Frequently Asked Questions

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Question:
Why are there 4 image data files within the Level 1 ALI product?

Answer:
The Level 1 radiometric processing produces four strips of image data recorded by the instrument's Sensor Chip Assemblies (SCAs). The four files can be mosaicked together from left to right, starting with the .M4R file through the .M1R file. The mosaicked file will represent geographic region of 37 kilometers in width. Please note that since this is a radiometric product only, geometric errors may exist along the mosaic lines. The absence of georeferencing in the data is also the reason the files must be mosaicked by pixel-based mosaicking.

The information needed for mosaicking will be contained within each image file's HDF dataset attributes 1-8, called the "Number of Cross Track Pixels". This number reflects the number of columns (samples) of the strip of image. This is the offset in the X (column) for each strip from its adjacent strip. There is an overlap between the four SCAs of approximately 10 pixels for the multispectral bands and approximately 30 pixels for the panchromatic band, which is the reason for the odd-numbered offsets in the sample direction. The offset for the Y (row) direction will always be 1 or 0, depending on what the software expects. The HDF dataset attribute 1-9, called the "Number of Along Track Pixels," reflects the number of rows (lines) of image data. The four strips should be similar, but not exact, in size, both in rows and in columns.

Example
"Number of Cross Track Pixels" is 315
"Number of Along Track Pixels" is 2740
The following offsets are reflected for the 4 strips:

.M4R offsets y=1, x=1
.M3R offsets y=1, x=316
.M2R offsets y=1, x=631
.M1R offsets y=1, x=946

Total mosaic size would be 2740 rows by 1260 columns.

Also note that the Panchromatic (PAN) band will need approximately 3X the offset, due to its 10-meter resolution in comparison to the 30-meter resolution bands.