Data Retrieval Method: Extract or Download?

[alineaf1]

Hello!

I'm learning how to use Aqua Modis data, and I have a question that I believe is basic, but I hope someone can help me:

When I download the 8day chlorophyll information for the period 2016-09-13 - 2016-09-20, using the Data Retrieval Method: download (i.e. global data), and load it into QGIS, I see the concentration minimum of 0.019694 and maximum of 80.466187

However, when I download the same information (chlo_a, 8day, 2016-09-13 - 2016-09-20), using the Data Retrieval Method: extract, and selecting the region: N: -23,340, W: -45,900, E: -44,500, S: -24,470

the minimum concentration is 0.197772 and the maximum is 1.429617

I thought that regardless of the region, the data would be the same, that when selecting the region, it would just be a crop/zoom, but I understand that they vary, correct?

So if I want to do an SST/Chlo analysis specific to a region, I should choose the "extract" method, correct?

downloaded file: AQUA_MODIS.20160913_20160920.L3m.8D.CHL.chlor_a.4km.nc
extracted file: AQUA_MODIS.20160913_20160920.L3m.8D.CHL.x_chlor_a.nc

[PODAAC - jmcnelis]

@alineaf1 :

Can you please clarify what tool/interface you're referring to? Where do you see the Extract/Download options? There's more than one way to "select" cells by geographic extent, and we would need to know what tool you are using to give any insight. Thank you

-Jack

[alineaf1]

Yes, sure.

https://oceancolor.gsfc.nasa.gov/l3/order/

In the Aqua modis L3/L4 Browser. Tks

[OB General Science - guoqingw]

Hi,

The discrepancies you've observed primarily stem from comparing local min/max values to global ones. When you download without extraction, you receive the Chlorophyll image for the entire globe. On the other hand, using the extract function allows you to isolate the specific region defined by the provided latitude and longitude coordinates. This means you can study a local area without having to download the global image. So, to answer your question, yes, if your focus is solely on your study area, using the extract function will conserve storage space on your computer, and the Chlorophyll range will precisely correspond to your study area.

An easy way to visualize the disparities between the two downloaded files is by opening them in SeaDAS (https://seadas.gsfc.nasa.gov). There, you'll notice that the file with the extension .chlor_a represents the entire global ocean, while .x_chlor_a pertains specifically to the region defined by your specified latitude and longitude coordinates.

I hope this clarifies things for you.
Guoqing