Use Rasterize Point Data to interpolate irregularly gridded data into a raster image. ENVI’s gridding function uses Delaunay triangulation of a planar set of points. After the irregularly gridded data points are triangulated, they are interpolated to a regular grid. You can use linear or smooth quintic polynomial interpolation. You can also select extrapolation for grid points outside of the triangulation area. ENVI reads the grid points from an ASCII file, and supports different input and output projections.

1. From the Toolbox, select Terrain > Rasterize Point Data. The Enter ASCII Grid Points Filename dialog appears.
2. Select the input ASCII file. The Input Irregular Grid Points dialog appears.
3. Enter the column numbers that contain the x and y positions and z data values.
4. Select the input projection type from the list and enter a Zone number.

To enter a custom projection type, click New.

5. Click OK. The Gridding Output Parameters dialog appears.
6. To change the projection of the upper-left coordinate only, click Change Proj.
7. If your output projection is geographic, define the output pixel and image sizes in degree units.
8. To create a custom projection type, click Change Proj then click New and enter the customized map projection information.
9. Use the X/Y Pixel Size and the Output X/Y Size fields to map coordinate or latitude/longitude information for the upper-left coordinate, pixel size, and output image size.
10. Use the choices in the Options drop-down button to manage the output projection and map extent settings.
11. Select the type of interpolation by clicking the Interpolation toggle button. The choices are: Linear (quintic polynomial) or Quintic (smooth quintic polynomial).
12. Select whether or not to extrapolate edges by clicking the Extrapolate Edges toggle button.

If you select Yes, ENVI uses quintic extrapolation.

13. Select the Output Data Type from the drop-down list.
14. Select output to File or Memory.
15. Click OK. ENVI adds the resulting output to the Layer Manager.