Strange reflectance values in S2A L1C images over sea ice

Optical Toolbox
#1

Hi,

I am dealing with L1C images over sea ice. I have downloaded some and was wondering about: When I look e.g. at Band 8A, there are some images where the reflectance over the water between the ice has realistic values of 0.4 (this is at the same time the lowest reflectance value in this image). But in other images, the whole water has a reflectance of almost zero, partially even exactly zero which is a rather unrealistic reflectance for water.
How can this be and is there a possibility to correct the low values?
And as we are already talking about it, how can reflectance values of 2.7 be? It is appearing in one of my images.

Thanks in advance,
Julius

#2

Hi Julius,

Can you provide details of the image (Orbit, Tile, Acquisition Date, Processing Baseline etc?).

Thanks in advance

Cheers

Jan

Jan Jackson

S2 MPC/CC Operations Manager

#3

Hi Jan,
I’ve just copied the metadata for you:

Driver: SENTINEL2/Sentinel 2
Files: S2A_OPER_MTD_SAFL1C_PDMC_20161121T174914_R093_V20161120T100152_20161120T100152.xml
Size is 512, 512
Coordinate System is `’
Metadata:
CLOUD_COVERAGE_ASSESSMENT=0.0
DATATAKE_1_DATATAKE_SENSING_START=2016-11-20T10:01:52.026Z
DATATAKE_1_DATATAKE_TYPE=INS-NOBS
DATATAKE_1_ID=GS2A_20161120T100152_007383_N02.04
DATATAKE_1_SENSING_ORBIT_DIRECTION=DESCENDING
DATATAKE_1_SENSING_ORBIT_NUMBER=93
DATATAKE_1_SPACECRAFT_NAME=Sentinel-2A
DEGRADED_ANC_DATA_PERCENTAGE=0
DEGRADED_MSI_DATA_PERCENTAGE=0
FOOTPRINT=POLYGON((-30.23121359203619 -73.88523809095295, -30.00996714138066 -73.8088375025861, -29.676127801902048 -73.69131716069539, -29.34714455052908 -73.57318214157297, -29.022775798383847 -73.45447781062563, -28.702796400730445 -73.3353750526158, -28.38676369098438 -73.21595783287603, -28.074812493826524 -73.09623347208702, -27.797548938970436 -72.98807425960285, -27.725592287870583 -72.98856027053438, -27.07738784481399 -73.11499774144512, -27.094751948140228 -73.12188515388958, -26.697344155371386 -73.19716967223816, -26.683165143945462 -73.97970634881884, -30.244249859642604 -73.95553305513582, -30.23121359203619 -73.88523809095295, -30.23121359203619 -73.88523809095295))
FORMAT_CORRECTNESS_FLAG=PASSED
GENERAL_QUALITY_FLAG=PASSED
GENERATION_TIME=2016-11-21T17:49:14.000114Z
GEOMETRIC_QUALITY_FLAG=PASSED
PREVIEW_GEO_INFO=BrowseImageFootprint
PREVIEW_IMAGE_URL=https://pdmcdam2.sentinel2.eo.esa.int/s2pdgs_geoserver/geo_service.php?service=WMS&version=1.1.0&request=GetMap&layers=S2A_A007383_N0204:S2A_A007383_N0204&styles=&bbox=-30.244249859642604,-73.97970634881884,-26.683165143945462,-72.98807425960285&width=1321&height=367&srs=EPSG:4326&format=image/png
PROCESSING_BASELINE=02.04
PROCESSING_LEVEL=Level-1C
PRODUCT_START_TIME=2016-11-20T10:01:52.026Z
PRODUCT_STOP_TIME=2016-11-20T10:01:52.026Z
PRODUCT_TYPE=S2MSI1C
PRODUCT_URI=S2A_OPER_MSI_L1C_TL_SGS__20161120T144941_A007383_T26CMD_N02.04
QUANTIFICATION_VALUE=10000
RADIOMETRIC_QUALITY_FLAG=PASSED
REFERENCE_BAND=B1
REFLECTANCE_CONVERSION_U=1.02339495218124
SENSOR_QUALITY_FLAG=PASSED
SPECIAL_VALUE_NODATA=0
SPECIAL_VALUE_SATURATED=65535
Subdatasets:
SUBDATASET_1_NAME=SENTINEL2_L1C:S2A_OPER_MTD_SAFL1C_PDMC_20161121T174914_R093_V20161120T100152_20161120T100152.xml:10m:EPSG_32726
SUBDATASET_1_DESC=Bands B2, B3, B4, B8 with 10m resolution, UTM 26S
SUBDATASET_2_NAME=SENTINEL2_L1C:S2A_OPER_MTD_SAFL1C_PDMC_20161121T174914_R093_V20161120T100152_20161120T100152.xml:20m:EPSG_32726
SUBDATASET_2_DESC=Bands B5, B6, B7, B8A, B11, B12 with 20m resolution, UTM 26S
SUBDATASET_3_NAME=SENTINEL2_L1C:S2A_OPER_MTD_SAFL1C_PDMC_20161121T174914_R093_V20161120T100152_20161120T100152.xml:60m:EPSG_32726
SUBDATASET_3_DESC=Bands B1, B9, B10 with 60m resolution, UTM 26S
SUBDATASET_4_NAME=SENTINEL2_L1C:S2A_OPER_MTD_SAFL1C_PDMC_20161121T174914_R093_V20161120T100152_20161120T100152.xml:PREVIEW:EPSG_32726
SUBDATASET_4_DESC=RGB preview, UTM 26S
Corner Coordinates:
Upper Left ( 0.0, 0.0)
Lower Left ( 0.0, 512.0)
Upper Right ( 512.0, 0.0)
Lower Right ( 512.0, 512.0)
Center ( 256.0, 256.0)

Thanks for your help,
Julius

#5

Hi Julius,

Skip my last request (I have deleted it). I have identified the Tile (under PRODUCT_URI). My fault for trying to multitask while in a teleconference!

Cheers

Jan

#6

So do you have any idea? :smiley:

#7

Hi Julius,

I’m trying to get a profile plot from SNAP, but it takes some time for some reason. I’ve looed at a histogram for profile plot for the Tile in the XML you copied, and can find that the histogram for B12 appears to show 0 values , but when I zoom in to the the 0,0 intercept, it shows no pixels with exactly zero.

image.jpg5760×1080 1.13 MB

Am I doing something wrong? :slight_smile:
Cheers

Jan

#8

Hi Jan,

I get the same histogram for B12. :smiley: Although there are no zeros, the values are still much lower than in some other images. This is what I was wondering about…

Cheers,
Julius

#9

Hi all
I have replied to this question on stackexchange.

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