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Abstract

The Envisat-Meris S10 or "EM10" are near-global, 10-daily composite images are synthesised from the "best available" observations registered in the course of every "dekad" by the orbiting earth observation system Envisat-Meris. VITO ingests all the daytime registrations of ENVISAT-MERIS and further processes them into global, 10-daily synthesis images, very comparable with the S10 of SPOT-VEGETATION.

Within this project only the TOC NDVI (Top of Canopy) and the fAPAR (Fraction of Absorbed Photosynthetically Active Radiation) are computed out of the MERIS Reduced Resolution data. 'Reduced Resolution (RR)' products are generated by aggregating the FR data onboard to a nominal resolution of 1200 m. The MERIS Reduced Resolution 3rd reprocessing dataset was used to construct this archive. The archive ranges from April 2002 till April 2012. Since ENVISAT is no longer operational (see message of ESA) there will be no near-real time products.

The state and evolution of the terrestrial vegetation concern a large number of users in specific applications such as agriculture, forestry, environmental monitoring etc.. Few of the parameters that express this can be directly observed from satellites, but fAPAR is one of those that can be observed. The Fraction of Absorbed Photosynthetically Active Radiation (fAPAR) acts as an integrated indicator of the status and health of the plant canopy and has been recognized as a fundamental surface parameter for environmental studies by international organizations like the Global Climate Observing System (GCOS). The value represents the fraction of the solar energy which is absorbed by vegetation and therefore plays the role of a battery during the plant photosynthetic process; they are dimensionless and scaled between 0 and 1. (Gobron, et al). Since the fAPAR values is available in the level 2 MERIS RR images (band toa_veg, band nr: 20) the main production step is the compositing of these bands for the specific region of interest.

 

Processing steps (top)

MERIS Top Of Canopy Normalized Difference Vegetation Index (TOC-NDVI)

The Normalized Difference Vegetation Index (NDVI) provides an index to present the vitality of the vegetaton on the earth’s surface. The algorithm uses the abrupt rise of the reflection level of 0.7 mm. The NDVI results from the following equation

(Rs,NIR–Rs,RED)/ (Rs,NIR+Rs,RED),

with Rs,RED and Rs,NIR the atmospherically corrected surface reflectances in the RED and Near Infrared (NIR) bands.  The processing chain uses the rectified NIR (band 13) and rectified RED (band 8) as part of the MERIS Reduced Resolution (RR) level 2 dataset. These rectified bands are generated by the FAPAR processor, based on the Gobron method (see http://fapar.jrc.ec.europa.eu/), using the MERIS Level1b TOA reflectances and angles Tie Point Grids.

The processing chain at VITO consists of extracting both rectified bands, remapping them, performing the NDVI calculation and compositing the resulting values for a certain area of interest for a specific period. 

During the selection step only land pixels are considered that are not flagged as SUSPECT, HIGH_GLINT nor LOW_SUN (using the standard MERIS L2 flags, as described in the MERIS Handbook). During the Maximum Value Compositing processing step, all pixels over water and desert are flagged using a static land cover input mask.

 

 

MERIS Fraction of Absorbed Photo synthetically Active radiation (fAPAR)

 

The Normalized Difference Vegetation Index (NDVI) provides an index to present the vitality of the vegetaton on the earth’s surface. The algorithm uses the abrupt rise of the reflection level of 0.7 mm. The NDVI results from the following equation

(Rs,NIR–Rs,RED)/ (Rs,NIR+Rs,RED),

with Rs,RED and Rs,NIR the atmospherically corrected surface reflectances in the RED and Near Infrared (NIR) bands.  The processing chain uses the rectified NIR (band 13) and rectified RED (band 8) as part of the MERIS Reduced Resolution (RR) level 2 dataset. These rectified bands are generated by the FAPAR processor, based on the Gobron method (see http://fapar.jrc.ec.europa.eu/), using the MERIS Level1b TOA reflectances and angles Tie Point Grids.

The processing chain at VITO consists of extracting both rectified bands, remapping them, performing the NDVI calculation and compositing the resulting values for a certain area of interest for a specific period. 

During the selection step only land pixels are considered that are not flagged as SUSPECT, HIGH_GLINT nor LOW_SUN (using the standard MERIS L2 flags, as described in the MERIS Handbook). During the Maximum Value Compositing processing step, all pixels over water and desert are flagged using a static land cover input mask.

 

MERIS-RR processing scheme

 

Input

Input 1: All ENVISAT-MERIS segments (N1 format) for a 10 daily period .

Output

Output 1: ENVI header (*.hdr)

The header file contains standard information allowing to easily read it through ENVI software, it contains the following information (see also image format and names):

- description: general description

- samples: number of columns

- lines: number of rows

- flags: special values in the image (flagged)

- bands: number of bands in the image (usually 1)

- map info: the projection system and coordinates

- sensor type: the sensor based on which the product is generated

- Program through which the product is generated

Output 2: binary image (*.img)

This file contains the actual image values and can be imported into any software able to read binary images (such as arcgis, idrisi, envi, etc.) when the appropriate header file is available.

 

 

Technical description (top)

Temporal aspects:

Every month is divided in three "dekads". The first two always comprise ten days (1-10, 11-20), the third one has variable length as it runs from day 21 until the end of the month. The distinction between "days" is based on UT/GMT criteria. And every "dekad" a new EM10 is generated. The time series of EM10, distributed by VITO, starts in March 2002 and goes until April 2012. Since ENVISAT is no longer operational (see message of ESA) there will be no near-real time products.

Spectral aspects:

Each EM10 composite comprises six separate image layers, whose characteristics are listed in the table below. All layers have data type “8-bit, unsigned byte”, so the digital values V in the images vary (potentially) between 0 and 255.

IMAGE Physical Values Y Scaling Digital Values V
VVV CONTENT UNIT Ylo Yhi Y = A + B*V Vlo Vhi Vflag

FAP

FAPAR

-

0 → 1.25

Y=0+0.005*V

0 → 250

254-255

FAP_DAY

Day in dekad

-

1 → 10

Y=V

1 → 10

255

FAP_TCO

Nr of clear observations

-

1 → 255

Y=V

1 → 255

0

TOCNDV

Top of Canopy NDVI

-

-0.08 → 0.92

Y=-0.08 + 0.004*V

0 → 250

254-255

TOCNDV_DAY

Day in dekad

-

1 → 10

Y=V

1 → 10

255

TOCNDV_TCO

Nr of clear observations

-

1 → 255

Y=V

1 → 255

0

Remarks:

  • V=digital value as indicated in the image, Y=physical meaning.
  • Vlo-Vhi=Significant V-range. The scaling only holds for this range. Values beyond Vlo-Vhi are flags.
  • Vflag: Per image, there is only one flag to indicate all "aberrant" states (Sea, NoData, NoValidData, Error). The Status Map image provides more information.

Spatial aspects:

In a spatial sense, the near-global EM10 are identical to (and congruent with) the ones of SPOT-VEGETATION:

Geodetical datum WGS84
Projection Unprojected Geographic Lon/Lat
Resolution ΔLon=ΔLat=1°/112=0.00892857  (± 1 km along a great circle)
Centre of edge pixels

LONmin: -180° → LONmax+179.991071°

LATmin: -56° → LATmax+75°

Ncol / Nrec / Npix 40 320 / 14 673 / 591 615 360

However, the near-global imagery is not distributed as such. Users can only select one of the ten pre-defined "windows" listed in the table below. These windows are the same as used for the free distribution of SPOT VEGETATION products.

ROI def

 

www

WINDOW

LONmin

LONmax

LATmin

LATmax

Ncol

Nrec

Npix

AMn

North America

-180

-13

40

75

18704

3920

73 319 680

AMc

Central America

-125

-50

0

50

8400

5600

47 040 000

AMs

South America

-93

-33

-56

25

6720

9072

60 963 840

EUR

Europe

-11

62

25

75

8176

5600

 45 785 600

AFR

Africa

-26

60

-35

38

9632

8176

78 751 232

ASw

Asia West

25

98

5

50

8176

5040

41 207 040

ASn

Asia North

45

180

40

75

15120

3920

59 270 400

ASe

Asia East

68

147

5

55

8848

5600

49 548 800

ASi

Asian Islands

92

170

-12

29

8736

4592

40 115 712

AUS

Australasia

95

180

-48

10

9520

6496

61 841 920

 

The listed LON/LAT-values indicate the geographical longitude and latitude of the centre of the edge pixels in each window. ‘Ncol’ and ‘Nrec’ are the number of columns and rows in each window and ‘Npix’ is the total number of pixels (Npix = Nrec x Ncol).

Image format and names

The images are distributed in “flat binary” format without header/trailer bytes. “Flat” means that each layer of a certain composite is stored in a separate image file. These files have the fixed extension *.IMG. Because all layers have the byte data type (1 pixel = 1 byte), the total number of bytes in each image is equal to the number of pixels (see Npix in the table). This type of imagery can be easily imported in any standard software for image processing.

Associated to each image file is a small ASCII-formatted annotation file, which has the same basename as the image but extension *.HDR. It provides all the basic ancillary information to import the imagery directly into the ENVI software. Similar annotation files can easily be created for other packages. Below is an example of the HDR of the NDVI image of EUR-window extracted from the EM10 of the second dekad of January 2008. The sequence “1.5, 1.5, -11, 75” in “map info” specifies that the centre of the top-left pixel (Col=1.5, Rec=1.5) is located at the geographical position with Lon=-11°, Lat=75°.

Contents of annotation file MER_RR_20080111_S10_EUR_TOCNDV.HDR

ENVI
description = { 10 day MAX composite of NDVI in - }
samples = 8176
lines = 5600
bands = 1
band names = { 0 }
header offset = 0
file type = ENVI Standard
data type = 1
x start = 0
y start = 0
FLAGS = { 254=sea, 255=compo }
MAP INFO = { Geographic Lat/Lon, 1, 1, -11, 75, 0.0089285710, 0.0089285710, WGS-84, units=Degrees }
SENSOR TYPE = ENVISAT-MER-RR
VALUES = {NDVI, -, 0, 250, 0, 250, -0.08, 0.004}
interleave = bsq

The filenames are as follows: MER_RR_YYYYMMDD_S10_www_vvv.eee, with:

  • YYYYMMDD = start date of the concerned dekad (DD can only be 01, 11 or 21).

  • www = the label of the selected geographical window (see table above).
  • vvv = the label of the concerned image layer (see the first table).
  • eee = the filename extension (IMG or HDR).

In addition to the IMG/HDR files, each EM10 composite also comes with the following two files:

  • MER_RR_YYYYMMDD_S10_www_vvv.tiff: a quicklook map.

  • MER_RR_YYYYMMDD_S10_www_vvv.xml: metadata for the composite as a whole.

The EM10 are transferred to the users in ZIP-form (see PRODUCTS→DISTRIBUTION). Each ZIP thus comprises 16 files: six IMGs, six HDRs, two XML and two TIFFs.

Distribution (top)

General

The EM10 products are freely available without limitations for non-commercial use. But when they are used for reports, publications or similar, please mention the reference: "by courtesy of the CVB project, produced by VITO" and inform us (Helpdesk) for which purpose they are used.

The EM10 products can be ordered via the on-line catalogue and/or via subscription. Selections can be made in time (e.g. the 36 dekads of 2009) and in space (e.g. one or more of the ten pre-defined windows), but not in a thematic sense (e.g. only NDVI). Each individual EM10-composite thus always comprises all of the twelve constituent images (see “Technical description”). The EM10 are always delivered in ZIP-form.

Product catalogue

The catalogue can be consulted to discover and order available products online.
Specific queries can be defined as follows:

  • A time interval can be specified via the fields ‘Start Date’ and ‘End Date’. Clicking on the field opens a standard calendar box. Remind that the EM10 series goes from 2002 till 2012. If days other than 01/11/21 are selected, they are reset to the start date of the concerned dekad (e.g. 10 January and 31 March are respectively reset to 1 January and 21 March).
  • One of the ten pre-defined windows can be selected by activating the “Draw rectangle” command (third icon from right) and delineating an area of interest (AOI). Clicking on the “Search” button then shows the window(s) covering this region. Some AOIs may only fall in one window (e.g. Canada in www=AMn), others may even be covered by three different windows (e.g. Iraq in www=ASw, EUR and AFR).

After submitting the query by means of the ‘search’ button, result pages are shown with an overview of the matching products. For each EM10 product, the following buttons are available:

  •  “Show on map”: the area of interest (AOI) of the selected product will be highlighted in yellow on the map above.
  • “Add this item to basket”: add the product to the shopping basket, which allows you to order the product. Later on you can finalise the order containing all products into the basket. This functionality is only available for registered users. If you are not yet logged in, you are invited to do so.

  • “Add all items to basket”: add all products on this page to the shopping basket.

  • “Go to basket”: navigate to the shopping basket. Here you get an overview of all products you selected for ordering. You can remove products or finalise the order by clicking on the ‘Proceed’ button.

As soon as the products are ready for download over FTP, you will receive an e-mail. Please notice that you can use the same userId/password to download the products from the FTP-server, as the one you use to log on on the catalogue. The catalogue entry “Max Records” allows to limit the number of matching products returned on each result page. The “Expand tabs” arrow on the top-right of the map allows to adapt the map contents and to manage the selected AOIs.

Massive order

If you are interested in getting the whole archive of EM10 via FTP, please make a request through the Helpdesk, indicating:

  • The geographical window: any of the ten predefined windows (www in the concerned table of “Technical description”).

  • The start and end dates of this subscription

  • Contact details:

    • Username

    • Company and address

    • Contact person

    • E-mail address

    • Purpose of using the data

You will be contacted in a few days. When the subscription is accepted, you will be informed by e-mail whenever new products are available on the FTP-server. (top)