Image Math expressions

[Llion Preston]

Hi,

 

I have an NDVI Image which is stored as a one byte (8-bit) with a range of 0-255. The valid range of the data is 0-250, with 251 and 255 being quality flags. I wish to convert the images using the formula I was provided with [NDVI(real) = (NDVI- 125)/125] using the ImageMath function but when I run the below script it just produces a binary image. How would I produce the correct expression to remove values > 250 and then (image – 125)/125?

 

import os

import rsgislib

from rsgislib import imagecalc

from rsgislib.imagecalc import BandDefn

 

print("Converting NDVI range from 255 to -1-1")

 

# convert NDVI from 255 to -1 to 1

outputImage = 'ndvi_corrected.kea'

gdalformat = 'KEA'

datatype = rsgislib.TYPE_32UINT

expression = '(b1-125)/125'

imagecalc.imageMath('NDVI_20160101.kea', outputImage, expression, gdalformat, datatype)

 

 

Many Thanks,

Llion

[Pete Bunting [pfb]]

Hi Llion,

The error is your output data type 'datatype = rsgislib.TYPE_32UINT’ should be 'datatype = rsgislib.TYPE_32FLOAT’

Best wishes,

Pete

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[Llion Preston]

Hi Pete,

Brilliant, thought it would be something silly like that. Is there a way to alter the expression so that values > 250 are changed to NoData?

Thanks,

Llion

[Pete Bunting [pfb]]

Hi Llion, 

You need to put an ‘if’ statement into your expression:

b1>150?-9999:(b1-125)/125

Cheers, Pete

****************************************************
* Dr Pete Bunting
* Reader in Remote Sensing
* Earth Observation and Ecosystem Dynamics Group
* Department of Geography and Earth Sciences
* Aberystwyth University
* Aberystwyth
* Ceredigion
* SY23 3DB
* UK
* 
* Ph: +44 (0) 1970 622615
* Mob: +44 (0) 7917 842743
* Email: p...@aber.ac.uk
* ORCID: http://orcid.org/0000-0002-7435-0148
****************************************************

"Please consider the environment before printing this email or any documents attached”

On 30 Aug 2016, at 14:19, Llion Preston <llion....@gmail.com> wrote:

Hi Pete,

Brilliant, thought it would be something silly like that. Is there a way to alter the expression so that values > 250 are changed to NoData?

Thanks,

Llion

[Llion Preston]

Hi Pete,

Worked perfectly.

Thanks again,

Llion

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