l2_flags viewer

[ryan458]

Hopefully this is a quick and simple question. I'm trying to figure out how to view the l2_flags (1km MODIS Aqua) in SeaDAS 7.4. I haven't examined flags in quite some time and I'm unsure how to do it in the BEAM-based SeaDAS. For clarity, I want to know which flags have been tripped at each pixel, bit just the floating point value.

Thank you,
Ryan

[gnwiii]

Try SeaDAS Tutorial: Masks (English only, unfortunately).

[OB SeaDAS - knowles]

Hi Ryan,

The Pixel Info tool will show you all flag states for a selected or mouse-hovered pixel.

See slide #42 of the "SeaDAS Walkthrough" presentation:
https://seadas.gsfc.nasa.gov/docs/SeaDAS_Walk_Through.pdf

and/or
slide #8 of the "SeaDAS Tools presentation:
https://seadas.gsfc.nasa.gov/docs/SeaDAS_Tools.pdf

thanks
Danny

[ryan458]

Thanks for that information. Unfortunately I'm not seeing the flag values in the flag viewer (or the masks in Mask Manager). The L2 files I'm trying were downloaded from the Level 2 Browser. The only processing I've done to them is remap them using gpt.sh and bl2map_mosaic. See below for my XML processing file (abbreviated). Would that processing had messed up how Seadas sees the l2_flags? I also noticed that in the tutorials you posted the l2_flags band has 'F' on the band icon in the File Manager. I am not seeing that on my files.

Thanks,
Ryan

<graph id="bl2map_mosaic">
  <version>1.0</version>
  <node id="mosaicNode">
<operator>Mosaic</operator>
<sources>
   <sourceProducts>${sourceProducts}</sourceProducts>
</sources>
<parameters>
    <variables>
        <variable>
            <name>chlor_a</name>
            <expression>chlor_a</expression>
        </variable>
        <variable>
            <name>l2_flags</name>
            <expression>l2_flags</expression>
        </variable>
    </variables>
    <conditions/>
    <combine>OR</combine>
    <crs>PROJCS["WGS 84 / World Equidistant Cylindrical",
         GEOGCS["WGS 84",
         DATUM["World Geodetic System 1984",
         SPHEROID["WGS 84", 6378137.0, 298.257223563, AUTHORITY["EPSG","7030"]],
         AUTHORITY["EPSG","6326"]],
         PRIMEM["Greenwich", 0.0, AUTHORITY["EPSG","8901"]],
         UNIT["degree", 0.017453292519943295],
         AXIS["Geodetic longitude", EAST],
         AXIS["Geodetic latitude", NORTH],
         AUTHORITY["EPSG","4326"]],
         PROJECTION["Equidistant_Cylindrical", AUTHORITY["EPSG","9842"]],
         PARAMETER["central_meridian", -68.007004],
         PARAMETER["latitude_of_origin", 42.9215],
         PARAMETER["standard_parallel_1", 42.9215],
         PARAMETER["false_easting", 0.0],
         PARAMETER["false_northing", 0.0],
         UNIT["m", 1.0],
         AXIS["Easting", EAST],
         AXIS["Northing", NORTH],
         AUTHORITY["EPSG","32663"]]
    </crs>
    <orthorectify>false</orthorectify>
    <elevationModelName>GETASSE30</elevationModelName>
    <resampling>Nearest</resampling>
    <westBound>-72.778999</westBound>
    <northBound>46.3950</northBound>
    <eastBound>-63.2350</eastBound>
    <southBound>39.44800</southBound>
    <pixelSizeX>1000.0</pixelSizeX>
    <pixelSizeY>1000.0</pixelSizeY>
</parameters>
</node>
</graph>

[gnwiii]

Mapping requires stretching and or shrinking some regions of the level-2 image, so there is not a 1-to-1 correspondence between pixels of a mapped image and the pixels of a the original level-2 image. Mapped pixels are generally some (weighted) average of values from the input level-2 pixels.  An average of geophysical variables still makes sense as a geophysical variable, but simple averaging doesn't make sense for flags, so you would be limited to nearest-neighbor mapping.  General-purpose mapping software doesn't attempt to preserve flags, and doesn't generally attempt to preserve all the metadata that goes with flags even if you try ti include them in the list of variables.

[ryan458]

Ok - thank you for the clarification. Much appreciated.

Ryan

[OB SeaDAS - knowles]

Hi Ryan,

As mentioned, flags would lose their original meaning if they are reprojected.  Note: you can mask out undesired data by applying the level-2 flags during the Mosaic step. 

Danny