HANTS

For many remote sensing applications which focus on land surface properties, cloud cover has disastrous effect on the results.
The NLR HANTS (Harmonic ANalysis of Time Series) algorithm accomplishes two tasks:

  1. Screening and removal of cloud contaminated observations;
  2. Temporal interpolation of the remaining observations to reconstruct gapless images at a prescribed time.

The basic idea is to calculate a Fourier series to model a time series of pixelwise observations, while at the same time identifying outliers relative to the model of time series. The algorithm eliminates such outliers and replaces them with the value given by the Fourier series.

 

 
In this example data from three bands of the SPOT-VGT sensor of the year 2002 have been used, separate images were combined into a 10-day composite. Bands B2 (red), B3 (near infrared) and MIR (mid-infrared), were assigned to the blue, red and green colourguns, respectively. This band combination results in purple snow, red vegetation, green soils, white sands (Sahara) and black water.

HANTS was set to remove all high outliers, which are mostly related to clouds. The SPOT-VGT SM data layer (status map) was used to further remove any unreliable data.

The main differences between the images are due to the varying snow cover and the growth of vegetation (agricultural crops).

Have also a look at another HANTS example: Search for trend in changes - Southeast Asia

 

Local installation 

Install HANTS (1.98MB).


Change log

25 Sep 2009:

  • The Analysis control parameter List of frequencies has been removed. Only the Number of frequencies (n) has to be entered and then the first n harmonics will be calculated.
  • An error in the FFT processing has been repaired.

10 Sep 2009:

New installer program which works also under Windows 64-bits.

ATTENTION: From 4 till 10 September 2009 by accident an old version of HANTS has been placed on the site. If you downloaded HANTS in that period, please uninstall it and download and install the latest version.


Errors

  • Latest release:

None known.

  • Before 25 Sep 2009:
    • The following error message may be received when installing HANTS:

      The system DLL user32.dll was relocated in memory. The application will not run properly. The relocation occurred because the DLL C:\Windows\System32\Hhctrl.ocx occupied an address range reserved for Windows system DLLs. The vendor supplying the DLL should be contacted for a new DLL.

      This problem is explained on http://support.microsoft.com/kb/935448 where also a solution to this problem is given.
    • The Help button may not work. Start the Hants help manually: go to the HANTS install directory and start Hants.chm (Compiled HTML Help).
    • When runing HANTS, the created bat file starts with setting the path variable to include the path to the HANTS software. That might go wrong. Instead of adding the HANTS software directory, the user home directory or working directory is added to the path.

      Adjust set path manually, using the Edit button, to include the actual HANTS path. For example:
      SET PATH=%PATH%;C:\Progam Files\Hants 
    • The FFT part of HANTS seems not to be working correctly.

Climate Indicators from Time Series of NDVI images (CITISEN)

HANTS has been used for the project CITISEN. In this project NOAA NDVI data has been processed with HANTS and analysed with additional software to discover trends in vegetation growth patterns. Have a look at the final report.