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.
- With the Soil_Properties_01.RwDat file still displayed in the datasheet, select the Map | Grid-Based Map | Based on XYZ Data menu option.
- Input Columns: The prompts along the left side of the window should still read the following (from the previous lesson); if the wrong column title is displayed, just click on the down-arrow and scroll up or down to the correct column title.
- X (Easting): Easting
- Y (Northing): Northing
- Z (Elevation): Elevation
- Data Source: Choose Datasheet. (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 in the Utilities datasheet by reading directly from the external ASCII file.)
- Grid Model: This heading contains important settings for how the program will interpolate the elevations in the grid model.
Create New Grid: Select this option, to tell RockWorks that you want to interpolate a new grid model for the soil data. Expand this heading.
- Grid Name: Click to the right to type in a name for the grid model which will be created: elevations
RockWorks will append the ".RwGrd" file name extension automatically.
- Gridding Method: Click the Options button here. The left side of this window llists the gridding methods, and the right side the additional gridding options.
- Algorithms: These options determine the method that will be used to interpolate the grid model.
- Triangulation: Click in this radio button. This method builds a network of triangles similar to EZ Map, and then calculates the slope of each triangle to determine the elevation at each grid node location.
- Interpolate Edge Nodes using Inverse Distance: Click in this radio button. 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.
- Dimensions: Click on this tab along the right side of the window.
- Based on Output Dimensions: This should be the default setting. The surfaces will be dimensioned as per the settings under the Output Dimensions. If you’d like to double-check these settings, you can click the Adjust/Examine Output Dimensions button to view the window you saw back in the Set Output Dimensions lesson.
! 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 Dimensions heading.
Confirm Grid Dimensions: Check this. The program will display a summary window prior to creating the model.
- Additional Options: Set these options.
- Decluster: On
Logarithmic: Off- High Fidelity: Off
- Polyenhance: Off
- Smooth: On (Default size and Iterations = 1)
- Densify: Off
- Maximum Distance Filter: Off
- Z=Color: Off (This is used for color modeling only.)
- Faulted: Off
- Click the OK button to accept these gridding settings.
- Create 2-Dimensional Grid Diagram: Check this box to request a map. Click on the Options button to the right to access the settings for the map.
- Here you should see almost the exact same map settings as you saw for the EZ Map, and the settings you established for that map should still be shows as default.
- Symbols, Symbol Labels, Contour Lines, Colored Intervals, and Border should all still be checked.
- Background Image, Labeled Cells, Fault Polylines, and Polygon(s) can remain un-checked.
- Click OK to close the Map Options window.
- Create 3-Dimensional Grid Diagram: Uncheck this.
- Create Grid Statistics Report: Check this.
- Click the Process button at the bottom of the window.
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.
- Click OK to accept these.
Next, it 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 Gridding Options window.
- Click OK to accept these grid dimensions.
The program 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 name elevations.RwGrd. A summary of this model's statistics will be displayed in a text tab.
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.
- Do some window-arranging: Make this plot window smaller (by clicking and dragging on the lower-right corner of the window). Move this window to the upper-left corner of your screen.
- Now 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 EZ contour map you made in the previous step. Arrange the two on the screen so that you can see them both.
- Compare the two images. Note how the EZ 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 an EZ 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.
- 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.
- Save the grid-based contour map: choose File | Save and assign this map the name: grid map
- Click in the Grid Statistics tab which should be displayed next to the map tab.
This will display a summary of the data in the interpolated grid model: dimensions, total nodes, grid area.
! In your own work, if the grid model's Z values represent thickness, refer to the Volume item for isopach volume computation.
- Leave this window open - we're going to create one more diagram in this Grid Based Map window.
Grid-Based Maps, Gridding Reference, Triangulation Gridding
See also: RockPlot2D Tutorial for lessons on editing graphics.
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