VanderSat Data Flags

Introduction

VanderSat strives to provide the best and most reliable data. Therefore our data comes with a set of flags that supply additional information on our data. These flags indicate possible data issues as well as information on data usage and quality. This document lists the flags that come with our Soil Moisture, Vegetation Optical Depth and Land Surface Temperature products and how to interpret them.

How to retrieve the data flags

VanderSat product have an associated data flags file. The flags can be retrieved via de VanderSat API (VanderSat API User Guide). The API name for the flag file associated with a products can be derived as follows by prefixing the product name with QF-. So if the product name is SM-LN_V001_100 the associated flag file can be retrieved using the product name QF-SM-LN_V001_100.

There is an exception to this rule: the flag file for VOD and SM are shared per band. This means that there are different flag files for each band (C, L, X, the second part of the product name) but there is only a flag file created for the soil moisture product. For VOD and LST the soil moisture flag file should be used. For all other products the flag file is unique per product.

Example flag file product mappings
Product Flag file
SM-LN_V001_100 QF-SM-LN_V001_100
SM-XN_V001_100 QF-SM-XN_V001_100
VOD-LN_V001_100 QF-SM-LN_V001_100
VOD-XN_V001_100 QF-SM-XN_V001_100
DRZSM-XN-T10_V001_100 QF-DRZSM-XN-T10_V001_100

For details on how to retrieve the actual data files see the API Guide (API-spatialmaps).

Available flags

Complex (steep) terrain

The complex terrain flag indicates areas in which the complex terrain (steep topography) may have influenced our algorithm. In many cases the data is still fine but care must be taken when using the data.

Affected products:

  • Soil Moisture
  • VOD
  • Land Surface Temperature

Build-up area

The build-up area flag indicates paved/build-up areas. Although this does not hamper our algorithm as such, the obtained values for soil moisture and VOD cannot be interpreted the same as for natural areas. Land Surface Temperature is not affected.

Affected products:

  • Soil Moisture
  • VOD

Possibly influenced by water

This flag indicates area that are either in close proximity to open water (approx 400m) or may be inundated part of the time and missed by our open water algorithm. In most cases the results are fine but the data should be treated with care.

Affected products:

  • Soil Moisture
  • VOD
  • Land Surface Temperature

Open water

This flag indicates areas that are deemed to be open water (rivers, lakes seat etc). For areas marked as open water no Soil Moisture, VOD or Land Surface Temperature is given they are filtered out during the processing of the raw satellite data.

Affected products:

  • Soil Moisture
  • VOD
  • Land Surface Temperature

Snow

This flag indicates areas where snow is detected. For areas marked as snow no Soil Moisture, VOD or Land Surface Temperature is given they are filtered out during the processing of the raw satellite data.

Affected products:

  • Soil Moisture
  • VOD
  • Land Surface Temperature

Frozen Soil

This flag indicates areas where frozen soil is detected. For areas marked as frozen soil no Soil Moisture, VOD or Land Surface Temperature is given they are filtered out during the processing of the raw satellite data.

Affected products:

  • Soil Moisture
  • VOD
  • Land Surface Temperature

mpdi < 0.0001

This flag indicates if the mpdi of brightness temperatures at the given frequency is too low for the algorithm to converge. The formula is: \(\frac{BT_V-BT_H}{BT_V+BT_H} < 0.0001\)

Affected products:

  • Soil Moisture
  • VOD

Dense Vegetation

This flag indicates high vegetation cover. For those areas soil moisture retrieval is less reliable. Depending on the frequency, the flags value for vegetation corrected soil moisture and vod are applied: C/X band: medium VOD threshold = 0.45 / high VOD threshold = 0.6 L band: medium VOD threshold = 0.3 / high VOD threshold = 0.4

Affected products:

  • Soil Moisture
  • VOD

Severe Rain

This flag indicates areas high intensity rain is detected. For areas marked no Soil Moisture, VOD or Land Surface Temperature is given they are filtered out during the processing of the raw satellite data.

Affected products:

  • Soil Moisture
  • VOD
  • Land Surface Temperature

RFI

If a footprint is contaminated more than 25% with RFI it is removed from processing. Footprints will less contamination than 25% are retained but the data is marked as influenced by RFI.

Affected products:

  • Soil Moisture
  • VOD

No algorithm Convergence

If the soil moisture and VOD retrieval algorithm fails to converge no data is produced and the area is flagged.

Affected products:

  • Soil Moisture
  • VOD

Instrumental Flaws (e.g. along the edges of the swath)

If the brightness temprerature at 36.5GhZ V produces values either over 400K or under 0.9 * the water temperature, the data is considered as unrealistic and is removed.

No Coverage

If one or more brightness temperature frequency is missing (due to a combination of different satellite swaths), the no coverage flag is produced

Affected products:

  • Soil Moisture
  • VOD
  • Land Surface Temperature

DRZSM quality indicator

The Derived Root Zone Soil Moisture (DRZSM) quality indicator is an indicator for the amount of days for which a valid soil moisture measurement was used. Like the DRZSM product, it is calculated recursively and depends on the amount of days with a valid measurement and the chosen T value. This indicator ranges from 0-100 % and describes for each pixel the level of confidence that the DRZSM has had sufficient data of acceptable quality within the characteristic time scale T. The quality indicator is calculated according to:

\[q(t) = c + q(t-1) * e^{(-dt/T)}\]

with

\[c = (1-e^{-(1/T)})\]

if a valid measurement is used and otherwise

\[c = 0\]

Note that for satellite products that do not have daily coverage, the quality maximum will converge to a value lower than 100 %.

Affected products:

  • Derived Root Zone Soil Moisture

Flags data file description for LST, VOD and SM

During the processing of the data for each product an associated flag file including all the corresponding flags is also created. The relation between product name and flag name is described in How to retrieve the data flags.

This file is a float64 map with each number associated with one or multiple flags. The float values contain decimal numbers (1 and 0), the position of which indicates the flag. Below, the flags are split into three sections, each refering to individual processing steps though the sections are always added to each other in the data-flag product.

Conversion of VanderSat data flags
Section 1 Section 2 Section 3
0111 111111 111111
Content of the first section
Section 1 Flag
x 1e12  
100 Build-up Area
10 Complex Terrain
1 Water Influence
Content of the second section
Section 2 Flag
x 1e6  
100000 Frozen Soil
10000 No Coverage
1000 mpdi < 0.0001
100 Medium VOD
10 High VOD
1 Algorithm Convergence (SM=0 or SM=0.9)
Content of the third section
Section 3 Flag
100000 Potential RFI contamination at X band
10000 Potential RFI contamination at C1 band
1000 Open Water
100 Instrumental Flaw
10 Severe Rain
1 Snow

For example, a value in the final quality flag map of 100100100100001.0 would point to:

  • 100 from Section 1 Built-up Area
  • 100100 from Section 2 Frozen Soil and Instrumental flaw
  • 100001 from Section 3 Potential X band RFI and Snow

When only a small value is given, then only the latter sections apply. E.g. 10.0 would mean:

  • 000010 from Section 3 Severe Rain

Critical Flags list

Soil Moisture:

The following flags are considered “critical” and the soil moisture data associated is discarded, replaced by the no data value and saved in a second band:

  • Snow / ice
  • Frozen
  • OpenWater
  • RFI (removed in V 1.0)
  • Soil moisture = 0 or 1
  • mpdi < 0.0001
  • no coverage
VOD:

The following flags are considered “critical” and the VOD data associated is discarded, replaced by the no data value and saved in a second band:

  • Snow / ice
  • Frozen
  • OpenWater
  • RFI (removed in V 1.0)
  • Soil moisture = 0 or 1
  • mpdi < 0.0001
  • no coverage
LST:

The following flags are considered “critical” and the LST data associated is discarded, replaced by the no data value and saved in a second band:

  • Snow / ice
  • Frozen
  • OpenWater
  • no coverage

Flags data file description for DRZSM

The quality indicator is stored as a byte integer map without scaling and includes a value between 0 and 100 % indicating the estimated data quality.