Slope / Aspect Analysis

RockWorks | Utilities | Grid |
Directional Analyses | Slope/Aspect

This program is used to compute the changes in Z-values (elevations, concentrations, etc.) between neighboring nodes in an existing grid file, and store these in up to three new grid files:

• A slope grid which contains the change (or slope) between node Z-values, expressed in degrees, percent, or radians.
• An aspect grid which contains the direction of the slope, expressed in azimuth degrees.
• A second-derivative grid which contains the change in slope (the slope of the slope grid) to illustrate bends.

Your "basic" grid file containing structural elevation values, for example, can generate a nice looking contour map from which you can determine the elevation for any particular location. Or, a grid file containing contaminant data can be used to produce a contour map illustrating the concentrations as they are distributed within a study area.

The Slope/Aspect Analysis tool can take this information one step further. A slope grid can tell you the steepness of a tightly-contoured structural face, and an aspect grid can inform you of the direction it is facing. Its second-derivative grid can tell you how quickly a contaminant's concentration changes in space.

The grid files that result from the Slope/Aspect Analysis utility can be used to create the directional maps also available from the Grid menu.

Menu Options
Step-by-Step Summary

Menu Options

• Input File: Click here to enter the name of the existing grid model (.RwGrd file) for which slope and direction are to be computed. 
  Note: The horizontal and vertical units within this model must be in the same units. For example, if the X and Y coordinates are in longitude/latitude and the elevations are in feet, the results will be meaningless.  See the Coords menu tools for coordinate translation information.
• Normalize Nodes Prior to Analysis:  Leave this unchecked if your grid model represents topographic elevations. 
  If your grid model represents something other than topography (e.g. geochemical concentrations), you should check this box so that the Z values can be normalized to map units prior to computing the slope and aspect.  For example, if a grid node spacing is 10 meters and the user is analyzing geochemical data than ranges between zero and 0.01, the maximum slope would be less than one degree in which case the program would produce a blank diagram. With normalization, however, the data is now rescaled to resemble a three-dimensional cube in which the slope will have a wide range (i.e. zero to 90 degrees).
  • Automatic: Using this normalization scheme, the program will rescale the grid node values such that the minimum node value will represent zero while the maximum node value will be equal to the map distance from the southwest corner to the northeast corner of the grid model.  The map below illustrates an upgradient vector map based on normalized geochemical data that initially ranged between zero and 0.01.
  • Manual: Choose this option if you want to enter your own min and max range within which the grid node values are to be normalized before the analysis is run.
• Output: 
  • Slope: Insert a check in this check-box to create a slope grid model of the input model named above.  
    • File: Click to enter a name for the slope grid model, such as "slope.RwGrd."
      • Units: Select the desired units of the output slope grid file by clicking in the appropriate radio button.  Degrees are useful for hydrology and structure.  Percent is useful for civil engineering.  Radians are useful for geometric analyses.
  • Aspect: Check this item to create an "aspect" or direction grid model of the input model named above. 
    • File: Enter a name for the aspect grid, such as "aspect.RwGrd." Direction is recorded as azimuth degrees.
  • 2nd-Derivative: Check this item to create a "second-derivative" grid model, which represents the slope of the slope model.
    • File: Enter a name for the model, such as "2nd.RwGrd." The output units for this model are declared under Units, below.

Step-by-Step Summary

This program requires that a real number RockWorks grid model already exists.

1. Access the RockWorks Utilities program tab so that you can see the Grid menu. It is not necessary to open a datasheet.
2. Select the Grid | Directional Analyses | Slope/Aspect Analysis menu option.
3. Enter the requested menu settings, described above.
4. Click the Process button to proceed.

The program will read the source grid file, and compute the following (as requested)

• The change between adjacent grid node values, storing this slope calculation using the units you selected, under the indicated slope grid file name.  
• The downhill direction of slope for each grid node, saving this information under the indicated aspect grid file name.
• The change in node values in the slope grid, storing these values under the indicated second-derivative grid file name.

Tips

• To illustrate any of these as a 2D or 3D surface, expand the Grid Models heading in the Project Manager, and double-click on the name of the model.
• You can also use the Map | Grid-Based Map tool, choosing Use Existing Grid, specifying the slope, aspect, or second-derivative file names. 
• Use the Grid | Directional Analyses mapping options to display up- or downhill gradient maps, flow maps, and more.
  
  

Back to Grid Menu Summary

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