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Occasional Contributor
Posts: 7
Registered: ‎06-14-2017
Accepted Solution

Determining correct NDVI sensor & settings

Hi all,

 

I am looking to carry out NDVI on the Environment Agency's (EA) 4-band RGBN aerial photophy. According to the EA (link), the band order is as follows: Band 1: Red, Band 2:Green, Band 3:Blue, Band 4: Near Infrared. When I go to carry out NDVI I am unsure whether I need to specify a sensor...as there is no option for EA. If I leave it blank it seems to still automatically display options for what it thinks the NIR and R band are, however it autopopulates to band 3:red and band 4:infrared, which is not the order the EA states its bands are. I'm looking for advice on how to clear this up and carry out the NDVI correctly.

 

Thanks,

 

Terindviphoto.PNG

Technical Evangelist
Posts: 832
Registered: ‎10-01-2015

Re: Determining correct NDVI sensor & settings

[ Edited ]

Hi Teri,

 

Sounds like the EA is using the same band ordering as is used for NAIP imagery (RGBN), rather than the more common "increasing wavelength" order of BGRN. So, you can either use that sensor, or you can add one easily to ERDAS IMAGINE specifically for the EA data by going into the <IMAGINE_HOME>/etc directory and creating a .saf for the data. Easiest way it to copy an existing one that's close to what you want, e.g. NAIP_RGBN.saf. Rename the file, change the display names in the text and alter the wavelengths (if necessary). Restart ERDAS IMAGINE and you should have  a new sensor available on the sensor pull-downs.

 

E.g.

 

ea_rgbn {
SensorName "EA RGBN";
BandMeasurement "Wavelength";
BandUnits "nanometers";
BandCount 4;
/* These wavelengths are derived based on Leica's ADS40 scanner, a commonly used system for generating NAIP imagery.
If a different camera was used to create the NAIP then the wavelenghts may need to be adjusted accordingly */
Band 1 { Value 635.0; Width 50.0; }
Band 2 { Value 560.0; Width 50.0; }
Band 3 { Value 460.0; Width 60.0; }
Band 4 { Value 860.0; Width 50.0; }
}

 

Or - just use the Cell Array on the dialog you screenshotted to pick the appropriate bands that represent Red and NIR by clicking in the appropriate box. We provided that option specifically for sensors that may not be already defined (or for users who may wish to perform normalised index calculations using other wavelengths).

 

Cheers

 

Ian

Ian Anderson
Chief Product Owner, Desktop Remote Sensing
Hexagon Geospatial
Occasional Contributor
Posts: 7
Registered: ‎06-14-2017

Re: Determining correct NDVI sensor & settings

Thanks for the speedy reply, that was very helpful Ian!