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MODIS Vegetation Indices QA Science Data Sets
The QA information below is excerpted from the MODIS Vegetation Indices Users Guide available on-line at http://tbrs.arizona.edu/project/MODIS/UserGuide_doc.php. All information can be assumed applicable to both Terra (MOD13) and Aqua (MYD13) VI products.
The first section describing MOD13A1 contains comprehensive information applicable to all MODIS VI products, including the 250 m MOD13Q1. QA specific to other VI products are targeted here: MOD13A2, MOD13A3.
MOD13A1 500m Vegetation Indices QA Science Data Sets
The MOD13A1 product includes 2 QA science data sets (SDSs), which are also called the per-pixel QA: the 250/500m 16 days NDVI Quality SDS for the NDVI and the 250/500m 16 days EVI Quality SDS for the EVI. Their dimensions (the number of samples and lines) are the same as those of the VI SDSs and each pixel consists of 10 bit-fields (a total of 16 bits). Their contents are explained in Figure 6 below. Table 4 lists the names of the bit-fields (parameter names), the number of bits assigned for the bit-field, the bit combinations, and their corresponding descriptions. As seen in Table 4, the per-pixel QA bits are designed to document the conditions under which each pixel was acquired and processed. The NDVI and EVI per-pixel QA bits have an identical format.
Table 4. Descriptions of the VI Quality Assessment Science Data Sets (QA SDS)
| Bit No. | Parameter Name | Bit Comb. | Description |
|---|---|---|---|
| 0-1 | VI Quality (MODLAND Mandatory QA Bits) | 00 | VI produced with good quality |
| 01 | VI produced but with unreliable quality and thus examination of other QA bits recommended | ||
| 10 | VI produced but contaminated with clouds | ||
| 11 | VI not produced due to bad quality | ||
| 2-5 | VI Usefulness Index | 0000 | Perfect quality (equal to VI quality = 00: VI produced with good quality) |
| 0001 | High quality | ||
| 0010 | Good quality | ||
| 0011 | Acceptable quality | ||
| 0100 | Fair quality | ||
| 0101 | Intermediate quality | ||
| 0110 | Below intermediate quality | ||
| 0111 | Average quality | ||
| 1000 | Below average quality | ||
| 1001 | Questionable quality | ||
| 1010 | Above marginal quality | ||
| 1011 | Marginal quality | ||
| 1100 | Low quality | ||
| 1101 | No atmospheric correction performed | ||
| 1110 | Quality too low to be useful | ||
| 1111 | Not useful for other reasons (equal to VI quality = 11: VI not produced due to bad quality) | ||
| 6-7 | Aerosol Quantity | 00 | Climatology used for atmospheric correction |
| 01 | Low | ||
| 10 | Intermediate | ||
| 11 | High | ||
| 8 | Atmosphere Adjacency Correction | 0 | (No) No adjacency correction performed |
| 1 | (Yes) Adjacency correction performed | ||
| 9 | Atmosphere BRDF Correction | 0 | (No) No atmosphere-surface BRDF coupled correction performed |
| 1 | (Yes) Atmosphere-surface BRDF coupled correction performed | ||
| 10 | Mixed Clouds | 0 | (No) No mixed clouds |
| 1 | (Yes) Possible existence of mixed clouds | ||
| 11-12 | Land/Water Mask | 00 | Ocean/inland water ? Shallow ocean ? Moderate and continental ocean ? Deep ocean ? Deep inland water |
| 01 | Coastal region ? Ocean coastlines and lake shorelines ? Shallow inland water |
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| 10 | Wetland ? Ephemeral water |
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| 11 | Land | ||
| 13 | Snow/Ice | 0 | (No) No snow/ice |
| 1 | (Yes) Possible existence of snow/ice | ||
| 14 | Shadow | 0 | (No) No shadow |
| 1 | (Yes) Possible existence of shadow | ||
| 15 | Compositing Method | 0 | BRDF composite method used for compositing |
| 1 | Constraint view angle MVC (CV-MVC) method used for compositing |
The first two bits are used for the MODIS Land Science Team (MODLAND) mandatory per-pixel QA bits that summarize the VI quality of the corresponding pixel locations. Percentages of sums of its four possible values (bit combinations) over a tile will give the MODLAND mandatory QA metadata object values (Table 5).
Table 5. Relationship between the MODLAND Mandatory per-pixel QA Bits and QA Metadata Objects
| VI Quality Bit Combination | Corresponding QA Metadata Object |
|---|---|
| 00: VI produced, good quality | QAPercentGoodQuality |
| 01: VI produced, unreliable quality | QAPercentOtherQuality |
| 10: VI produced, contaminated with cloud | QAPercentNotProducedCloud |
| 11: VI not produced due to bad quality other than cloud | QAPercentNotProducedOther |
The 2nd QA bit-field is called the VI usefulness index. The usefulness index is a higher resolution quality indicator than the MODLAND mandatory QA bits (16 levels) and its value for a pixel is determined from several conditions, including 1) aerosol quantity, 2) atmospheric correction conditions, 3) cloud cover, 4) shadow, and 5) sun-target-viewing geometry (Table 6). As shown in Table 6, there is a specific score that is assigned to each condition and a sum of all the scores gives a usefulness index value for the pixel. An index value of 0000 is corresponding to the highest quality, while the lowest quality is equal to a value of 1100 (i.e., 13 levels). The three largest values are reserved for three specific conditions which are shown in Table 4. There are relationships between the VI usefulness index and the MODLAND mandatory QA bits. Pixels with the index value of 0000 and 1111 always have the MODLAND QA bit values of 00 and 11, respectively.
Table 6. VI Usefulness Index Scaling Method for the MOD13A1 Product
| Parameter Name | Condition | Score |
|---|---|---|
Aerosol Quantity |
If aerosol climatology was used for atmospheric correction (00) | 2 |
| If aerosol quantity was high (11) | 3 | |
| Atmosphere Adjacency Correction | If no adjacency correction was performed (0) | 1 |
| Atmosphere BRDF Correction | If no atmosphere-surface BRDF coupled correction was performed (0) | 2 |
| Mixed Clouds | If there possibly existed mixed clouds(1) | 3 |
| Shadow | If there possibly existed shadow (1)/td> | 2 |
| View zenith angle (qv) | If qv<> 40o | 1 |
| Sun zenith angle (q<s) | If qs> 60o | 1 |
The next three QA bit-fields document atmospheric correction scenarios of each pixel. The bits 6-7 are used to indicate aerosol quantity, and the bits 8 and 9 indicate whether an adjacency correction and atmosphere-surface BRDF coupled correction, respectively, are applied or not.
The bit 10 indicates a possible existence of mixed clouds. As the original spatial resolutions of the red and NIR bands are 250 m, these two bands were spatially aggregated to a 500 m resolution before the computations of VIs. The mixed cloud QA bit is flagged if any of the 250 m resolution pixels that were used for the aggregations were contaminated with cloud.
The bits 10-11 are used for the land/water mask. The input land/water mask to the MOD13A1 VI product has 7 land/water classes. These 7 classes are lumped to 4 classes upon the generations of the VI products, including the ocean/inland water, coastal region, wetland, and land classes. The VIs are not computed for pixels only over the ocean/inland water class.
The bits 13 and 14 indicate possible existences of snow/ice and shadow, respectively.
The last QA bit field indicates a compositing method used for each pixel as the MODIS VI compositing algorithm utilizes two compositing methods (the BRDF compositing and CV-MVC methods) interchangeably from pixel to pixel. Currently, the BRDF compositing algorithm has been deactivated and, thus, this QA bit is always set to 1.
Figure 6: MODIS VI Quality Assurance SDS bit map.
MOD13A2 1km 16-Day Vegetation Indices QA Science Data Sets
As the atmospheric adjacency effects are no longer of importance for a 1 km distance, the QA bit field for the adjacency atmospheric correction (the bit 8) is used instead to indicate a possible existence of adjacency cloud in the MOD13A2 QA SDSs (Table 10). Accordingly, the condition of the atmospheric adjacency correction is not considered in the VI usefulness index computation (Table 11).
Table 10. Description of the 8th QA Bit for the MOD13A2 Product
| Bit No. | Parameter Name | Bit Comb. | Description |
|---|---|---|---|
| 8 | Adjacency Cloud | 0 | (No) No adjacency cloud detected |
| 1 | (Yes) Possible existence of adjacencycloud |
Table 11. VI Usefulness Index Scaling Method for the MOD13A2 Product
| Parameter Name | Condition | Score |
|---|---|---|
| Aerosol Quantity | If aerosol climatology was used for atmospheric correction (00) | 2 |
| If aerosol quantity was high (11) | 3 | |
| Atmosphere BRDF Correction | If no atmosphere-surface BRDF coupled correction was performed (0) | 2 |
| Mixed Clouds | If there possibly existed mixed clouds(1) | 3 |
| Shadow | If there possibly existed shadow (1) | 2 |
| View zenith angle (qv) | If qv> 40o | 1 |
| Sun zenith angle (qs) | If qs> 60o | 1 |
MOD13A3 1km Monthly Vegetation Indices QA Science Data Sets
As for the MOD13A2 product, the QA bit field for the adjacency atmospheric correction (the 8th bit) is used to indicate a possible existence of adjacency cloud in the MOD13A3 QA SDSs (Table 15).
The other difference is that the 14th bit is used to indicate whether mixed compositing methods were used, as the MOD13A3 product is derived as a weighting average of 2 ? 3 MOD13A2 products of which 16 days periods overlap with a month (Table 15). Currently, the BRDF compositing algorithm has been deactivated and, thus, the 14th and 15th QA bit are always set to 0 and 1, respectively.
| Bit No. | Parameter Name | Bit Comb. | Description |
|---|---|---|---|
8 |
Adjacency Cloud |
0 | (No) No adjacency cloud detected |
| 1 | (Yes) Possible existence of adjacency cloud | ||
| 14 | Mixed Compositing Methods | 0 | (No) Mixed compositing method not used |
| 1 | (Yes) Mixed compositing method used |
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