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, all you need are the grid file and a polygon (either drawn in a base layer, or in a vector file format).
The steps are:
- Grid the data over the entire rectangular bounding box. You can choose to create a map from the grid file, if desired.
- Define the polygon for the area(s) you want to calculate the volume within. You can either:
- Add an empty base layer to the map and draw the polygon(s) you want.
- Obtain a vector file of the polygon in a vector format (e.g. DXF, SHP, BLN, KML, MIF, etc). You can choose to add this to the map as a base layer, if desired.
- Create a vector file containing the polygon. If you need help creating one, please see the article How can I create a BLN file in Surfer?.
- Click Grids | Calculate | Volume.
- In the Grid Volume dialog,
- In the Upper Surface section, choose a grid-based map from the dropdown list, or click Browse to navigate to and select your grid file.
- Since volume is calculated between two surfaces, in the Lower Surface section, either choose a different map layer or browse to a grid file for the lower surface, or set a constant Z value.
- If your z units are not the same as your x and y units, enter the appropriate Z Scale Factor to convert from the z units to the x and y units.
- In the Polygon Boundary section, choose a base map containing your polygon from the dropdown list, or click Browse to navigate to and select your vector file.
- Toggle Volume Inside, and choose whether to calculate for all polygons on the map layer or just the selected polygon(s).
- Click OK and a Grid Volume Computations report is generated.
- Save this file if desired by clicking File | Save As in the report window.
In Surfer 13 and previous, you will need to blank the grid file outside the polygon defining the area using Grid | Blank, and then 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 using a BLN file that describes the boundary of the polygon.
- Grid the data over the entire rectangular bounding box.
- Create a BLN file containing the polygon you want to blank outside of. If you need help creating a BLN file, please see the article How can I create a BLN file in Surfer?.
- Once the BLN file is created, be sure to open the BLN file in the Surfer worksheet and confirm that cell B1 shows 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 click Grid | Blank, select the grid file and click Open.
- Select the BLN file and click Open.
- Save the blanked grid to a new name.
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):
- Click Grid | Volume.
- Select the grid file and click Open.
- 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.
- The Volume Report is generated. The volume calculations will not include the areas that are blanked.
Note that the units of the volume are in X*Y*Z. If the X, Y and Z coordinates in the grid file are all in meters, then the volume is reported in m3 (and surface area is X*Y, or m2).
- 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?
Updated April 8, 2020