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Formula for Enhanced Vegetation Index (EVI) in Raster Calculator?

StephenChignell

Hi everyone,
I'm trying to derive Enhanced Vegetation Index (EVI) from Landsat 5 TM data using the raster calculator and I can't seem to get the output I want. Here is a link to the description and formula for EVI:

http://en.wikipedia.org/wiki/Enhanced_vegetation_index

The output should be a raster with values ranging from -1 to 1, but I get values from like 30-40 which is incorrect.

Right now my formula looks like this:

2.5 * (Float("%Band 4%" - "%Band 3%") / Float("%Band 4%" + 6 * "%Band 3%" - 7.5 *"%Band 1%" + 1))

Does anyone see something in here that doesn't make sense-- or even better--a formula I could copy/paste?

Thanks!

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RushatiDey

Hello, could you calculate the EVI values? I'm facing the same issue. Even after ToA correction,the values are not between -1 to +1.

Luke_Pinner

The EVI formula only works with reflectance (from 0 - 1) because of the coefficients in the formula - L=1, C1 = 6, C2 = 7.5, and G (gain factor) = 2.5

NDVI has no such coefficients, so you can calculate it regardless of whether your data is scaled or not.

Note that that scale factor I referred to is specifically for the MODIS MYD09A1 surface reflectance product. MODIS MYD09A1 surface reflectance values are multiplied by 1000 and then stored as 16-bit integer data to reduce file size/storage space. The scale factor converts the 16 bit values to floating point values from 0.0 - 1.0

If you want Landsat EVI, you can just download it these days. If you want to calculate it yourself, you need to use reflectance data (not DN) which you can either download (the surface reflectance product) or calculate it yourself (either simple top of atmosphere reflectance or atmospheric correction to surface reflectance).

SalmanAhmed3

Can you tell me what is the purpose of this scale factor? Ive calculated NDVI from Landsat TM and never applied this scale factor. Ive always got the correct values nevertheless. Thats why Im a bit confused why to use this?

ahmedzaeen1

Please, I tired to do it, but unfortunately, I have got map has the same color. just one color?????

MeganMcSherry

Are you applying the MYD09A1 scale factor of 0.0001?

2.5 * (("%Band 4%" * 0.0001 - "%Band 3%" * 0.0001)) / (("%Band 4%" * 0.0001) + 6 * ("%Band 3%" * 0.0001) - 7.5 * ("%Band 1%" * 0.0001) + 1))

No I hadn't been...the numbers make so much more sense now! Thank you for bringing that to my attention, I'm still learning. Much appreciated.

Luke_Pinner

Are you applying the MYD09A1 scale factor of 0.0001?

2.5 * (("%Band 4%" * 0.0001 - "%Band 3%" * 0.0001)) / (("%Band 4%" * 0.0001) + 6 * ("%Band 3%" * 0.0001) - 7.5 * ("%Band 1%" * 0.0001) + 1))<SPAN class="line-numbers-rows"><SPAN>‍</SPAN></SPAN>

MeganMcSherry

What MODIS product are you using?

I am using MYD09A1 images.

curtvprice

I strongly suggest extracting some pixel values and doing the calculation by hand before doing battle with arcpy on this.

Luke_Pinner

What MODIS product are you using?

MeganMcSherry

What data type are the bands? If you have byte or unsigned integer data, you might be getting integer overflows as you're applying the float conversion after the operations. As an example - say you have unsigned 16bit integers, band 4 = 500 and band 43 = 750.
Float(500 - 750) will return 65286 but Float(500) - 750 will return -250.0.

Try:
2.5 * ((Float("%Band 4%") - "%Band 3%")) / (Float("%Band 4%") + 6 * "%Band 3%" - 7.5 *"%Band 1%" + 1))

Hi lpinner,

I am having a similar problem in creating the formula for EVI using MODIS bands. No matter which way I enter it (with the float conversion in various positions), I still get an index from -10405 to 9610. All of the bands I am using are 32-bit unsigned integers. Any tips on how to enter the EVI equation using these MODIS bands?

Thanks very much.

StephenChignell

What data type are the bands? If you have byte or unsigned integer data, you might be getting integer overflows as you're applying the float conversion after the operations. As an example - say you have unsigned 16bit integers, band 4 = 500 and band 43 = 750.
Float(500 - 750) will return 65286 but Float(500) - 750 will return -250.0.

Try:
2.5 * ((Float("%Band 4%") - "%Band 3%")) / (Float("%Band 4%") + 6 * "%Band 3%" - 7.5 *"%Band 1%" + 1))

Thanks Luke,
I'll give this code a try once (if) I've resolved the issues with my actual correction coefficients...

Cheers,
Steve

StephenChignell

The coefficients you are using are hard-coded for MODIS data sets (as stated in the wikipedia article). Additionally, the values required are not raw Landsat dataset DN values (0-255) but instead must be atmospherically-corrected reflectance values.

Here's a journal article that discusses the processing involved.
http://southwestnwrsnatresources.files.wordpress.com/2011/12/sesnie-et-al-2011.pdf

Hi Curt,
Thanks for the advice--I do plan on using atmospherically corrected imagery in my eventual run--I forgot to mention that.

Thanks too for the article, it was very informative. However, do you know if EVI correction factors have been developed for TM data? I can't seem to find anything that suggests anyone has derived EVI from Landsat.

Thanks again,
Steve

curtvprice

I'm trying to derive Enhanced Vegetation Index (EVI) from Landsat 5 TM data using the raster calculator and I can't seem to get the output I want. Here is a link to the description and formula for EVI:

http://en.wikipedia.org/wiki/Enhanced_vegetation_index

The output should be a raster with values ranging from -1 to 1, but I get values from like 30-40 which is incorrect.

The coefficients you are using are hard-coded for MODIS data sets (as stated in the wikipedia article). Additionally, the values required are not raw Landsat dataset DN values (0-255) but instead must be atmospherically-corrected reflectance values.

Here's a journal article that discusses the processing involved.
http://southwestnwrsnatresources.files.wordpress.com/2011/12/sesnie-et-al-2011.pdf

Luke_Pinner

What data type are the bands? If you have byte or unsigned integer data, you might be getting integer overflows as you're applying the float conversion after the operations. As an example - say you have unsigned 16bit integers, band 4 = 500 and band 43 = 750.
Float(500 - 750) will return 65286 but Float(500) - 750 will return -250.0.

Try:

2.5 * ((Float("%Band 4%") - "%Band 3%")) / (Float("%Band 4%") + 6 * "%Band 3%" - 7.5 *"%Band 1%" + 1))

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