FAQ
SPOT 2.5 m and 5 m colour
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Q1 - What are the production options for the merged products? The merged 1A and 2A products are available as full scenes, in a bicubic resampled GeoTIFF image format. There are many other options available (concerning size, radiometric enhancement, among others) available in the SPOTView Precision (level 2B) or Ortho (level 3) ranges. [+] More |
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Q2 - Which viewing configurations are acceptable for the merged 1A and 2A products? To guarantee the best possible quality, the merged product only uses Spot 5 1A panchromatic and colour (XS) scenes acquired simultaneously by the same instrument. |
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Q3 - How are the B&W and colour scenes merged for level 1A given that, even though simultaneously acquired, they are not registered? The B&W and colour scenes are acquired with a slight difference in viewing angle, with the B&W sensor pointed a little to the front and the colour sensor pointed a little towards the rear; the difference between the two is about 1.058 degrees which corresponds to about 15 km on the ground. This difference leads to local geometric offsets as well as a slight time difference (There is a delay of 2.25 seconds between the acquisition of a landscape line by the B&W sensor and the acquisition of the same line by the colour sensor). The merger process fixes the geometric deviations between the two level 1A scenes in the following way:
This level 1A processing is possible due to our thorough knowledge of image acquisition conditions on the Spot 5 satellite as well as the very low residual offsets of simultaneously acquired images. Processing with this configuration does not require the use of external data (ground control points or digital terrain model). Such external data are still necessary for all other cases (scenes which are not taken at the same time.). |
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Q4 - What is the geometry of the merged 1A and 2A product? The merged product keeps the geometry of its B&W source product. The values of the additional geometric data are exactly the same for both products. Physical modelling of the data thus gives identical results. |
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Q5 - What are the radiometric characteristics of the merged 1A and 2A product? The merged product is produced with the same radiometric range as the 10 m colour source product. The values of the additional radiometric data (PHYSICAL_GAIN, PHYSICAL_BIAS) are the same as those for the 10 m colour source product. All information which is no longer relevant has been removed (Pixel_Parameters, Dead_Detectors, Bad_Lines). However, since the objects were not observed using the same scale for both products, the spectral content may not be exactly the same (whence the slight differences in the histograms). |
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Q6 - Which spectral bands are supplied with a merged 1A and 2A product? The merged product supplies the spectral bands which are correlated with the panchromatic spectral band (0.48 mm-0.71 mm) : principally XS1 (B1), XS2 (B2) and XS3 (B3) for colour rendering. This is why the SWIR band is not provided. Another reason is to limit the volume of the product. SPOT Image also offers the 10 m colour source product with the SWIR band for a very reasonable cost. This band can be used once it has been adjusted to fit the merged product. |
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Q7 - The edges of the image of the merged 1A and 2A scenes are crenellated, is this normal? Yes. It is due to the correction grid used to adjust the colour source scene to fit the B&W scene. The grid is of course not homogenous for the entire scene. A given pixel is only assigned a value if it exists on both 1A source scenes after correction, otherwise the value assigned to the image is 0. |
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Q8 - I already have a B&W SPOT Scene and a colour SPOT Scene. How can I merge them easily? Merging at the 1A level is a very tricky operation which cannot be done by any of the software available on the market. We suggest that the two scenes first be orthorectified using external data (ground control points, tie points, digital terrain models , etc.) so that they can be perfectly registered point by point. The next step is the radiometry merger which can be done by most off-the-shelf software. |
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Q9 - What is the format of the merged 1A and 2A products? The merged 1A products (2.5 m colour and 5 m colour) come in the Dimap format. The meta-data for these products are described in the 'SPOT Scene profile'. The merged 2A products (2.5 m colour and 5 m colour) come in the Dimap format and their meta-data are described in a 'SPOTView profile'. The two profiles for the DIMAP format are described at the following address:www.spotimage.fr/dimap/spec/dimap.htm |