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| I want to use a BLN blanking file to blank the area outside a polygon, but when I load the BLN file as a base map, I can only fill inside the polygon. |
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Q: I want to use a [.BLN] blanking file to blank the area outside a polygon, but when I load the [.BLN] file as a base map, I can only fill inside the polygon. How can I blank outside my polygon?
A. The Map | Base Map method does not have the ability
to fill outside a polygon, but there are other methods to consider:
- If you are making a map with a
[.GRD] or [.DEM] file, use the Grid | Blank command to blank the grid
nodes that are outside the polygon. Set the header flag to 0
to blank outside. Create a new map with the new [.GRD] file.
-or-
- Modify the [.BLN] file to add a
frame around the internal polygon. For example, if you want to
fill the area outside a square that extends from (1,1) to
(2,2), and the map ranges from (0,0) to (9,7), then the [.BLN]
format is as follows:
|
Example of
modified file with frame around the internal polygon. |
Modified file in [.BLN] format
11, 1
|
|
"Total number of values, blanking flag 1 (blank inside)
|
| 0.0, 0.0 |
|
"x1, y1 |
| 9.0, 0.0 |
|
|
| 9.0, 7.0 |
|
|
| 0.0, 7.0 |
|
|
| 0.0, 0.0 |
|
"Repeat x1, y1 to close
outer polygon |
| 1.0, 1.0 |
|
"Start internal polygon |
| 2.0, 1.0 |
|
|
| 2.0, 2.0 |
|
|
| 1.0, 2.0 |
|
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| 1.0, 1.0 |
|
"Close internal polygon |
| 0.0, 0.0 |
|
"Repeat x1, y1 to close
outer polygon |
The [.BLN] format draws the connecting line from the internal polygon to
the repeated (x1, y1) point at (0,0). Set the line style to Invisible for
this base map, or use the [.BNA] format, which does not draw the connecting
line:"Start internal polygon.format draws the connecting line from the internal polygon to
the repeated (x1,y1) point at (0,0). Set the line style to Invisible for
this base map, or use the [.BNA] format, which does not draw the connecting
line:
Modified file in [.BNA] format
"","",11
0.0 0.0
9.0 0.0
9.0 7.0
0.0 7.0
0.0 0.0
1.0 1.0
2.0 1.0
2.0 2.0
1.0 2.0
1.0 1.0
0.0 0.0
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