How do I make the blanked edge of a contour or image map smooth in Surfer?

In some cases, when you blank a grid file by assigning a NoData value to some nodes, there can be a jagged edge between the map data and where the blanked line is. This can happen when blanking a grid via Grids | Edit | Blank, or when gridding data via Grids | New Grid | Grid Data and incorporating a fault, selecting a gridding method that blanks the grid outside the data limits, or selecting to blank the grid outside the convex hull of the data. 

jagged blanked edge

Blanked edges can sometimes have a jagged or rough appearance.


The jagged appearance along a blanked edge is caused by the low resolution of the grid file. When the blanking boundary goes through a grid cell, the entire cell is blanked. The larger the grid cells, the more jagged it will look. The smaller the grid cells, the more smooth it will look. 


To reduce the jagged appearance of the blanked edge of a grid file, you can reduce the size of the grid cells by increasing the grid resolution (number of grid nodes). You can do this following one of these methods:


Method 1:  Regrid the data. Click Grids | New Grid | Grid Data and regrid the original data with more grid nodes, and then use the higher resolution grid file for blanking. The default is 100 grid lines in the longest direction. In the Grid Data dialog, increase the # of Nodes by a factor of 2, 5, or 10 for increasingly higher resolutions and smoother blanked edges.


Method 2: Smooth the grid. Smoothing the grid file using Grids | Edit | Spline Smooth to insert more nodes in the grid file prior to blanking.


Method 3: Resample the grid. Inserting more grid nodes by clicking Grids | Resize | Mosaic, selecting the grid file and clicking Open. In the Grid Mosaic dialog, increase the # of Nodes values and click OK.


Once the grid has a higher resolution, update the map with the higher resolution grid file. If you have a BLN file of the blanking boundary, you may also want to consider adding a base layer on top of the contour map, setting the line width for the base layer to a slightly larger width.


Updated February 13, 2017

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