Vrije Universiteit amsterdamGlobal evaluation of four AVHRR–NDVI data sets: Intercomparison and...

vrije Universiteit amsterdamGlobal evaluation of four AVHRR–NDVI data sets: Intercomparison and assessment against Landsat imagery

Global evaluation of four AVHRR–NDVI datasets: Intercomparison and assessment against Landsat imagery

Global evaluation of four AVHRR–NDVI datasets: Intercomparison and assessment against Landsat imagery

Hylke E. Becka,*, Tim R. McVicarb, Albert I.J.M. van Dijkb, Jaap Schellekensc, Richard A.M. de Jeua, L. Adrian Bruijnzeela

a VU University Amsterdam, The Netherlandsb CSIRO Land and Water, Australiac Deltares, The Netherlands

* E-mail: [email protected]

Corresponding publication:

Beck, H.E., et al., Global evaluation of four AVHRR–NDVI data sets: Intercomparison and assessment against Landsat imagery, Remote Sensing of Environment (2011), doi:10.1016/j.rse.2011.05.012


(1) Introduction

(2) AVHRR-NDVI dataset intercomparison

(3) AVHRR-NDVI dataset validation

(4) Conclusion/Summary

OutlineOutline


(1) Introduction(1) Introduction

• NOAA’s AVHRR sensors operating since 1981• Confused as to which global AVHRR-NDVI dataset to use:

PAL, GIMMS, LTDR, FASIR, GVI, PAL-II, or …?• All are based on the AVHRR Global Area Coverage archive• Significant differences between these datasets!• Which one do I use?• Validation studies limited (e.g., regional, small sample size)• Idea: validate using forest cover change data?• Better idea: use FAO global database of Landsat samples


(1) Introduction(1) Introduction

• Four global AVHRR-NDVI datasets:– PAL (8 km, 10 days)– GIMMS (8 km, 15 days)– LTDR V3 (8 km, 10 days)– FASIR (12 km, 10 days)

• GIMMS the most popular• LTDR still in development


(2) AVHRR-NDVI dataset intercomparison(2) AVHRR-NDVI dataset intercomparison

AVHRR-NDVI dataset intercomparison:•1982-1999•Annual means of ‘growing season’ months•Global assessment at 0.5° resolution•Where do the datasets agree/disagree •Median, variance, trend, and correlation (here only trend is discussed)



Trends in desert areas!Large differences in Congo and Sahel!

GIMMS dataset distinctly different patterns!



AVHRR-NDVI dataset intercomparison:•Average for 0.5° latitude bands•LTDR V3 overestimates variance 40°S-30°S•GIMMS has lowest trends•GIMMS higher trends in tundra•Positive trends over almost whole latitudinal range for all datasets


AVHRR-NDVI dataset intercomparison:•Kruskal-Wallis test used for hypothesis of equal trends•Blue: similar trends•Red: different trends•Inconsistent trends in Africa and Europe•Highly consistent trends in Australia




AVHRR-NDVI dataset intercomparison:•The most popular dataset (GIMMS) is also the most different•Greening almost the whole latitudinal range and in most regions for all datasets•Most greening in Europe•More favorable conditions globally for vegetation growth


(3) AVHRR-NDVI dataset validation(3) AVHRR-NDVI dataset validation

AVHRR-NDVI dataset validation:•FAO Landsat database of 20 x 20 km2 samples•11,764 Landsat-5 samples covering all major land-cover types•Landsat suitable for validation•NDVI from bands 3 and 4•Absolute-values comparison (see paper)•Temporal-change comparison•MODIS-NDVI for verification

Every dot represents one or more Landsat samples!



AVHRR-NDVI dataset validation:•Landsat sample pairs•x-axis: AVHRR- or MODIS-NDVI change•y-axis: Landsat-NDVI change•Root Mean Square Difference (RMSD) indicates performance•GIMMS second best•MODIS best



AVHRR-NDVI dataset validation:•Higher RMSD in dense canopy land-cover types•NDVI saturation and non-linearity•Tropical forests: water vapor and clouds•Boreal forests: large SZA’s


AVHRR-NDVI dataset validation:•LTDR V3 most accurate in terms of absolute values (see paper)•GIMMS probably (!) most accurate in terms of temporal change•MODIS more accurate than all AVHRR-NDVI datasets, confirms method•Interesting: simple average of the AVHRR-NDVI datasets is better than GIMMS, information is lost by maximum-value compositing?



(4) Conclusion/Summary(4) Conclusion/Summary

• Significant differences in trends for almost half of the total land surface• Dataset choice has large implications• PAL and LTDR V3 lack calibration• GIMMS (the most popular dataset) is the most different• GIMMS probably has the best calibration• However, LTDR dataset still in development; may surpass GIMMS


Thank you!

Questions?

Also check our publication, ask for a hardcopy:Beck, H.E., et al., Global evaluation of four AVHRR–NDVI data sets: Intercomparison and assessment against Landsat imagery, Remote Sensing of Environment (2011), doi:10.1016/j.rse.2011.05.012

Vrije Universiteit amsterdamGlobal evaluation of four AVHRR–NDVI data sets: Intercomparison and...

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