I am having a hard time figuring out how exactly to do DOS. In the past I just used to locate a dark body of water and minus those values from bands 1,2,3,4,5 & 7. But more recently I heard it is better to look at a histogram and find the point where the DN values rise dramatically and us that as the constant value to subtract from each band. Chavez's method seems too complicated for me.
Also do I need to worry about just bands 1,2 & 3 or all bands?
Please advise.
Dear Emranul,
you have to worry about all bands, event though shorther wavelenghts are more scattered than longer wavelenghts. Actually the method from
CHAVEZ JR, P. S., 1988: An improved dark-object subtraction technique for atmospheric scattering correction of multispectral data. - Remote Sensing of Environment 24 (3): 459-479.
is not that complicated. It is just a wavelenght dependet relative scattering model. When I remember correctly the paper gives a "step by step" description. Just try to follow it. You can calculate the relative dependence quite easily with Excel or Open Calc.
I suggest you take the example values from Chavez' paper and try to follow his calculations. Then you just have to replace them with your starting values and haze estimations.
best regards,
Felix
Dear Emranul,
You need to check the minimum and maximum reflectance values of each band for example from your image histogram. To perform DOS for your study area image, normally you next to check whether in your study area image there is dark object feature such as deep water bodies (i.e. water bodies totally absorption process happened). So if you know min and max values for each bands of your image you can perform it for example using modeler tool in ERDAS.
Hope this helps.Thank you.
Regards,
Aizam.
Dear Emranul,
you should correct all bands when applying the DOS method.
The selection of the dark object spectral characteristics (i.e. the value to subtract for each band) can be done visually (e.g. selecting deep waters like suggested, if present) or looking at the histogram, following the procedure described by Chavez as said.
You will find useful this webpage:
http://www.gisagmaps.com/landsat-8-atco-guide/
which provides a step-by-step guide to apply various atmospheric correction models also giving you information on their strengths and weaknesses.
Regards,
Giulio
Hello Everyone,
I am the person who developed the 'Improved DOS' and 'COST' models. I wanted to say that it is absolutely critical that a VALID DARK OBJECT exist in the image that is being used to select the min value for haze corrections. Some of the better valid dark objects are sloped areas that are in complete shadow; dark water areas are at times good min dark objects and at times can not be good ones to use. This will depend on how sediment free / clear the waters are because in the visible bands (particularly the blue and green) when the waters are not completely clear they do not represent valid dark object and have surface reflectance values greater than 1%.
Good Luck with your atmospheric corrections and radiometric calibrations.
Pat
Hi,
I am trying to perform an automatic way of process Landsat 8 from DN to surface reflectance. One of the 'pains' have been to get the kl value automatically, because I refused to use the minimun value of the whole scene.
So what I finally did was to get a DTM (I have to cut a bigger one with the extension of the current scene, because is not always the same) and run Fmask (https://code.google.com/p/fmask/) whit throws a very good water mask (and cloud, cloud-shadow, snow-ice, and clean terrain pixel).
Once I get these two rasters (Hillshade and Fmask) I masked every band getting only the values within the terrain shadows and water bodies, then I choose the minimun of these masked values as the kl value.
For me, this would be correct, but I'd appreciate it if someone could let me know its opinion.
Ps. The python code for this is here: https://github.com/LAST-EBD/Protocolo/tree/master/Metodos/Get_kl_csw
You can use threshold for remove dark objects from your data. for this procedure, you must check histogram and determine a best threshold and mask this value from all of your bands.
good luck
Please, Is there anyone can drop codes used via Google Earth Engine for DOS from Sentinel-1 SAR?
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