Global land cover mapping: conceptual and historical background

Transcription

1 Space Research Institute Russian Academy of Sciences Global land cover mapping: conceptual and historical background Sergey BARTALEV with acknowledged contribution from: Etienne Bartholomé and Philippe Mayaux, EC JRC, Italy Pierre Defourny, Universite Catholique de Louvain, Belgium Martin Herold, Friedrich Schiller University Jena, ESA GOFC-GOLD, Germany METIER Graduate Training Course Remote Sensing of the Land Surface April 2007, University of Leicester, UK

2 What is land cover? The definition of land cover is fundamental, because in many existing classifications and legends it is confused with land use: Land cover is the observed (bio)physical cover on the earth s surface. When considering land cover in a very pure and strict sense, it should be confined to the description of vegetation and man-made features. Consequently, areas where the surface consists of bare soil are land itself rather than land cover. Also, it is disputable whether water surfaces are real land cover. However, in practice, the scientific community usually includes these features within the term land cover. after Antonio Di Gregorio

3 What is land use? Land use is characterized by the arrangements, activities and inputs people undertake in a certain land cover type to produce, change or maintain it. Definition of land use in this way establishes a direct link between land cover and the actions of people in their environment. The following examples are a further illustration of the above definitions: grassland is a cover term, while rangeland or tennis court refer to the use of a grass cover; and recreation area is a land use term that may be applicable to different land cover types: for instance sandy surfaces, like a beach; a built-up area like a pleasure park; woodlands; etc. after Antonio Di Gregorio

4 Growing interest for global land cover mapping Earth Science and Climate Change Carbon cycle Water and energy fluxes Climate and Ecosystem modelling International Conventions, such as: UN FCCC UN CBD UN CCD International environmental and security programmes, such as: Millennium Assessment FAO Global Forest Resources Assessment Global crop monitoring for food security

5 Main Earth observation sensors for global land cover mapping NOAA-AVHRR spatial resolution - 1 km 5 spectral bands SPOT-Vegetation spatial resolution 1 km 4 spectral bands Terra/Aqua-MODIS spatial resolution 0.25 km / 0.5 km / 1 km 36 spectral bands Envisat-MERIS spatial resolution 0.3 km 15 spectral bands

6 Existing global land cover products A constant but not really cumulative effort AVHRR, 1 resolution, 1994 AVHRR, 8 km, 1998 IGBP-DIS, AVHRR, 1 km, 2000 GLC-2000, VEGETATION, 1 km, 2002, 22 classes MODIS, 1 km, 2002, 17 classes Vegetation Continuous Fields, 1 km, 2002 Forthcoming : a 300-m global LC map for 2005 from 15-band FRS MERIS time series After Pierre Defourny

7 The IGBP Programme has coordinated the development of global land data sets from AVHRR data. The first 1 km spatial resolution land cover product is based on monthly Normalized Difference Vegetation Index composites from 1992 and The map legend contains 17 classes. Mapping method involved unsupervised classification with post-classification refinement using ancillary data.

8 historical reference: IGBP 1km Land Cover product Heavy data preparation process Fixed legend with limited number of classes Homogeneous classification procedure Statistically-based validation procedure

9 Comparison between the IGBP global landcover map and the forest map of Russia

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11 SPOT-Vegetation: spectral bands Relative response Reflectance Wavelength Blue Red NIR SWIR Typical vegetation reflectance Typical soil reflectance Blue: mkm Red: mkm NIR: mkm SWIR: mkm

12 SPOT-Vegetation instrument Spatial resolution at nadir observations is 1.15 km and at +/-50 0 off nadir observations degrade down to 1.7 km Geometrical accuracy: Inter-channels co-registration km Multitemporal co-registration km Absolute geographical location km Frequency of observation: 2-3 times a day in the boreal zone Radiometric accuracy: absolute, multitemporal and inerband calibration accuracy doesn t exceed the range of 2-5%.

13 SPOT-Vegetation standard products VGT-P: physical values of spectral reflectance on the top-of-atmosphere VGT-S1: daily maximum of NDVI composite of spectral reflectance at the top-of-canopy VGT-S10: ten-day maximum of NDVI composite of spectral reflectance at the top-of-canopy VGT-D10: ten-day composite of directionally normalised spectral reflectance at the top-of-canopy

14 GLC 2000 approach A new global land cover reference database for year 2000, produced by an international partnership of 30 institutions Source: Bartholomé et al. (2002) EUR EN, pp. 55.

15 GLC 2000 project structure WG Legend GLC 2000 project Chair Co-ordination WG Methods WG Validation DOC WWW WG Burn scars INTERNAL PRODUCTION SIBERIA AFRICA S. AMERICA SE ASIA BURN SCARS DATA INFORMATION SUPPORT CENT. ASIA ext. partner A EUROPE-MED ext. partner B N. AMERICA ext. partner C CHINA ext. partner D AUSTRALASIA ext. partner E Intern GVM WG EXTERNAL PRODUCTION (TBC with Partners) After Alan Belward

16 GLC 2000 Project implementation Global Land Cover map from SPOT VEGETATION data in 2000: the VEGA 2000 S1 data set covers the period 1st November 1999 to 31st December regional windows have been extracted and distributed to 30 GLC 2000 partners FAO - Land Cover Classification System as common tool for legend definition: the Global legend has been agreed upon A range of methods have been used to generate first-draft data classifications for regions including Eurasia, Africa, South America, North America, SE Asia and Europe Implementation: +/- (according to regions) Impact on product quality: +/- (according to regions) Impact on product acceptance: ++ Recently validated by statistically-sound approach => The reference LC map for the year 2000

17 Classification method Mainly unsupervised classification with supervised grouping of clusters into LC classes. Marginally: explicit thresholds on time series Neither method take full benefit of the information content for objects with a size close to IFOV Final output quality directly proportional to analyst s expertise and time spent

18 Minimum Requirements: Natural vegetation (terrestrial) Life form Cover Leaf type Phenology Layering Woody closed-open Trees closed-open closed open broadleaved needleleaved evergreen deciduous Shrubs closed-open closed open broadleaved needleleaved evergreen deciduous Herbaceous closed-open no (more) layer sparse shrubs sparse trees Lichens & Mosses After H.-J. Stibig

19 GLC Global Land Cover Classes 21 land cover classes are displayed at the global level GLC2000 Global Classes: Forest Tree Cover, broadleaved, evergreen (LCCS >15% tree cover, tree height >3m ) Tree Cover, broadleaved, deciduous, closed (or open to closed) Tree Cover, broadleaved, deciduous, open (LCCS 15% - 40% tree cover, tree height >3m ) IGBP correspondence / overlap Evergreen Broadleaved Forest (>60%, tree height >2m) overlap with woody savannas and savannas Deciduous Broadleaved Forest overlap with woody savannas and savannas Woody Savannas and Savannas (30-60% tree cover and 10-30%) Tree Cover, needle-leaved, evergreen Tree Cover, needle-leaved, deciduous Tree Cover, mixed phenology or leaf type Tree Cover, regularly flooded Mosaic: Tree cover / Other natural vegetation Evergreen Needleleaved Forest overlap with woody savannas and savannas Deciduous Needleleaved Forest overlap with woody savannas and savannas Mixed Forests overlap with woody savannas and savannas Evergreen broadleaved After H.-J. Stibig

20 GLC Global Land Cover Classes GLC2000 Global Classes: Non-Forest Shrub Cover, closed-open, evergreen (with or without sparse tree layer) Shrub Cover, closed-open, deciduous (with or without sparse tree layer) Herbaceous Cover, closed-open Sparse Herbaceous or sparse shrub cover Lichens & Mosses Regularly flooded shrub and/or herbaceous cover Cultivated and managed areas Mosaic: Cropland / Tree Cover Mosaic: Cropland / Other natural vegetation / Tree cover Bare Areas Water Bodies (natural & artificial) Snow and Ice (natural & artificial) Artificial surfaces and associated areas IGBP correspondence /overlap Shrubland- Open- closed Savannas Grasslands Persistent Wetlands Croplands Cropland / Other Vegetation Mosaic Barren or sparsely vegetated Water Snow & Ice Urban and built-up areas After H.-J. Stibig

21 16.0% 14.0% 12.0% 10.0% 8.0% 6.0% 4.0% 2.0% 0.0% Land-cover class area Tree cover needleleaved deciduous Tree cover mixed Flooded forest Shrubland evergreen Shrubland deciduous Open-closed grassland Sparse grassland Regularly flooded grass- & shrublands Lichen & mosses Cultivated & managed areas Tree cover / other natural vegetation Cropland / Tree cover Cropland / other natural vegetation Bare soil Waterbodies Snow & Ice Cities After Philippe Mayaux Tree cover needleleaved evergreen Tree cover broadleaved deciduous Tree cover broadleaved evergreen

22 GLC 2000 validation strategy Confidence-building method Systematic review of the regional products by experts and comparison with reference data Design-based method Quantitative accuracy assessment of the global product using a stratified random sampling of high-resolution sites After Philippe Mayaux

23 GLC2000 quality control to avoid macroscopic errors before the quantitative accuracy assessment to improve the global acceptance of GLC2000 products Systematic descriptive protocol to document the verification per cell Use of ancillary data (maps, Landsat & SPOT images, aerial photographs ), expert knowledge and intrinsic properties of the dataset (temporal profiles, colour composites ) After Philippe Mayaux

24 GLC2000 quality control: an example for Russia After Philippe Mayaux

25 Distribution of the Landsat-ETM+ sampling for validation of GLC2000 After Philippe Mayaux

26 Classical confusion matrix of the 21 GLC 2000 classes After Philippe Mayaux

27 Thematic distance for GLC2000 classes After Philippe Mayaux

28 Global accuracy assessment using thematic distance If we call B the confusion matrix, the global accuracy can be written: If δ is substituted by the fuzzy thematic distance ϕ we get the fuzzy global accuracy: After Philippe Mayaux

29 Confusion matrix of GLC 2000 classes after weighting by thematic distance After Philippe Mayaux

30 GLOBCOVER projet objectives Develop and demonstrate a production service of a global land cover map for 2005 at 300 m resolution using MERIS with FAO Land Cover Classification System Deliverables A hardware & software system Validated products Time composited surface reflectances 300-m Global Land Cover map for the year 2005 After Pierre Defourny

31 GLOBCOVER Challenges Main challenges : Full chain development from pre-processing to final LC product ~20 TByte of FRS MERIS data acquired from Dec.04 to June 06 Automatic and operationnal classification process No thermal nor short wave infra-red bands Uneven acquisition plan => irregular time series Key lessons learnt from previous experiences : Regionally-tuned approach more efficient Temporal and spectral information required for LC discrimination Critical role of consistent reflectance time series and refl. composites Great contribution of international experts network Typology and Validation strategy to be defined from the very beginning After Pierre Defourny

32 Processing flowchart Preprocessing Level 1B MERIS FR Geometric Corrections Cloud screening Atmospheric Corrections BRDF correction and time compositing LCCS Land Cover Map FR Validation Classification Labeling Spectrotemporal classification Level 3 MERIS Mosaics FR After Pierre Defourny

33 Seasonal composite: Spain/Morocco (September November) After Pierre Defourny

34 Classification concept GLOBCOVER Classification process is an automatic classification algorithm embedded in a processing chain globally consistent but regionally tuned repeatable designed to take advantage of the MERIS time series information content (300 m, 15 bands) After Pierre Defourny

35 Legend : 23 LCCS classes After Pierre Defourny

36 Classification steps A priori stratification equal-reasoning regions Per-pixel classification algorithm homogeneous land cover objects Per-object temporal profile discrimination consistent spectro-temporal classes NDVI 4,5 4 3,5 3 2,5 2 1,5 1 0, temps Labeling rule-based procedure LCCS land cover classes Validation classification accuracy After Pierre Defourny

37 Global Stratification Split the world in 21 equal-reasoning regions from ecological and remote sensing point of view Each region separatetly processed using regionally tuned classification parameters After Pierre Defourny

38 Classification scheme Each region will be processed independently using regionally tuned classification parameters clustering n classes NDVI R (%) Phenological characterisation of each class t Annual or seasonal composites & stratification file (21 areas) Unlabelled Product Clustering x classes Step 1 Seasonal composite Calculating mean of each indicator per class Min Max Amplitude Step 3 Step 2 Neo-channels After Pierre Defourny

39 Calibration/Validation by an international experts network Labelling Calibration Independent Validation Unlabeled Polygon Set + + Ancillary data: - Geocover data set - Global networks - Temporal profiles LCC Legend Calibration Point Set Vegetation Expert = Unlabeled GlobCover Product LCC Legend Labeled Non-Validated GlobCover Product Thematic accuracy over a given period of time After Pierre Defourny

40 Annual Spring Good discrimination between pastures and croplands Bad discrimination between pastures and croplands Summer After Pierre Defourny Autumn

41 Selection of best MERIS bands for each equal-reasoning area Grassland (Fr) Cropland (Ro) Irrigated crops (Fr) Needleleaved forest (Sp) Broadleaved forest (Ro) Mixed forest (Fin) Shrubland (Sp) Water (Fr) Sandy desert (Sp) Snow (Ro) Urban (Ro) Reflectance Ch1 Ch2 Ch3 Ch4 Ch5 Ch6 Ch7 Ch8 Ch9 Ch10 Ch12 Ch13 Ch14 Bands After Pierre Defourny

42 Test of Classification Method using SPOT VEGETATION TS Eurasian strata Cultivated and Managed areas (level 1) Rainfed herbaceous crops (level 2) Closed to open grassland Closed broadleaved deciduous forest Closed needle-leaved evergreen forest Artificial surfaces and associated areas (urban areas) Water bodies Wetlands Sparse vegetation Bare areas After Pierre Defourny

43 First GLOBCOVER classification from MERIS FSR data England Eurasia Mediterranean area Desert After Pierre Defourny

44 GLC 2000 Globcover MERIS Globcover VGT Corine After Pierre Defourny

45 Forest areas in global land cover maps Forest definitions: IGBP legend : percent tree cover >60% / tree height >2m GLC2000 legend : percent tree cover >15% / tree height >3m After Martin Herold

46 Assessment of agreement between global land cover products and FAO FRA-2000 Differences in forest area in comparison to FRA-2000 (%) After Anatoly Schvidenko IGBP GLC UMD MODIS

47 Vegetation continuous fields products from MODIS data 0% 100% Percent tree cover threshold

48 Existing global land cover datasets Common ground for land characterization Common land cover classifiers (LCCS) Cover type/ life form Trees Shrubs Herbaceous Bare Snow & Ice Artificial Leaf longevity Leaf type Needle-leaved Broadleaved Cultivated and managed/ (semi-)natural Cultivated/ managed Evergreen Deciduous Terrestrial / aquatic+ regularly flooded Aquatic/ flooded After Martin Herold

49 End of the 1-st lecture