Why does hicO's Lt(top of atmosphere) data exceed 1?

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comet0607
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Why does hicO's Lt(top of atmosphere) data exceed 1?

by comet0607 » Mon Nov 22, 2021 3:53 am America/New_York

Hello!Recently, I am using HICo to do research on true color image synthesis, which requires 70 channel data between HICO 380-780. The L1 data of HICO is downloaded and Lt (top of atmosphere) data is used. I found that many reflectance data were greater than 1, and even reached 7.96. May I ask what processing method or formula HicO adopted to obtain Lt data? Why is it so large?What should I do with it if I want to use it?

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OB.DAAC - amscott
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Re: Why does hicO's Lt(top of atmosphere) data exceed 1?

by OB.DAAC - amscott » Mon Nov 22, 2021 8:21 am America/New_York

Hello,

Information for HICO can be found on the Ocean Color web: https://oceancolor.gsfc.nasa.gov/hico/. At the bottom of the page, meetings are listed where presentations are linked and available for download. They discuss characteristics of the instrument and may have the answer to your question.

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Re: Why does hicO's Lt(top of atmosphere) data exceed 1?

by OB SeaDAS - xuanyang02 » Mon Nov 22, 2021 9:21 am America/New_York

The unit of Lt in HICO L1B is W/m^2/micrometer/sr.

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Re: Why does hicO's Lt(top of atmosphere) data exceed 1?

by comet0607 » Tue Nov 23, 2021 2:01 am America/New_York

Hi Alicia,

Many thanks for your answer !
I have checked your reply and realized that Lt in HICO's L1b data is not reflectance data but radiance data.
I choose one scene with cloud image,the maximum Lt (Top of Atmosphere radiance) of it was 26.2140.Since I need to use HICO and MODIS data in my research, I can obtain a model that is also applicable to MIDOS by applying HICO data and results through machine learning. Therefore, HICO and MODIS input data must be the same, e.g. radiance or reflectance data. But I looked at the MODIS-MOD021KM-radiance data with ENVI, and the maximum values were even three or four hundred.
Why is the same data set for different sensors so different?
Where can I get the top reflectance of HICO?
If you need to convert radiance data to Reflectance, where can you find the solar irradiance required in the conversion formula?

Thanks,
Comet
Last edited by comet0607 on Tue Nov 23, 2021 2:45 am America/New_York, edited 1 time in total.

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Re: Why does hicO's Lt(top of atmosphere) data exceed 1?

by OB.DAAC - SeanBailey » Tue Nov 23, 2021 10:29 am America/New_York

If correctly read, the Lts from both the HICO L1B files and the MODIS L1B files will be in W/m^2/um/sr and will be consistent. Looking at a typical HICO scene, I found plenty of data over clouds with values in the 300-400 W/m^2/um/sr range.

The easiest way to get TOA reflectance for both MODIS and HICO is to use the l2gen program provided with SeaDAS.
Output the rhot_nnn product (the _nnn will be expanded by the program to product the rhot for each wavelength).

If you want to do the math yourself, you'll need the solar irradiance data. We recently updated our webpage with this information, but have yet to add HICO - however, we will soon. Until then the table below has the HICO solar irradiance data (in mW/cm^2/um/sr, so you'll have to multiply by 10 to get W/m^2/um/sr).

The equation to use is:
TOA_reflectance = pi * TOA_radiance / solar_irradiance / cos(solar_zenith_angle)

The solar_irradiance is the mean extraterrestrial irradiance multiplied by the Earth-Sun distance correction (you'll have to compute that based on day of year)

Regards,
Sean

# wavelength(nm) solar_irradiance(mW/cm^2/um/sr)
353 103.124
358 103.54
364 111.97
370 117.938
375 114.635
381 108.559
387 106.168
393 113.436
398 140.94
404 168.523
410 170.866
416 173.272
421 172.957
427 165.001
433 164.175
438 178.116
444 192.336
450 202.441
456 206.849
461 206.985
467 204.482
473 205.679
479 206.556
484 198.884
490 195.167
496 196.009
501 193.913
507 193.126
513 186.25
519 180.145
524 182.698
530 185.934
536 186.693
542 185.689
547 186.486
553 185.329
559 181.205
564 179.117
570 179.431
576 180.067
582 179.465
587 176.563
593 175.139
599 174.523
605 172.647
610 169.338
616 166.081
622 165.271
627 164.663
633 163.38
639 161.5
645 159.199
650 155.939
656 151.186
662 151.793
668 151.92
673 149.895
679 147.849
685 146.482
690 146.007
696 144.854
702 143.274
708 140.976
713 137.985
719 135.85
725 134.144
731 132.334
736 130.081
742 128.144
748 127.591
753 126.517
759 125.219
765 123.475
771 121.287
776 118.919
782 116.75
788 114.976
794 113.546
799 112.359
805 111.277
811 110.118
816 108.757
822 107.245
828 105.672
834 103.99
839 102.011
845 99.684
851 97.503
857 96.164
862 95.642
868 95.333
874 94.853
880 94.093
885 93.054
891 91.818
897 90.491
902 89.199
908 88.004
914 86.836
920 85.676
925 84.648
931 83.761
937 82.832
943 81.803
948 80.758
954 79.747
960 78.791
965 77.891
971 77.015
977 76.146
983 75.278
988 74.398
994 73.466
1000 72.475
1006 71.504
1011 70.645
1017 69.872
1023 69.111
1028 68.307
1034 67.465
1040 66.647
1046 65.888
1051 65.145
1057 64.349
1063 63.504
1069 62.67
1074 61.88
1080 61.089

comet0607
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Re: Why does hicO's Lt(top of atmosphere) data exceed 1?

by comet0607 » Wed Nov 24, 2021 9:32 pm America/New_York

Does HICO's L1b data include (Lt *0.02) as radiance? Are there any other steps needed to process the data?
I chose a scene with cloud HICO data (H2010304222425.l1b_iss.nc), and the maximum value of Lt is only 26.2140. Is it my method of data processing wrong?

Thanks again for your kind help!
Comet

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Re: Why does hicO's Lt(top of atmosphere) data exceed 1?

by comet0607 » Thu Nov 25, 2021 8:54 pm America/New_York

I am not sure if my reply has been sent, because it is not shown on my side. Here is my reply (Wed Nov 24, 2021 9:32pm America/New_York) :

Does HICO's L1b data include (Lt *0.02) as radiance? Are there any other steps needed to process the data?
I chose a scene with cloud HICO data (H2010304222425.l1b_iss.nc), and the maximum value of Lt is only 26.2140. Is it my method of data processing wrong?

Thanks again for your kind help!
Comet

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Re: Why does hicO's Lt(top of atmosphere) data exceed 1?

by OB.DAAC - SeanBailey » Mon Nov 29, 2021 10:41 am America/New_York

If your netCDF reader already applied the scale_factor / add_offset, then you don't need to reapply it. I can confirm the file does include values much higher than 26.
Screen Shot 2021-11-29 at 10.33.55 AM.png
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Screen Shot 2021-11-29 at 10.33.55 AM.png
Screen Shot 2021-11-29 at 10.33.55 AM.png (358.03 KiB) Not viewed yet
Regards,
Sean

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Re: Why does hicO's Lt(top of atmosphere) data exceed 1?

by comet0607 » Thu Dec 23, 2021 2:47 am America/New_York

I still don't quite understand how this data should be used. After I opened it with Matlab, the value of Lt*0.02 was very small. I want to know how to use HICO's L1b Lt (atmospheric top radiance data). What is the problem that caused the low value of data? Because I see that the program about true color synthesis of HICO data on the website is only [SLOPE = 0.02; LTData = SLOPE*double(LTdata)]. How can I use SeaDAS to get this image you drew?

Thanks,
Comet

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Re: Why does hicO's Lt(top of atmosphere) data exceed 1?

by OB.DAAC - amscott » Mon Dec 27, 2021 9:44 am America/New_York

If you visit the Ocean Color Web: https://oceancolor.gsfc.nasa.gov/

You can navigate to:
Software & Tools > SeaDAS Documentation
Software &. Tools > SeaDAS Informational Videos

To learn how to use the features of the SeaDAS software.

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