McIDAS User's Guide
Version 2019.1
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A subset of the MODIS ADDE image servers in McIDAS-X and -XRD version 2011.1 and later contain the ability to correct the "bowtie effect" in MODIS imagery. The correction is activated by the user by including the keyword NAVTYPE=RECT in their IMGCOPY, IMGDISP, IMGFILT or IMGOPER command. See below for more information about the bowtie effect and how to configure your MODIS ADDE image servers to apply the correction using the MRTSwath software package.
The scanning geometry associated with the MODIS instrument introduces oversampling artifacts at the far ends of each swath of data. This is commonly referred to as the "bowtie effect". These artifacts can be eliminated from the image by remapping the native satellite projection into a standard geographic projection.
A common application used to perform this reprojection is MRTSWath (MODIS Reprojection Tool - Swath, package courtesy of the U.S. Geological Survey). At the heart of MRTSwath is the swath2grid executable which can be run using a simple command line interface. It is this interface that forms the harness linking the McIDAS ADDE service to MRTSwath.
A subset of the MODIS ADDE image servers in McIDAS-X and -XRD version 2011.1 and later contain modifications that allow them to use the MRTSwath extension and its swath2grid executable. There are no special installation instructions for the servers; the mechanism used to run swath2grid only requires that the MRTSwath environment be installed within the scope defined by MCPATH. The package can be obtained from the Land Processing DAAC at the following address:
https://lpdaac.usgs.gov/lpdaac/tools/modis_reprojection_tool_swath
The MRTSwath package is self contained with support for numerous platforms / operating systems. At SSEC, we have installed and tested the package on OS X, Linux32 and Linux64 systems. During the installs we specified /usr/local/MRTSwath as the installation directory and accepted the path to the Java binary when prompted. We recommend following those steps so the installer sets the environment (e.g., adds /usr/local/MRTSwath/bin to the PATH and sets the MRTSWATH_HOME and MRTSWATH_DATA_DIR environment variables) to run the program successfully.
Not all MODIS ADDE image servers contain the MRTSwath extension; see the Limitations section below for more details. The servers that do allow MRTSwath reprojection do so by spawning a child process from the ADDE server. Execution of the ADDE server is suspended while the reprojection tool runs, producing a binary file containing the corrected image. Once the reprojection is completed, control is passed back to the ADDE server and processing continues. The returned image is a remapped version of the original data that resides on disk. Thus, IMGDISP of MODIS data using the reprojection tool will display an image but the digital data used to create the image only resides in memory, not on disk. This means that users who want to use the IMGPROBE command with the corrected image will need to use IMGCOPY with NAVTYPE=RECT to copy it to the local disk, then display it with IMGDISP and then probe it with IMGPROBE.
McIDAS users can request bowtie corrected image segments through ADDE client applications IMGCOPY, IMGDISP, IMGFILT and IMGOPER by adding keyword NAVTYPE=RECT to their command request. Specifying NAVTYPE=RECT signals the MODIS ADDE image servers to invoke swath2grid to perform a bowtie correction and reprojection of the data from satellite to Rectilinear projection.
A subset of the MODIS ADDE image servers have been extended to permit users to specifically request bowtie corrected image segments from the online datasets. MRTSwath's resolution restrictions limit the number of Level 2 products that can be reprojected. See below for the list of MODIS ADDE image servers that have been extended to request MRTSwath bowtie correction if the user specifies the NAVTYPE=RECT keyword in their IMGCOPY, IMGDISP, IMGFILT or IMGOPER command.
The image files used with MRTSwath must match the MODIS direct broadcast naming convention (e.g., a1.yyddd.hhmm.1000m.hdf and t1.yyddd.hhmm.500m.hdf) and the MODIS navigation files (e.g., a1.yyddd.hhmm.geo.hdf and t1.yyddd.hhmm.geo.hdf) must also be present in the same directory.
Although both McIDAS and MRTSwath provide numerous geographic projections, inconsistencies between the definitions within each system tend to introduce warping. To avoid these errors, the servers are limited to a Rectilinear transform. Because of the way the Rectilinear projection is defined, it eliminates the potential for computational errors when moving from MRTSwath to McIDAS.
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