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| How can I calculate the volume and/or area inside an irregular polygon? |
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To simply calculate the planar area inside an irregular polygon, please see: How can I measure the distance/length or area on a map?
If you want to calculate the volume or surface area inside a polygon, blank the grid file outside the polygon defining the area and use the Grid | Volume command. The blanked areas are not included in the volume calculations and are separated out in the surface area calculations.
Step 1: Blank the Grid File to the Desired Area
The area outside the desired polygon area can be blanked in a number of ways:
Option 1: Use a BLN file to blank the GRD file.
Grid the data over the entire rectangular bounding box and blank the area inside or outside a polygon defined in an ASCII BLN file. If you need help creating a BLN file, please see instructions on how to create a BLN file. Once the BLN file is created, be sure to open the BLN file in the Surfer worksheet and confirm that cell B1 includes a 0 (zero), which tells Surfer to blank outside the polygon in the BLN file. If it is a 1, change it to a 0, save and close the file. Then use the Grid | Blank command to blank the grid file with the boundary in the BLN file.
Option 2: Grid the data and specify a reduced search radius.
Grid the data with a method that supports Search Radii. Gridding methods have this option are: Inverse Distance to a Power, Kriging, Modified Shepard’s Method, Nearest Neighbor, Radial Basis Function, Moving Average, Data Metrics, and Local Polynomial. The grid is blanked when data are not within the search radii.
To use this method, go to Grid | Data, select your data file and click Open. In the Grid Data dialog, select one of the above gridding methods and click on the Advanced Options button, then click on the Search tab. Remove the check mark from the No Search (use all of the data) if necessary, and specify the Radius 1 and Radius 2 values.
Option 3: Choose a gridding method that limits interpolation to the convex boundary around the data
Some gridding methods do not create data where there was no data to begin with. Go to Grid | Data, select your data file and click Open. In the Grid Data dialog, choose either Triangulation with Linear Interpolation or Natural Neighbor as the gridding method.
Step 2: Calculate the Volume and Area
Once the grid is blanked, so that there is only data inside the area you want (you may want to create a contour map first to confirm this):
1. Go to Grid | Volume.
2. Select the grid file and click Open.
3. In the Grid Volume dialog, the grid file will be entered as the Upper Surface. The volume calculations calculate the volume between two surfaces. You can calculate between a grid file and a horiztonal surface, or between two grid files. Enter your desired input for the Upper Surface and Lower Surface and click OK.
4. The Volume Report is generated. The volume calculations will not include the areas that are blanked. The units of the volume are in X*Y*Z. If your X, Y and Z data are all in meters, then the volume is reported in m^3 (and surface area is X*Y, or m^2).
See Also:
How can I create a BLN file?
How can I grid only the area inside my data limits?
How can I create a map inside an irregular boundary?
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