geoscience australia md anisul islam geoscience australia evaluation of imapp cloud cover mapping...

Geoscience Australia

Md Anisul Islam


Evaluation of IMAPP Cloud Cover Mapping Evaluation of IMAPP Cloud Cover Mapping Algorithm for Local application. Algorithm for Local application.

Australian Government



Objective of the study:

Evaluation of IMAPP Cloud Cover Mapping algorithm (Collection 4 & 5) for local application to utilise the cloud mask for generating higher level Land application products.


Ancillary data

Ecosystem file Land/water mapDEMDaily snow/ice SSTGlobal data assimilation system (GDAS1)

Image Pixel

Labelling the pixels to Surface types

Water Land Desert Snow/Ice

10 Spectral Cloud tests

Group 1 Group 2 Group 3 Group 4 Group 5

Initial Cloud mask obtained from Clear sky conservative approach

Final Cloud mask

Clear Sky Restoral tests

Sunglint regionShallow waterDesert & Land

Cloudy Pixels

Schematic diagram of IMAPP MODIS Cloud Cover Algorithm

Clear Pixels


Group 1 (IR bands):

BT11, BT13.9 & BT6.7

Group 2 (Thermal band differences):

Trispectral TestBT11- BT12, BT11 – BT3.9

Group 3 (Reflective bands):

R0.66 or R0.87 & R0.87/R0.66

Group 4 (NIR thin Cirus):

R1.38

Group 5 (IR thin Cirus): BT3.7 – BT12

Clear sky conservative approach of Initial cloud mask generation

Gi=1,5 = min[Fi] & Initial Cloud mask confidence level = (Product of Gi=1,5)-5

Where, G = Group

Three thresholds used in cloud screening

Confidence Level (F): > 0.95 & ≤ 0.99 - Uncertain clear≥ 0.66 & ≤ 0.95 - Uncertain cloud


Difference between Collection 4 and Collection 5 Cloud Mask Algorithms

Surface type Labelling scheme:

Pixels belonging to Ecosystems: Savanna (Woods), Hot & Mild Grasses andShrubs, Woody Savana are labelled as surface type Land in Collection 4 algorithm, are labelled as Desert for Australian Continent (latitude 11.0 - 40.0 (S)& longitude 110.0 -155.0 (E)) in Collection 5 algorithm.

Major Effects on the above pixels in Collection 5 algorithm are :

Subject to different threshold values (for Desert: -20, -18, -16) of BT11-3.9 spectral test in Collection 5 algorithm as against threshold values of Land (-14, -12, -10) in Collection 4 algorithm

Subject to spectral test of Desert (R0.87) applied in Collection 5 algorithm asagainst spectral tests of Land (R0.66 & R0.87/R0.66 ) applied in Collection 4algorithm


Threshold values:

For BT13.9 test are 224, 226 and 228 (Kelvin) for Collection 4 algorithm and are 222, 224 and 226 (Kelvin) for Collection 5 algorithm

For R0.87/R0.66 test are 0.55, 0.40 & 0.30 for Water in Collection 4 algorithm and are 0.65, 0.55, & 0.45 in Collection 5 algorithm

Additional ancillary data used in Collection 5 Cloud Mask Algorithm:

NOAA optimum Interpolation (OI) Sea Surface Temperature (SST) V2 product at 1 degree resolution – helps to improve the cloud mask over ocean at night.

Global assimilation system (GDAS1) for retrievals of temperature and moisture profile – helps to improve the cloud mask over many areas of

the land.


Full swath images acquired from the Orbit covering maximum Land areas of Australia

Monthly MODIS image acquired between September 2004 to April 2005

Acquisition date Orbit no

2 Sept 2004 25043

20 October 2004 25742

21 November 2004 26208

23 December 2004 26674

9 February 2005 27373

30 April 2005 28538

MODIS DATA ASSESSED IN THE PROJECT:

MODIS TERRA images capturing high temporal and spatial variation of Land areas of Australia


Visual assessment of final Cloud Mask using:

RGB of visual bands Gray scale image bands used in the spectral test

Spectral Analysis of data from sample sites in areas where there are errors as determined by visual assessment

Convertion of image band Digital numbers (DN) to the units of the thresh values:

Convertion of reflective image band DN to reflectance unit & Thermal image band DN to Kelvin

Extraction of sample data from the images

Scatterplots of the bands used in spectral tests having errors versus sample site attributes to determine the amount of errors.

Methodology:


Relective bands 1 4 3 Inverse BT11 – BT3.9

Collection 4 Cloud Mask Collection 5 Cloud Mask

Confident Clear Probably Clear Uncertain Cloudy

20 October 2004


20 October 2004



21 November 2004



21 November 2004

Relective bands 1 4 3 Relective band 26 (R1.38 Thin Cirus test)

Inverse BT11 – BT3.9 Confident Clear Probably Clear

Uncertain Cloudy


23 December 2004



Confident Clear Probably Clear

Uncertain Cloudy

Relective bands 1 4 3

23 December 2004

Relective band 26 (R1.38 Thin Cirus test)

Inverse BT11 – BT3.9


9 February 2005




Uncertain Cloudy


9 February 2005




2 September 2004




Uncertain Cloudy


2 September 2004




Spectral plots of the cloud free samples taken from Multitemporalimages.

THCCLD - threshold corresponding to HighConfident Cloudy pixels (α)

TV - threshold value for pass or fail (β)

THCCLR - threshold corresponding to High Confident Clear pixels (γ)


Conclusions and Recommendations

Collection 5 cloud mask significantly reduces misclassification of the clear pixels as cloudy pixels relative to Collection 4 Cloud mask. However, collection 5 cloud mask may detect patchy Low cloud as uncertain cloud and uncertain clear pixels (October 04).

Collection 5 cloud mask may be highly sensitive in detecting High thin clouds (November 04, December 04), which may be better evaluated using additional data.

Collection 5 cloud mask may have small errors of misclassifying clear pixels as cloud in the surface type Land (February 05), which may be attributed to BT11-BT3.9 test. Modification of the threshold values of this test may further improve the cloud mask.

Clear pixels of bright desert and salt lake may be misclassified as uncertain clear pixels and cloudy pixels (Sep 04 & April 05), which may be attributed to Reflectance R0.87 test. Modification of threshold values of R0.87 test and or BT11 threshold values used in the clear sky restoral test may be modified to improve the results.

geoscience australia md anisul islam geoscience australia evaluation of imapp cloud cover mapping...

Documents

algorithm slide

geoscience australia

algorithm subject

spectral cloud tests

uncertain cloud slide

spectral tests of land

spectral test of desert

trispectral test bt