Hamm, Nicholas A.S.

Milton, Edward J.

Odongo, Vincent O.

English

Southampton (e-Prints Soton)

For more information:Vincent O. Odongo a , Nicholas, A. S. Hamm b  and Edward J. Milton aa Geography & Environment, University of Southampton, Southampton, Hampshire, SO17 1BJ, UKT: +44 (0)23 8059 3260, F: +44 (0)23 8059 3295, e.j.milton@soton.ac.uk b Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, P.O. Box 217, 7514 AE, Enschede, The NetherlandsOdongo: T: +31 (0) 53 4874 524, F: +31 (0) 53 4874 554, odongo18835@itc.nlHamm: T: +31 (0) 53 4874 330, F: +31 (0) 53 4874 335, hamm@itc.nlWhat are the scientific (or decision/policy) goals?•	To	perform	an	independent	assessment	of	Tuz	Gölü,	a	site	in	Turkey	proposed	for	the	vicarious	calibration	of	satellite	sensors,	in	terms	of	its	spatial	homogeneity	as	expressed	in	the	visible	and	near	infra-red	wavelengths	over	a	25-year	period	(1984-2009).Site description•	Tuz	Gölü	is	an	ephemeral	lake	in	the	central	Anatolian	region	of	Turkey,	approximately	150	km	south-east	of	the	city	of	Ankara	(lat.	38.50°,	long.	33.20°)	at	an	alti-tude	of	905	m	above	mean	sea	levelFigure 1: Tuz Gölü study siteWhat are the sources of data? (satellite, airborne, in situ, other)•	A	time	series	of	cloud-free	Landsat	TM	images	for	July	and	August	were	obtained	from	the	NASA	Land	Pro-cesses	Distributed	Active	Archive	Centre	(LPDAAC).		These	were	atmospherically	corrected	using	the	AT-COR-2	software	and	a	standard	atmosphere	(mid-lati-tude	summer).What other information and models are being used?•	Spatial	statistics	:		 •	 Getis	statistic	(Gi*)	 •	 Coefficient	of	Variation	(CV)What are the definitions and quality requirements for the key measurements and data outputs?For	a	surface	to	qualify	as	a	test	site	suitable	for	calibra-tion	and	atmospheric	correction,	it	has	to	satisfy	the	fol-lowing	criteria	(Thome,	2001,	Teillet	et	al.,	2007,	Kneu-buhler	et	al.,	2006).		1	The	site	should	have	high	spatial	uniformity	over	a		large	area	to	minimize	mis-registration	and	adjacency	effects	due	to	light	scattered	from	outside	the	target	region.	2	The	site	should	have	a	reflectance	factor	greater	than	0.3	across	all	wavelengths	averaged	over	all	angles	in	order	to	provide	higher	signal-to-noise	ratio	(SNR)	and	reduce	uncertainties	due	to	the	atmospheric	effects.	3	The	surface	of	the	site	should	have	flat	spectral	reflec-tance	to	reduce	uncertainties	due	to	different	spectral	response	profiles	when	multiple	sensors	are	involved	in	cross-calibration.	4	The	surface	properties	of	the	site	(reflectance,	BRDF,	spectral)	should	be	temporally	invariant	to	reduce	BRDF	and	spectral	surface	reflectance	effects.	5	The	site	should	be	situated	in	arid	regions	with	low	probability	of	cloudy	weather	and	precipitation.	For	this	study	(1),	(2)	and	(4)	lead	to	the	following	criteria:1	Criterion	1:	Gi*	>	0;2	Criterion	2:	CV	<	3%;3	Criterion	3:	HDRF	>	30%;4	Criterion	4:	Persistence	of	1	–	3	over	time.∗ ∑ ∗ ̅∗ ∗   (1)      (2) Figure 3:  (a) Getis statistic map (b) Getis statistic in TM bands  2, 3 and 4 for the months of August 1984 (top), August 2000 (middle) and August 2009 (bottom).What is the impact of not achieving the quality requirements/goals for the parameters? (as above)Any	significant	variation	in	points	1,	2,	3	and	4	will	result	to	disqualification	of	the	surface	as	a	candidate	test	site	for	VC.	Are quality indicators assigned at every step of the chain or only the end product?What are your recommendations for the way forward when thinking about quality of data within your study?Natural	targets	such	as	salt	pans	have	many	advantages	as	vicarious	calibration	(VC)	sites.	However,	they	are	func-tioning	geomorphological	systems,	and	as	such,	the	lo-cation	and	extent	of	sub-areas	which	are	suitable	for	use	as	calibration	sites	are	likely	to	vary	over	time.	Effective	use	of	such	sites	requires	understanding	of	the	processes	causing	spatial	and	spectral	variability	and	of	their	likely	impact	over	the	temporal	and	spatial	scales	involved.If you have read about or used QA4EO, provide feedback on the role this played in your re-search.QA4EO	inspired	this	research	as	part	of	a	broader	effort	to	provide	the	scientific	community	with	independent	evalu-ation	of	the	sites,	methods	and	practices	of	vicarious	cali-bration	in	EO.Figure 4: Two stage methodological approachFigure 5: Spatio-temporal stability map for Tuz Gölü showing the proposed calibration site.Figure 2: Landsat TM used in the studyTable 1: Landsat TM data used in the study obtained from LPDAACDate (YYYY-MM-DD) Time (GMT) Cloud Cover (%)  Solar Zenith Angle (degrees)  1984-08-03  7:50:51  0  33.39 1989-08-16  8:01:06  0  35.28 1998-08-26  8:00:04  0  36.77 2000-08-15  7:59:06  0  34.51 2003-08-08  7:58:19  0  33.03 2006-07-31  8:14:43  0  29.04 2006-08-16  8:14:54  0  32.31 2009-07-30  8:16:43  0  29.55 2009-08-15  8:16:57  0  32.73 2009-08-31  8:17:12  4  36.38 A BSpatio-temporal assessment of Tuz Gölü, Turkey as a potential vicarious calibration site

Spatio-temporal assessment of Tuz Golu, Turkey as a potential vicarious calibration site

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