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Inquiry about MODIS Data Time Stamps

Posted: Mon Oct 23, 2023 3:49 pm America/New_York
by moghadam.hadis
Hello all,

I hope this email finds you well. I am writing to seek clarification on the time information associated with MODIS data. I have been using MODIS data (MYD21C1v061) for my research, and I'm particularly interested in understanding the exact time of data acquisition for specific geographical coordinates (latitude and longitude).

I have noticed that MODIS data are provided at 10:30 am and 1:30 pm local time. However, my analysis requires that I align MODIS data with another dataset that provides time information in Coordinated Universal Time (Era5).

My specific question is: Is there a way to obtain precise time information for each grid point, which includes both longitude and latitude? This information would greatly assist me in synchronizing MODIS data with my other dataset.

I appreciate your support and any guidance you can provide regarding how to obtain the time information for individual grid points within the MODIS data.

Thank you for your time and assistance. I look forward to your response.



Re: Inquiry about MODIS Data Time Stamps

Posted: Mon Oct 30, 2023 2:07 pm America/New_York
by LP DAAC - lien
Hi Hadis,
As you know we were discussing this through the email. So I will just post the answer the below for the user community.

I am sorry, but there is not really a way to get that accurate of a time. For each pixel you will be able to get hourly average UTC of when it was acquired, but that is as close as it has. What makes it a little more problematic with the CMGs is that each pixel is an aggregate of several pixels. Please let me know if you have any questions.

Thank you for your email. I have a question regarding it, and I would greatly appreciate your assistance.

According to the Modis user guide, within the product, there is a variable named 'Day_view_time,' which represents the Average Daytime View Time (UTC). When I read the file with Matlab, for example, for this specific file: "MOD21C1.A2000055.061.2020037143439.hdf"

'long_name' 'Average Nighttime View Time (UTC)'
'units' 'hrs'
'format' 'scaled'
'coordsys' 'cartesian'
'valid_range' [0,120]
'_FillValue' 255
'_Scale' 0.200000000000000
'_Offset' 0

My question is: what do the hours represent, and how can I determine the time for each grid?

Once again, I appreciate your time and attention.

They represent the average UTC time that the data was collected for that specific pixel.

So for instance, I downloaded a file and loaded the Average_Day_Time_View_Time layer. I looked at the values of pixels in Eastern Spain as an example. Many had a value of 59, so when I take 59 and multiply it by the scale factor I come up with 11.8 and that sounds right in the acceptable range, that would mean the average time of those pixels acquisition is 11:48 UTC.
Please let me know if you have any questions.