How to add a custom Automated Water Extraction Index (AWEI) to the Indices tool in ERDAS IMAGINE?
A spatial model needs to be created to perform the band ratioing. This example references the research article Automated Water Extraction Index: A new technique for surface water mapping using Landsat imagery by Feyisa, Meilby, Fensholt and Proud – published 2013.
The custom index expression to be used in the spatial model is:
AWEIsh = ρband1 + 2.5 x ρband2 – 1.5 x (ρband4 + ρband5) – 0.25 x ρband7.
This formula was sourced from the research article referenced above. ρ is the reflectance value of spectral bands of Landsat 5 TM: band 1 (blue), band 2 (green), band 4 (NIR), band 5 (SWIR) and band 7 (SWIR).
Every BandSelection operator (Pband1, Pband2, etc.) will be left blank for use in the Indices tool. The spatial model created for the custom AWEI looks like this:
When the spatial model is ready, copy it to the folder: $IMAGINE_HOME\etc\models
Open a text editor and enter the following information:
AWEI Advanced Water Extraction Index
Pband1 + 2.5 * Pband2 - 1.5 * (Pband4 + Pband5) - 0.25 * Pband7
This text file needs to be saved with an extension .fpd and copied to the folder $IMAGINE_HOME\etc. The FPD file contains the information required for operator inputs, band selections, and band wavelength and width values.
With the custom model and .fpd file saved to the correct folders, launch IMAGINE's Indices tool.
ERDAS IMAGINE > Raster tab > Classification group > Unsupervised menu > Indices
The custom indices expression should now appear under the new Custom Indices category that was defined.
A key strength of mapping out the exact wavelengths and widths for the band selections is that it makes it automatically adapted for other sensors. Using this index for the intended Landsat 5 TM sensor will use bands 1, 2, 4, 5 and 7. If the sensor is changed to Landsat 8 MS the Indices tool will automatically re-map to the corresponding wavelengths. The same indices expression now uses bands 2, 3, 5, 6 and 7 as the best for the change in sensor.
This approach is highly valuable for researchers and academics. ERDAS IMAGINE Spatial Modeler has the ability to create, adapt and customize the software tools applicable for your research interests. For more information on the Hexagon Geospatial U – Educational Program refer to: http://www.hexagongeospatial.com/solutions/education
The spatial model and FPD file can be found in the file awei.zip attached to this article.