Dear Sir or Madam,
I am working with the MOD09A1 v 6.1 product over the Arctic Ocean. I am particularly using bands 1, 2 and 3 of the product. Especially in band 2 I encountered many negative reflectance values over open water. The same pixels are however valid in the other bands. Is there any suggestion how to proceed with these negative reflectance values?
Kind Regards,
Niklas
MODIS MOD09A1 Product negative reflectances
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LP DAACx - lien
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Re: MODIS MOD09A1 Product negative reflectances
Hi Niklas,
Sorry, but I don't think there is much that can be done about that, the MOD09 products are not recommended for water pixels.
The "anomalies" that you've identified, e.g. negative reflectance values,
are known issues in MODIS along with many other Surface Reflectance products such as Landsat surface reflectance.
The atmospheric correction and surface reflectance retrieval algorithms are not
ideal for water bodies due to the inherently low level of water leaving
radiances and consequently very low SNR. Quantitative remote sensing
retrievals of water column constituents requires different algorithms.
Similarly reflectance values greater than 1.0 can be encountered over
bright targets such as snow and playas. These are computational
artifacts. The surface reflectance QA bands and product user guides should
be consulted.
Please let me know if you have any questions.
Thanks,
Brett
Sorry, but I don't think there is much that can be done about that, the MOD09 products are not recommended for water pixels.
The "anomalies" that you've identified, e.g. negative reflectance values,
are known issues in MODIS along with many other Surface Reflectance products such as Landsat surface reflectance.
The atmospheric correction and surface reflectance retrieval algorithms are not
ideal for water bodies due to the inherently low level of water leaving
radiances and consequently very low SNR. Quantitative remote sensing
retrievals of water column constituents requires different algorithms.
Similarly reflectance values greater than 1.0 can be encountered over
bright targets such as snow and playas. These are computational
artifacts. The surface reflectance QA bands and product user guides should
be consulted.
Please let me know if you have any questions.
Thanks,
Brett
Re: MODIS MOD09A1 Product negative reflectances
Dear Sir or Madam,
I am experiencing a similar issue to Niklas's problem. I am a master's student and currently in the thesis stage. My thesis is about detecting harmful algal blooms. The platform I am using is Google Earth Engine. In one phase of my thesis, I need to calculate NDWI (McFeeters), for my study area. I am trying to use the MODIS collections 'MOD09GA.061' or 'MOD09A1.061.' However, due to the negative values in the green and/or NIR bands for NDWI, the resulting values are in a range such as (-200 / +230). Normally, the values must range from -1 to +1. As I understand your answer, using the collections I mentioned above is not very suitable for water studies.
My question is: Which MODIS version or collection can I use for NDWI or other indices (such as FAI, AFAI, NDVI, SABI, etc.) calculations on the Google Earth Engine platform? Is there a collection with raw images that has not undergone atmospheric correction available on Engine? I have looked, but I couldn’t find any. Please could you help me?
Best Regards.
I am experiencing a similar issue to Niklas's problem. I am a master's student and currently in the thesis stage. My thesis is about detecting harmful algal blooms. The platform I am using is Google Earth Engine. In one phase of my thesis, I need to calculate NDWI (McFeeters), for my study area. I am trying to use the MODIS collections 'MOD09GA.061' or 'MOD09A1.061.' However, due to the negative values in the green and/or NIR bands for NDWI, the resulting values are in a range such as (-200 / +230). Normally, the values must range from -1 to +1. As I understand your answer, using the collections I mentioned above is not very suitable for water studies.
My question is: Which MODIS version or collection can I use for NDWI or other indices (such as FAI, AFAI, NDVI, SABI, etc.) calculations on the Google Earth Engine platform? Is there a collection with raw images that has not undergone atmospheric correction available on Engine? I have looked, but I couldn’t find any. Please could you help me?
Best Regards.
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LP DAAC - dgolon
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Re: MODIS MOD09A1 Product negative reflectances
We'll tag this ticket with OBDAAC and PODAAC to see if they have any water specific MODIS data that will meet your needs. For questions related to the Google Earth Engine tool, you'll need to ask their team: https://developers.google.com/earth-engine/help
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