the X and Y directions. Note: If the pixel height varies significantly from the width, the slope will not be computed correctly.

Description

This parameter is a multiplicative factor for the resolution. It is used to convert the spatial units (pixels) to the correct distance units. Since the slope algorithm depends on cancelling out the units, the conversion factor from the z vertical (input pixel) units to the xy horizontal (spatial) units needs to be considered. For example, if the xy units are in meters and the z units are in kilometers the unit conversion factor should be 1000 as we are converting z kilometers to xy meters. The default presumes the units are the same.

Type	double
Default	1.0

Description

Defines both the X and Y pixel resolution to be used across the entire image. Note that if the pixel height varies significantly from the width, the slope will not be computed correctly.

Type	double