| Re: How to read ASTER in ENVI? [message #55542] |
Mon, 27 August 2007 07:51 |
James Kuyper
Messages: 425
Registered: March 2000
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Senior Member |
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Jeff N. wrote:
> On Aug 27, 3:21 am, highstone <gaolei...@gmail.com> wrote:
>> hello, I have a secen ASTER data named as "pg-
>> PR1B0000-2003102104_030_001"，which not have suffix ".hdf ", and no
>> other files that have the same filename with suffix such as ".met",
>> and so on. When I read it by ENVI from "file----open image file" or
>> "file ----open external file---eos---aster", all the bands can be
>> showed in "available bands", and the DN of VIR band is float type and
>> more than 1, which shows it is not reflectance. I want to know how to
>> get the reflectance for visible or infrared bands and the radiance for
>> Tir of such ASTER data in ENVI or ERDAS? please help me, thank you.
>
> Just as an FYI: just b/c your data have values greater than 1 does
> not necessarily mean you don't have reflectance data. FLAASH, for
> example, commonly outputs "scaled reflectance" data, which is
> reflectance x some scale factor (you choose it, but 10,000 is
> common). But I think Mort's right about your data being radiance data
> in this case.
MODIS L1B reflectances are also scaled. However, even after the
appropriate scale and offset are applied, the reflectances can still
be both greater than 1.0 or less than 0.0. They can be greater than
1.0 due to specular reflection like sunglint on the oceans being
analyzed as if it were diffuse reflection. They can be less than 0.0
due to background subtraction and statistical fluctuations in the
detected photon counts in very dim scenes - in such scenes, you should
include the negative values in averages over larger regions; otherwise
the average will be biased to the high side.
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| Re: How to read ASTER in ENVI? [message #55543 is a reply to message #55542] |
Mon, 27 August 2007 07:25  |
Jeff N.
Messages: 120
Registered: April 2005
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Senior Member |
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On Aug 27, 3:21 am, highstone <gaolei...@gmail.com> wrote:
> hello, I have a secen ASTER data named as "pg-
> PR1B0000-2003102104_030_001"，which not have suffix ".hdf ", and no
> other files that have the same filename with suffix such as ".met",
> and so on. When I read it by ENVI from "file----open image file" or
> "file ----open external file---eos---aster", all the bands can be
> showed in "available bands", and the DN of VIR band is float type and
> more than 1, which shows it is not reflectance. I want to know how to
> get the reflectance for visible or infrared bands and the radiance for
> Tir of such ASTER data in ENVI or ERDAS? please help me, thank you.
Just as an FYI: just b/c your data have values greater than 1 does
not necessarily mean you don't have reflectance data. FLAASH, for
example, commonly outputs "scaled reflectance" data, which is
reflectance x some scale factor (you choose it, but 10,000 is
common). But I think Mort's right about your data being radiance data
in this case.
Jeff
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| Re: How to read ASTER in ENVI? [message #55549 is a reply to message #55543] |
Mon, 27 August 2007 04:37 |
Mort Canty
Messages: 134
Registered: March 2003
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Senior Member |
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highstone schrieb:
> hello, I have a secen ASTER data named as "pg-
> PR1B0000-2003102104_030_001"锟斤拷which not have suffix ".hdf ", and no
> other files that have the same filename with suffix such as ".met",
> and so on. When I read it by ENVI from "file----open image file" or
> "file ----open external file---eos---aster", all the bands can be
> showed in "available bands", and the DN of VIR band is float type and
> more than 1, which shows it is not reflectance. I want to know how to
> get the reflectance for visible or infrared bands and the radiance for
> Tir of such ASTER data in ENVI or ERDAS? please help me, thank you.
>
From the filename, I believe that you are looking at an ASTER image from
2003 processed to Level 1B: registered radiance at the sensor. See
http://lpdaac.usgs.gov/aster/ast_l1b.asp
That meaans that the float data are in units of
watts/meter^2/steradian/micrometer
In order to get ground reflectance you would either have to apply
atmospheric correction yourself (e.g. with ENVI's FLAASH Addon) or order
the corresponding product from LPDAAC.
-Mort
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