# Presentation: 547707

## IKONOS, ETM, MODIS NDVI: comparison
Jeff Morisette, MODLAND, SSAI
Positive Systems for Appalachian Transect
Rob Sohlberb, MODLAND, UMd
Report from Stennis Space Center on SDP validation activities
Mary Pagnutti, SSC
One Ikonos DEM/Stereo Pair for Barton Bendish site
J. Peter Muller, MODLAND, ULC

**Jeff Morisette, MODLAND, SSAI**
**Positive Systems for Appalachian Transect**
**Rob Sohlberb, MODLAND, UMd**
**Report from Stennis Space Center on SDP validation activities**
**Mary Pagnutti, SSC**
**One Ikonos DEM/Stereo Pair for Barton Bendish site**
**J. Peter Muller, MODLAND, ULC**

**NASA’s: Science Data Purchase***** ***

**One approach to scaling**

**Comparing **
**ETM+, IKONOS, and MODIS NDVI products**
**Framed in the context of statistical hypothesis testing**
**J. Morisette**

## General validation procedure: correlative analysis(slide from 1999 validation mtg.)

- Field data

- “Tasked” acquisitions:
- Airborne and high res. Satellite

- Automatic acquisitions:
- reference data and products to be validated

- Compare:
- Need to consider all three elements as samples from unknown distributions, use each component to estimate the respective distribution, and compare *distributions*

- points to pixels
- parameters and distributions
- relationships
- surfaces

- Point

- Fine resolution

- Fine resolution

**Spectral Bands, Red and NIR**

- Difference may be important: Gitelson and Kaufman, 1998; Bo-Cai Gao, 2000;
- which compared MODIS to AVHRR and found large differences in NDVI
- *ASD spectrum from grass area near GSFC

- AVHRR

- IKONOS

- ETM+

- MODIS

- grass reflectance*

## Study Area: Konza Prairie

- Data:
- MODIS daily products: Sept. 11
- 	500m surface reflectance
- 	500m pointer file
- 	1km viewing geometry
- 	LDOPE tools to combine
- (available through EDC EDG)
- ETM+, Sept. 11
- IKONOS, Sept. 15
- Aeronet (Meyer)
- (available through Konza Prairie Core Site web page)
- Vermote et al.’s Six S code
- (for ETM+ and IKONOS)

## IKONOS area on MODIS 500mand ETM+

## Subset area in Sinusoidal Projection

## Sampling with MODIS “Tile Mapper”

- Done for both IKONOS and ETM+

## Sampled imagery

- IKONOS (30m)

- ETM+

## Comparison at Multiple-scales

- MODIS
- Pixel 1, 1

- ETM+
- 14, 16
- n=224

- IKONOS
- 116, 120
- n=13,920

- Considering all three as variable and subject to errors, consider MODIS pixel relative to the *distribution* from the higher resolution data

## IKONOS vs ETM+ 

- Correlation = .5639
- Reject hypothesis of zero correlation
- Using standard Pearson method (p value ~0)

## IKONOS vs MODIS

- Correlation = .3114
- Reject hypothesis of zero correlation
- Using standard Pearson method (p value ~0)

## ETM+ vs MODIS

- Correlation = .3401
- Reject hypothesis of zero correlation
- Using standard Pearson method (p value ~0)

## Do the data follow a normal distribution?

- Null Hypothesis: Normally distributed
- Test: Kolmogorov-Smirnov Goodness-of-Fit Test:
- MODIS data: **Reject** (p = .0079)
- ETM+ at 500m: **Reject** (p = .0004)
- IKONOS at 500m: **Reject** (p ~ 0)
- ETM+: **Reject** (p ~ 0)
- IKONOS at 30m: **Reject** (p ~ 0)
- So, should consider testing correlation with non-parametric methods.

## Non-parametric correlation 

- Null Hypothesis: Zero Correlation
- Test: Spearman's rank correlation
- IKONOS vs ETM+:	**Reject** (rho = .5791, p ~ 0)  *(**corr = .5639)*
- IKONOS vs MODIS: 	**Reject** (rho = .3099, p ~ 0)  *(**corr = .3114)*
- ETM+ vs MODIS: 	**Reject** (rho = .3362, p ~ 0)  *(**corr = .3401)*
- But we still might want to question the hypothesis being tested.

## Test for Paired Differences

- Null Hypothesis: average paired difference is zero
- Test: T test (assume normality and homogeneity of variance)
- Test: Wilcoxon Rank Sum Tests
- IKONOS vs ETM+
- IKONOS vs MODIS	 **Reject** all three pair-wise combination
- ETM+ vs MODIS	 based on either test.
**So, for these data we are somewhere in the middle: **
**There is positive correlation, **
**but the average difference is not zero**

## Normalized differences to include variability in validation data

- MODIS – IKONOS(average)

- Std. Dev (IKONOS ave.)

- = “z score”

## Z score analysis

- IKONOS vs ETM

## Z score analysis

- IKONOS vs MODIS

## Z score analysis

- ETM vs MODIS

## Do the z-scores follow a normal distribution?

- Null Hypothesis: Normally distributed
- Test: Kolmogorov-Smirnov Goodness-of-Fit Test:
- Z from IKONOS vs ETM+:	Reject (ks = 0.1955, p ~ 0)
- Z from IKONOS vs MODIS: 	Reject (ks = 0.0537, p = 0.0142)
- Z from ETM+ vs MODIS: 	Reject (ks = 0.0538, p = 0.013)
- So, should consider testing z-scoresw with at least both parameteric
- and non-parametric methods

## Test of z-score “centered” on zero 

***Non-Parametric***
- Null Hypothesis: Median value is zero
- Test: Wilcoxon Signed Rank Sum Tests
- Z from IKONOS vs ETM+:	**Reject** (Z =-46.493, p ~ 0)
- Z from IKONOS vs MODIS: 	**Reject** (Z = -9.9305, p ~ 0)
- Z from ETM+ vs MODIS: 	**Reject** (Z = -6.2677, p ~ 0)
***Parametric***
- Null Hypothesis: Mean value is zero
- Test: T test
- Z from IKONOS vs MODIS: 	**Reject** (t = -10.143, p ~ 0)
- Z from ETM+ vs MODIS: 	**Reject** (t = -5.4727, p ~ 0)

## Conclusions

- Assumption of normality is not always met
- Non-parametric methods are available
- Z-score method shows one possible way to scale up; which incorporates variability and considers the validation data with respect to its distribution
- There is a fundamental difference between the null hypothesis of the correlation being zero and the difference being zero
- There is closer statistical agreement between MODIS and either IKONOS and ETM+ than between IKONOS and ETM+
- There is a difference between statistical and practical difference

## Comments

- ETM+, IKONOS, MODIS and Sun photometer data were easily available
- Major difficulty was ISIN projection and georeferencing – coordination of  Jacqueline Le Moigne, GSFC might prove helpful.
- Results are planned to be communicated in the validation article in the Special Issue of RSE.