I am using MODIS Calibrated Radiances (MOD021KM see https://ladsweb.modaps.eosdis.nasa.gov/missions-and-measurements/products/MOD021KM/) in SNAP and I am having difficulty understanding what I am seeing.
I have brought the HDF imagery in via File > Import > Optical Sensors > MODIS LIB
If I just bring up a RGB image and right click it I can copy the values to the clipboard (see below).
Reading the manual the data should be scaled integers but these are floating point data.
Does SNAP automatically convert the DNs to Radiances?
Image-X: 573 pixel
Image-Y: 1497 pixel
Longitude: 109°10'57" W degree
Latitude: 32°39'12" N degree
BandName Wavelength Unit Bandwidth Unit Value Unit Solar Flux Unit
EV_1KM_RefSB_8: 412.0 nm 0.0 nm 0.09609 none
EV_1KM_RefSB_9: 443.0 nm 0.0 nm 0.08402 none
EV_1KM_RefSB_10: 488.0 nm 0.0 nm 0.06811 none
EV_1KM_RefSB_11: 531.0 nm 0.0 nm 0.06147 none
EV_1KM_RefSB_12: 547.0 nm 0.0 nm 0.06191 none
EV_1KM_RefSB_13lo: 667.0 nm 0.0 nm 0.06661 none
EV_1KM_RefSB_13hi: 667.0 nm 0.0 nm 0.10987 none
EV_1KM_RefSB_14lo: 678.0 nm 0.0 nm 0.06654 none
EV_1KM_RefSB_14hi: 678.0 nm 0.0 nm 0.11234 none
EV_1KM_RefSB_15: 748.0 nm 0.0 nm 0.16412 none
EV_1KM_RefSB_16: 869.0 nm 0.0 nm 0.19351 none
EV_1KM_RefSB_17: 905.0 nm 0.0 nm 0.08783 none
EV_1KM_RefSB_18: 936.0 nm 0.0 nm 0.04161 none
EV_1KM_RefSB_19: 940.0 nm 0.0 nm 0.05688 none
EV_1KM_RefSB_26: 1375.0 nm 0.0 nm 0.00072 none
EV_1KM_Emissive_20: 3750.0 nm 0.0 nm 0.40577 none
EV_1KM_Emissive_21: 3959.0 nm 0.0 nm 0.50208 none
EV_1KM_Emissive_22: 3959.0 nm 0.0 nm 0.52399 none
EV_1KM_Emissive_23: 4050.0 nm 0.0 nm 0.52449 none
EV_1KM_Emissive_24: 4465.0 nm 0.0 nm 0.17288 none
EV_1KM_Emissive_25: 4515.0 nm 0.0 nm 0.50029 none
EV_1KM_Emissive_27: 6715.0 nm 0.0 nm 2.12349 none
EV_1KM_Emissive_28: 7325.0 nm 0.0 nm 3.56100 none
EV_1KM_Emissive_29: 8550.0 nm 0.0 nm 7.53757 none
EV_1KM_Emissive_30: 9730.0 nm 0.0 nm 5.49986 none
EV_1KM_Emissive_31: 11030.0 nm 0.0 nm 8.07653 none
EV_1KM_Emissive_32: 12020.0 nm 0.0 nm 7.61204 none
EV_1KM_Emissive_33: 13335.0 nm 0.0 nm 5.15944 none
EV_1KM_Emissive_34: 13635.0 nm 0.0 nm 4.10899 none
EV_1KM_Emissive_35: 13935.0 nm 0.0 nm 3.54949 none
EV_1KM_Emissive_36: 14235.0 nm 0.0 nm 2.49691 none
EV_250_Aggr1km_RefSB_1: 645.0 nm 0.0 nm 0.06468 none
EV_250_Aggr1km_RefSB_2: 859.0 nm 0.0 nm 0.10448 none
EV_500_Aggr1km_RefSB_3: 469.0 nm 0.0 nm 0.07415 none
EV_500_Aggr1km_RefSB_4: 555.0 nm 0.0 nm 0.06234 none
EV_500_Aggr1km_RefSB_5: 1240.0 nm 0.0 nm 0.11337 none
EV_500_Aggr1km_RefSB_6: 1640.0 nm 0.0 nm 0.09779 none
EV_500_Aggr1km_RefSB_7: 2130.0 nm 0.0 nm 0.06475 none
Actually the values are reflectances. Modis files have conversion factors for radiances and reflectances.
The DNs are automatically converted to reflectances.
Hi Marpet, thank you for your reply but I am not sure if you are correct. If you are correct then I am still very confused!
According to the MODIS L1B product users guide ( see page 31 of (https://oceancolor.gsfc.nasa.gov/docs/format/M1054_RevC_PUG_022709_Update.pdf), the MODIS products
Level 1B generates two calibrated data products for the Reflective Solar bands (RSBs): the
reflectance and the Earth-exiting radiance. Writing these two products in floating-point format to
the Level 1B Earth view product files would make the file sizes prohibitively large.
Instead, Level 1B writes a 16-bit scaled integer representation of the calibrated digital signals
measured by the MODIS, from which the reflectances and radiances can be calculated from two
pairs of scale and offset terms written as attributes to the reflective band SDSs. These two pairs are
derived from the calibration parameters that are input to Level 1B as look up tables. Reflective
Solar Band radiance can also be calculated more precisely using calculated reflectances and
appropriate granule metadata; see Section 5.4.3.
So It sounds to me that the MODIS L1B MOD021 product should be a 16-bit integer that you can calculate either reflectances or radiances using formulas.
Reading a little further, it appears that the scaled integers (SI which is a 16-bit integer) are representations of dn** (which is a 32-bit floating point). So I am thinking that SNAP is actually showing me the dn**, not the SI?
Therefore I need to (1) convert dn** to SI and then (2) convert SI to radiances?
Interestingly when you inspect the information for just one band of the file (EV_1KM_RefSB_8) you get the following information.
These (I think) are the conversion factors to go from dn** to SI and I don’t see the radiance scaling/offset factors or the reflectance scaling/offset factors.
Description: Earth View 1KM Reflective Solar Bands Scaled Integers
Geophysical unit: none
Geophysical data type: float32
Raw data type: uint16
Raster width: 1354 pixels
Raster height: 2030 pixels
Scaling factor: 2.609128932817839E-5
Scaling offset: -0.008270213339695576
Is log 10 scaled: false
Is no-data value used: false
No-data value: 0.0
Geophysical no-data value: -0.008270213339695576
Valid pixel expression: null
Spectral band index: 0
Wavelength: 412.0 nm
Bandwidth: 0.0 nm
Solar flux: 0.0 mW/(m^2*nm)
Product name: MOD021KM.A2018314.1800.006.2018315073121.hdf
Product type: MODIS Level 1B
Product description: MOD021KM.A2018314.1800.006.2018315073121.hdf
Product format: MODIS-L1B
Product reader: MODIS L1B Products
Product reader class: gov.nasa.gsfc.seadas.dataio.L1BModisProductReaderPlugIn
Product reader module: NASA OBPG Ocean Color Product Reader - v1.8
Product file location: C:\Users\Don\Documents\Brian\Fire\LAADS\CAMP\MOD021KM.A2018314.1800.006.2018315073121.hdf
Product scene width: 1354 pixels
Product scene height: 2030 pixels
Product start time (UTC): 10-NOV-2018 18:00:00.000000
Product end time (UTC): 10-NOV-2018 18:05:00.000000
You can also investigat the HDF file with e.g. HDFView.
There you can see all the attributes.
We haven code which reads the MODIS file is taken from SeaDAS a while ago.
Maybe use SeaDAS directly, it could be that they have improved the reading meanwhile.
In SNAP are float values shown wit ha max of ~1.4.
This indicates reflectances.
This also could be suitable tool for reading MODIS MODIS and VIIRS Data Tools and Services at your Fingertips
Source : https://daac.ornl.gov/resources/tutorials/MODIS-VIIRS_Webinar_20180816.html