6 MOD13A2 (16-day 1-km) VI

This product is generated using the output of the daily, MODIS surface reflectance aggregation algorithm (MODAGG). The output file contains 12 SDS (Table 8).

Examples of the MOD13A2 MODIS products for the Southwest USA are included at the end of this document (Fig. 10).

6.1 Algorithm Description

The MOD13A2 VI algorithm, as in MOD13A1, operates on a per-pixel basis and requires multiple observations (days) to generate a composited VI. Due to sensor orbit overlap, multiple observations may exist for one day, hence the aggregation algorithm (MODAGG) precedes the VI algorithm. MODAGG will ingest all the daily projected (tile) surface reflectance data and generate a maximum of four observations based on quality, cloud cover, and viewing geometry. In theory, this can result in a maximum of 64 observations over a 16-day cycle, however, due to the presence of clouds and the actual sensor spatial coverage, this number will range between 64 and 0 with decreasing observations from higher to equatorial latitudes.

Please see Subsection 5.1 “MOD13Q1/A1 Algorithm Description” for full details of the MODIS VI compositing method followed also for the MOD13A2 product.

6.2 Scientific Data Sets

The 1-km VI product has the following 12 Science Data Sets (Table 8):

Table 8: Product MOD13A2: 16-day 1-km VI.

Science Data Set

Units

Data type

Valid Range

Scale factor

1km 16 days NDVI

NDVI

int16

-2000, 10000

0.0001

1km 16 days EVI

EVI

int16

-2000, 10000

0.0001

1km 16 days VI Quality detailed QA

Bits

uint16

0, 65534

NA

1km 16 days red reflectance (Band 1)

Reflectance

int16

0, 10000

0.0001

1km 16 days NIR reflectance (Band 2)

Reflectance

int16

0, 10000

0.0001

1km 16 days blue reflectance (Band 3)

Reflectance

int16

0, 10000

0.0001

1km 16 days MIR reflectance (Band 7)

Reflectance

int16

0, 10000

0.0001

1km 16 days view zenith angle

Degree

int16

-9000, 9000

0.01

1km 16 days sun zenith angle

Degree

int16

-9000, 9000

0.01

1km 16 days relative azimuth angle

Degree

int16

-3600, 3600

0.1

1km 16 days composite day of the year

Day of year

int16

1, 366

NA

1km 16 days pixel reliability

Rank

int8

0, 4

NA


6.3 Product Specific Metadata

A listing of the metadata fields used for QA evaluations of the MOD13A2 VI product is included in Table 9.

Table 9: Metadata fields for QA evaluation of MOD13A2.

I. Inventory Metadata fields for all VI products (searchable)

QAPERCENTINTERPOLATEDDATA

QAPERCENTMISSINGDATA

QAPERCENTOUTOFBOUNDSDATA

QAPERCENTCLOUDCOVER

QAPERCENTGOODQUALITY

QAPERCENTOTHERQUALITY

QAPERCENTNOTPRODUCEDCLOUD

QAPERCENTNOTPRODUCEDOTHER

II. Product specific metadata (searchable)

Product

Specific Metadata variable name (Best Quality)

MOD13A2

NDVI1KM16DAYQCLASSPERCENTAGE

MOD13A2

EVI1KM16DAYQCLASSPERCENTAGE

III. Archived Metadata (not searchable)

Product

Metadata variable name (Array of QA usefulness histogram)

MOD13A2

QAPERCENTPOORQ1KM16DAYNDVI

MOD13A2

QAPERCENTPOORQ1KM16DAYEVI


6.4 Global and Local Metadata Attributes

As in all MODIS products, the global metadata is written to the ouput file during the generation process and could be used for searching the archive about the product. A listing of relevant metadata is provided.

6.5 Quality Assurance

In principal, the QA metadata objects and QA SDS’s of the MOD13A2 product are the same as those of the MOD13A1 product. In this section, we only describe the differences of the MOD13A2 product QA from the MOD13A1 product QA.

QA Metadata

As the spatial resolution of the MOD13A2 product differs from that of the MOD13A1 product, the 4 VI PSA object names differ correspondingly. Table 10 lists the correspondences between these object names.

Table 10: Correspondence of the VI PSA QA Metadata Object Names between MOD13A2 and MOD13A1 products.

Object Name in the MOD13A2

Object Name in the MOD13A1

NDVI1KM16DAYQCLASSPERCENTAGE

NDVI500M16DAYQCLASSPERCENTAGE

EVI1KM16DAYQCLASSPERCENTAGE

EVI500M16DAYQCLASSPERCENTAGE

QAPERCENTPOORQ1KM16DAYNDVI

QAPERCENTPOORQ500M16DAYNDVI

QAPERCENTPOORQ1KM16DAYEVI

QAPERCENTPOORQ500M16DAYEVI


QA Science Data Sets

VI usefulness index computation is performed according the criteria showed in Table 11.

VI Pixel reliability is described in ’Pixel reliability summary QA’ SDS (Table 4), and detailed QA bit fields are described in ’VI Quality detailed QA’ SDS (Table 5).

Table 11: VI Usefulness Index Scaling Method for the MOD13A2 Product.

Parameter Name

Condition

Score

Aerosol Quantity (bits 6-7)

Low or average aerosols

0

 

Climatology aerosols

2

 

High aerosols

3

Atmosphere BRDF Correction

Performed

0

(bit 9)

Not performed

2

Mixed Clouds (bit 10)

No mixed clouds

0

 

Possible mixed clouds

3

Shadows (bit 15)

No shadows

0

 

Possible shadows

2

View zenith angle ($q_ v$)

If $q_ v >$ 40$^\circ $

1

Sun zenith angle ($q_ s$)

If $q_ s >$ 60$^\circ $

1