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| I want to blank outside several polygons on a map, but the entire map gets blanked, resulting in a horizontal planar grid. |
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Q: I want to blank the grid nodes outside several polygons on a
map, but the entire map gets blanked, resulting in a horizontal planar
grid.
A: It
may be that the area outside polygon A is inside polygon B, and
vice versa, causing the entire map to be blanked.
- Combine the two
polygons into a single polygon, and repeat the first vertex of the
first polygon at the end of the second polygon to create a single
combined polygon.
- Change the header of the combined polygon to
reflect the total number of vertices's in both simple polygons, plus
the repeated first vertex.
- If you load the combined polygon as a
base map, a line is drawn back to the first vertex, so you may
need to keep both versions of the [.BLN] available, the separate
polygons to use as a base map, and the combined polygon to use for
blanking purposes.
- It is important to repeat the original x1 y1 value after every polygon in order to avoid residual artifacts from the blanking process.
| Two separate polygons
5 0 'Polygon A (has 5 values)
1.0 1.0
1.0 2.0
2.0 2.0
2.0 1.0
1.0 1.0
5 0 'Polygon B (has 5 values)
3.0 3.0
3.0 4.0
4.0 4.0
4.0 3.0
3.0 3.0 | One combined polygon
11 0 'Polygon A (has 11 total values)
1.0 1.0 'x1 y1
1.0 2.0
2.0 2.0
2.0 1.0
1.0 1.0
3.0 3.0 'Polygon B
3.0 4.0
4.0 4.0
4.0 3.0
3.0 3.0
1.0 1.0 'repeat x1 y1 | 
Blanking the area
outside
multiple separate polygons blanks
the entire grid
| 
Editing the blanking file
to combine the polygons
resolves the problem.
|
In a more complex example, consider adding a third polygon.
One composite BLN file with three polygons:
17 0 Composite Polygon consisting of Polygon A, B, and C
1 1 Polygon A - Original X1 Y1
1 2 Polygon A
2 2 Polygon A
2 1 Polygon A
1 1 Repeat of X1 Y1 and closure of Polygon A
3 3 Polygon B
3 4 Polygon B
4 4 Polygon B
4 3 Polygon B
3 3 Polygon B
1 1 Repeat of X1 Y1
7 5 Polygon C
7 6 Polygon C
8 6 Polygon C
8 5 Polygon C
7 5 Polygon C
1 1 Repeat of X1 Y1
|  |
Please see the KB_GridBlank_w_MultiplePolygons.SRF example in the attached ZIP file below to see the result of using the Grid | Blank command with a composite BLN that defines three polygons.
Also see attached script which automatically converts a BLN file with multiple polygons into a BLN file with a single complex polygon. Download the script and open it in the Scripter program that comes with Surfer. Go to Script | Run and follow the prompts.
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