Let's say you want to color only the areas of the map where the Z value is less than 150 (so all areas where Z is greater than 150 are transparent). You could do this a few different ways.
If you have a map:
- If you are using a contour map, then simply set the contour maximum value to the value you want. For example:
- Select the Contour layer in the Contents window.
- In the Properties window on the Levels page, set the Maximum Contour to 150. No contours above that will be drawn.
- To eliminate the fill if the contours are filled, either:
- Change the Level method to Advanced, click Edit Levels, double click the Fill for the last layer, change the Opacity to 0%, and click OK twice.
- Click the Custom colormap (...) button to the right of Fill colors to open the Colormap dialog.
- In the Colormap dialog:
- In the Data to Color Mapping section, set the Maximum to 150.
- Select the last node.
- Uncheck the box next to Apply opacity to ALL nodes, then decrease the Opacity to 0%.
- Click OK and the new color map is applied.
- Select the Color relief layer.
- In the Properties window on the General page, click the Custom colormap (...) button to the right of Colors to open the Colormap dialog.
- Perform step 3a above.
If you want to edit the grid to remove the nodes above that cutoff value:
- Click Grids | Calculate | Math.
- In the Grid Math dialog, click the Add Grids button, select the grid file and click Open.
- Enter the function: IF (A>=150, 1.70141e+38, A). This tells Surfer that if the Z value in the grid is greater than or equal to 150, then convert it to the Surfer NoData (1.70141e+38), otherwise leave the grid value alone.
- Enter a new Output Grid name and click OK to generate the new grid.
- Create the map from the new grid.
Updated January 11, 2017