[erdas-l] Problem with ignoring background value

[Ophelia Wang]

Hi all,

I've been having troubles to get rid of background values (255) of aerial photo
mosaics (.img) in Erdas Imagine. I've tried "mosaic" (which has options of
ignore certain input and output background and 0 values), "rescale" (ignore 0
and 255), ArcGIS copy raster (ignore 0 and 255), etc. etc. Everything that has
an option to ignore both 0 and 255 that I've tried.

None of those worked in terms of removing the white background of the aerial
photo mosaics. I have nearly 50 images, so I can't manually create AOI and then
subset each mosaic based on AOI.

Any suggestions? Thanks for you help.

Ophelia

--
Ophelia Wang
PhD candidate
Department of Geography and Environment
University of Texas at Austin
210 West 24th Street, Austin, TX 78712, USA
Phone: 1-512-471-5116
Fax: 1-512-471-5049
Email: ophel...@mail.utexas.edu

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[Emmanuel Bina]

I've had similar problems when mosaicking some quickbird images.

The AOI is irregularly shaped so all of the scenes have black (0) background. After mosaicking the scenes, the background just turns white when viewing in ArcGIS but still has a value of 0. The only solution I could think of is to use spatial modeler and add 1 to every pixel on the images.

Emmanuel

Ophelia Wang <ophel...@mail.utexas.edu> wrote:

Hi all,

I've been having troubles to get rid of background values (255) of aerial photo
mosaics (.img) in Erdas Imagine. I've tried "mosaic" (which has options of
ignore certain input and output background and 0 values), "rescale" (ignore 0
and 255), ArcGIS copy raster (ignore 0 and 255), etc. etc. Everything that has
an option to ignore both 0 and 255 that I've tried.

None of those worked in terms of removing the white background of the aerial
photo mosaics. I have nearly 50 images, so I can't manually create AOI and then
subset each mosaic based on AOI.

Any suggestions? Thanks for you help.

Ophelia

--
Ophelia Wang
PhD candidate
Department of Geography and Environment
University of Texas at Austin
210 West 24th Street, Austin, TX 78712, USA
Phone: 1-512-471-5116
Fax: 1-512-471-5049
Email: ophel...@mail.utexas.edu


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[Timo Ikola]

>The AOI is irregularly shaped so all of the scenes have black (0) background. After mosaicking the scenes, the background just turns white when viewing in ArcGIS but still has a value of 0.

 

You are right but your solution is not a good one. The basic story is as you say – black image background is and stays as a zero in Erdas mosaic process. The main problem is that Erdas shows zeros as a black (or transparent) and ArcGIS for some reason shows zeros as a white. My guess is that the main reason for this is  that ArcGIS has a specific “NoData” value which Erdas does not. When ArcGIS meets a pixel value 0 it is not a “nodata” or background to it and it shows it as a white. So zero does not represent background to ArcGIS and it then shows it as a white.

 

Erdas on the other hand does not has such fixed limitations and it can use any value as a background using that “ignore some value in process” option. Normally zeroes are considered as a background in Erdas but as told any other value can also be the background.

 

However there is in Erdas Imageinfo tool a setting where you can set background as a nodata. I do not know what it actually does in Erdas point of view but I guess that it might solve the case for ArcGIS. So open the final mosaic in Erdas Viewer. Start ImageInfo tool and there edit – set no data value. There you can define the no data value and when opening same file in ArcGIS it shold then work.

 

Another workaround if you happen to have thematic data is to give full opacity to zero pixel values using raster attribute editor. But as told this works just with thematic data and you have aerial imagery. But this same issue has be seen with raster maps and there this opacity workaround solves the case also.

 

I do not have ArcGIS in my hands at the moment but if I am not wrong it has just tools of doing this with thematic data and with aerial imagery it just shows it as is without much possibilies to affect on it. (Ok  contrast tools maybe)

 

Third workaround is to found out that ArcGIS NoData pixel value and transfer all zeroes to that…some softwares uses -9999, but I do not know what ArcGIS uses.

 

This is annoying problem and you will see variations of this same issue every here and then. The good point here is that Erdas keeps zeroes as a zero and problem comes more on from how different tools handle different values. If you do this rough solution (zeros to one) you change data quite dramatically. If you change all zeroes to one how you can then separate it from real pixel values with value one.

 

 

timo

 

 

 

 

 

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[Ophelia Wang]

Hi all,

Thanks for all the responses. I finally fixed the problem by creating a binary
mask that sets the aerial photos as 1 and the white background (layer 1=255,
layer 2=254 and 255, layer 3=255) as 0. I then used the mask function in Erdas'
utilities to mask out the unwanted background.

Ophelia

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[Emmanuel Bina]

Thanks. I’ll try your suggestions. The image will be just used for background in GIS so pixel values in not really much of a concern.

 

Emmanuel

 

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[Jay D. Davis]

I have my preferences [session|viewer|Background Transparent (checked)] set.
Perhaps that would work as well.

jd

-----Original Message-----
From: Ophelia Wang [mailto:ophel...@mail.utexas.edu]
Sent: Sunday, May 25, 2008 5:04 PM
To: ERDAS Discussion List
Subject: RE: [erdas-l] Problem with ignoring background value

Hi all,

Thanks for all the responses. I finally fixed the problem by creating a
binary
mask that sets the aerial photos as 1 and the white background (layer 1=255,
layer 2=254 and 255, layer 3=255) as 0. I then used the mask function in
Erdas'
utilities to mask out the unwanted background.

Ophelia

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