Tutorial: Create a Grid-Based 2D Contour Map

Estimated time: 4 minutes.

Contour maps illustrate elevations or other measured data in the study area using lines or colored intervals. "Grid-based" contours are constructed from an intermediate numeric model that the program computes for the given data points, extrapolating data into areas without control points. This model contains X,Y,Z values in a regularly-spaced grid (hence its name, "grid model") which can serve as the basis for 2D and 3D contour maps, for volumetric computations, and more.

! This lesson assumes that you have completed the previous lesson before continuing. All of the mapping settings will be the same as those used in that lesson.

1. With the Maps_Soil_Properties file still displayed in the datasheet, locate the ModOps program grouping in the main menu ribbon. Select the Grid | Create | XYZ - Grid | Based on XYZ Data menu option.
   
2. Set up the Grid Model settings by clicking on that tab.
   1. Data Source: Be sure the Datasheet is set as the input source. (In your own work, if you want to create a grid model from a huge list of XYZ points, you can bypass the loading of the points into the datasheet by reading directly from the external ASCII file.) The following columns should still be selected.
      • X (Easting): Easting
      • Y (Northing): Northing
      • Z (Elevation): Elevation
   2. Grid File: Click this tab to enter the name for the output grid model: Elevations      RockWorks will append the ".RwGrd" file name extension automatically.
   3. Dimensions: Click on this tab.
      • Based on Project Dimensions: This should be the default setting. The surfaces will be dimensioned as per the settings under the Project Dimensions.
        ! In your own work, we recommend you choose this option so that the grid and solid model dimensions are consistent. However, the program does offer the option to vary the model dimensions, under the Variable heading.
      • Confirm Model Dimensions: Check this. The program will display a summary window prior to creating the model.
   4. Algorithm: Click the here to choose the gridding method.
      • Triangulation: Click in this radio button. This method builds a network of triangles similar to the previous map, and then calculates the slope of each triangle to determine the elevation at each grid node location.
        • Interpolate Edge Nodes using Inverse Distance: Choose this option. This tells the program to compute grid node values outside the control points using the Inverse Distance algorithm, as opposed to assigning them a constant value. Refer to the links below for more information about gridding and Triangulation gridding.
      • ! Note that the options which are visible will change if you choose a different gridding method.
   5. Options: Set these options.
      • Faulted: Off
      • Decluster: On
      • Logarithmic: Off
      • High Fidelity: Off
      • Polyenhance: Off
      • Densify: Off
      • Maximum Distance Filter: Off
      • Z=Color: Off (This is used for color modeling only.)
      • Smooth: On (Default size and Iterations = 1)
      • 
        
3. 2D Grid Map: Check this box to request a map. Click on the tab to access the settings for the map. Here you should see almost the exact same map settings as you saw for the Triangulation Contour Map, and the settings you established for that map should still be shows as default.
   • Colored Intervals, Contour Lines, Labeled Axes, Map Overlays (Symbols and Symbol Labels), and Peripherals should all still be checked.
   • Labeled Cells, Gradient Vectors, Background Image, Other 2D Files, and Border can remain un-checked.
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4. 3D Grid Diagram: Uncheck this.
   
5. Click Continue to proceed.

   Because the "confirm intervals" option was activated earlier, the program will display a window summarizing the Z (elevation) values found in the datasheet, and some recommended labeled and unlabeled line contour intervals, and color-contour intervals.

6. Click OK to accept these contour intervals.

   Next, RockWorks will display a summary of the grid dimensions. This window will summarize the map coordinates at the north, south, east, and west boundaries of the grid model, and the spacing of the grid nodes in these map units. In your own work, seeing this window can be helpful in knowing what the program will be doing. Or, if it's annoying, you can turn it off in the Grid Model | Dimensions tab.

7. Click OK to accept these grid dimensions.

   RockWorks will scan the datasheet, determine the location coordinates for each sample site, and build a grid model of the data using the Triangulation gridding method. The grid model will be saved on disk under the specified RwGrd file name.

   RockWorks will then start to build the map, creating line contours and color-filled contour intervals to represent the surface elevations stored in the grid model. It will overlay point symbols and labels to illustrate the drill hole locations, and annotate the border with titles, ticks, and coordinates. The completed map will be displayed in a new RockPlot2D tab.

8. Find the other map window: Hover over the RockWorks icon on the Windows taskbar, and click in the RockPlot2D window from the previous lesson. This is the Triangulation Contour map you made in the previous step. Arrange the two on the screen so that you can see them both.
   
9. Compare the two maps. Note how the triangulation contours extend only to the edges of the known points, while the grid-based contours extend to the edges of the map rectangle.
   • In your own work, you may find it handy to create a quick triangulation contour map first for a quick look - the contours may not be pretty but they'll honor the control points.
   • You may then create grid-based contours, comparing gridding algorithms and how well they represent the data. There are many benefits of having a good grid model - you can view it as a surface in 3D, you can filter it, you can use it to constrain a solid model, you can compute volumes... The Grid menu in the Utilities is dedicated to manipulation and visualization of grid models.
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10. Use the Zoom In tool if you want a closer look at either map. To use this tool, click the Zoom In button on the toolbar, and either single-click where you want to zoom in, or draw an enlargement rectangle on the screen.
    Use the Undo Last Zoom button to step back one zoom level, or the Zoom Out button to see the entire map.
    
11. Save the grid-based contour map: Select File | Save and assign this map the name: Grid Contours
    
12. Close this window by clicking the X button in the upper-right.
    

 Grid-Based Maps, Gridding Reference, Triangulation Gridding
 

See also: RockPlot2D Tutorial for lessons on editing graphics.

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