Calculate the volume or area inside an irregular polygon in Surfer

The ability to calculate the area or volume within an irregular polygon in Surfer is valuable for accurately analyzing real-world data, especially when dealing with natural or complex site boundaries. This feature helps users quickly estimate quantities such as material volumes or affected areas, using precise site-specific shapes. It streamlines workflows, supports regulatory reporting, and enables better decision-making by providing reliable, easy-to-export measurements directly within the software.

If you would like to calculate the planar area inside an irregular polygon, please see: Take measurements of your Map in Surfer

To calculate the volume or surface area inside a polygon, you will need both a grid file and a polygon (either drawn in a base layer, or in a vector file format).

To begin, you will need to ensure your grid data and polygon have been properly added to your Surfer project file. To do so:

1. Open your grid file by clicking File | Open.
2. Define the polygon for the area(s) you want to calculate the volume within. There are a few ways this can be done:
   1. Add an empty base layer to the map and draw the polygon(s) you want.
      1. Select your Map object.
      2. Click Home | Add to Map | Layer | Empty Base.
   2. Obtain a vector file of the polygon in a vector format (e.g. DXF, SHP, BLN, KML, MIF, etc). You can then add this file to the map as a base layer.
      1. Select your Map object.
      2. Click Home | Add to Map | Layer | Empty Base. Then select your vector file.
   3. Create a vector file containing the polygon; then, follow step 2b. If you need help creating a vector file, please see: How can I create a BLN file in Surfer?

From here, you can use the following steps to calculate the volume or surface area within your defined polygon.

1. Click the Grids | Calculate | Volume command or the  button.
2. From the Grid Volume dialog:
   1. 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.
   2. In the Lower Surface section, choose a different map layer or browse to a grid file for the lower surface; or, you can also choose to set a constant Z value.
      1. If the X, Y, and Z units are different, the resulting volume calculations are meaningless. If, for example, your XY values are in meters but your Z values are in feet, the volume results are square meters times feet, rather than cubic units. In the Options section, Z Scale Factor can be used to adjust for this problem. In this example, setting the Z Scale Factor to 0.3048 (number of meters in a foot) results in cubic meters for volume calculations. You will need to know the conversion factor to use this field.
   3. 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.
      1. Click Volume Inside, and choose whether to calculate for all polygons on the map layer or just the selected polygon(s).
3. Click OK in the Grid Volume dialog and the results are generated in a report.
4. Click File | Save As to save the report, or you can copy the information to the clipboard and paste it into another application.

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.

1. Grid the data over the entire rectangular bounding box.
2. 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?.
3. 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.
4. Then click Grid | Blank, select the grid file and click Open.
5. Select the BLN file and click Open.
6. 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):

1. Click 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 horizontal 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.

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).