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Adding a custom index to the Indices tool

by angelamanchester on ‎10-27-2016 06:57 AM (1,304 Views)

Question

How to add a custom Automated Water Extraction Index (AWEI) to the Indices tool in ERDAS IMAGINE?

 

Answer

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:

 

awei_gmdx.jpg

 

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 SH
AWEI Advanced Water Extraction Index
Pband1 + 2.5 * Pband2 - 1.5 * (Pband4 + Pband5) - 0.25 * Pband7
awei_sh.gmdx
nanometers
Raster Input.Filename
Raster Output.FilenameIn
5
Pb1
Pband1
485
70
Pband1.BandRange
Pb2
Pband2
560
80
Pband2.BandRange
Pb4
Pband4
830
140
Pband4.BandRange
Pb5
Pband5
1650
200
Pband5.BandRange
Pb7
Pband7
2215
270
Pband7.BandRange
0
Custom Indices

 

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.

 

indices_landsat5.jpg

 

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.

 

indices_landsat8.jpg

 

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.