Earthdata Forum

Hello,
I encountered a problem when running l2gen in batch to process hundreds of MODIS-aqua images. When I used the subsetting options with the parameters given as follows:

north=25.2
south=19.8
east=120.2
west=109.8
resolution=250

8 of the outputs (A2003115060500.L2_LAC, A2003293055500.L2_LAC, A2003307060500.L2_LAC, A2006061055500.L2_LAC, A2013319051500.L2_LAC, A2014321061000.L2_LAC, A2015019052500.L2_LAC, A2015315061500.L2_LAC) were upside down, and left and right reversed, i.e., the values of longitude decreased from left to right, and the values of latitude decreased from bottom to top. The same error occured when run l2gen in GUI. When the subsetting option was not set, and other l2gen parameters were kept the same, there was no such problem. When the subsetting options were set as above, and resolution was set as 1000, there was no such problem as well.
I used SeaDAS 7.5.3. I will be very grateful if anybody help or give me some suggestions. Thanks.

The issue here is that l2gen will extract sub-scans.  This combined with the bow-tie effect can results in the determination of the orbit orientation (ascending vs descending) to be incorrect.
The data are correct, but the metadata in the netCDF file, specifically the "startDirection" and "endDirection"  attributes may not be. This is exactly what is happening to you.  The display determines the orientation of the file based on these attributes.  Since the startDirection is listed as "Descending", the orientation of the display is inverted from what it should be.

There are two possible solutions for you: 
1) edit the attributes to both be "Ascending"
2) use l1aextract_modis to generate an extracted L1A file and process that to l2gen (without the subsetting paramters, since the input will already be subset).  This results in extracts that have full scans, which should help l2gen get the metadata written properly :grin:

Sean

Thank you, Sean.
I viewed the metadata, and the startDirection was exactly "Descending". However, the attributes seem to be uneditable in SeaDAS, I'm a newbie. So I tried the second solution and it worked :grin:.

Thunder