SPOT High Resolution Visible and Near Infrared Image Data and Descriptions of LEVEL 2 and S Products from SPOT IMAGE of Toulouse, France

SPOT has two similar instrument on each bus; SPOT 1 launched in
February 1986, SPOT 2 launched in January 1990 and SPOT3 launched in
September 1994 . The High Resolution Visible Imaging System (HRV) on
board on SPOT consists of push broom instruments.

Each instrument has two spectral modes: 1) panchromatic (P), black and
white from 510-730.nanometers; and 2) multispectral (XS), color with
three spectral bands, from 500-590, 610-680 and 790-890
nanometers. The resolution in P mode is 10 meters while the resolution
in XS mode is 20 meters. The P mode number of pixels per line can vary
from 6000 to 10400 and the number of lines per scene from 6000 to 9800
depending on the surveying method (vertical or oblique) and the
prepocessing level (note, for LEVEL 2, scenes are aligned with true
North and resampled accordingly). The volume of data per scene can
therefore vary from 36 to 100 Mbytes. In XS mode, the number of pixels
per lines varies from 3000 to 5200 and the number of lines per scenes
from 300 to 4900 for each of the three spectral bands. The total
volume of data can therefore vary from 27 to 76.5 Mbytes. Each SPOT
scene acquired in P or XS mode covers an area of 60 X 60 km2 in
near-vertical viewing, and up to 60 x 80 km2 in oblique viewing at
viewing angles up to 27 degrees either side of the vertical. The
oblique viewing offers two possibilities: 1) to increase the
revisiting capability up to five times per 26 days; and 2) to acquire
stereoscopic pairs by imaging the same portion of landscape from two
different revolutions. SPOT circles the Earth 14 and 5/26 times each
day at an altitude of 830 km in a sun-synchronous (polar) orbit.

All scenes are nominally centered in latitude on the J rows of the
SPOT Grid Reference System (GRS). By opting for a SAT
(Shift-Along-Track) scene, rather than a standard scene, the customer
can, however, obtain scenes that are, as the name implies, shifted
along the satellite track. The center of a SAT scene lies on a line
parallel to the satellite track at a latitude corresponding to a
whole-number multiple of one-tenth of the standard scene length
(i.e. 6 km) relative to the GRS (K,J) position. These positions are
numbered from 0 to 9, from North to South, between a given (K,J)and
the next (K,J) to the South.

The LEVEL 2 differs from the LEVEL 1B preprocessing as follows: 1)
geometry and location accuracy enhanced by internal data; 2) pixel
resampling along X and Y axes according to a map projection system
selected by the client (LEVEL 2 scenes are aligned with North); 3)
location of a point by its rectangular coordinates using CARTOGRAPHIC
marks (these replace the geographic marks used in LEVEL 1B); and 4)
possibility of joining two LEVEL 2 scenes, pixel to pixel, by
processing them as a single strip. A series of consecutive scenes from
the same satellite pass can be modeled just once (maximum 10
scenes). RADIOMETRIC corrections are the same as for LEVEL
1B. GEOMETRIC corrections are bidirectional and you can use an image
deformation law established from a viewing model. The model is
computed from the SPOT system internal data and can be applied to a
single scene or a strip of several scenes.

The LEVEL 2A product is a precision preprocessing level. In addition
to LEVEL 1B radiometric corrections, bidirectional geometric
corrections are performed in order to plot the scene according to a
given map projection (conformal Lambert, transverse Mercator, oblique
equatorial, polar stereographic, polyconic). Corrections are made
using satellite attitude data and viewing geometry. GCPs are not used
which means that topographical maps are not required. Although the
internal accuracy of the image is enhanced (0.5 10-3) absolute
location accuracy remains the same as for LEVEL 1B (about 800 m).
Location accuracy can, however, be optimized by a simple XY adjustment
based on known points.

The LEVEL 2B product is a precision preprocessing level in which
bidirectional geometric corrections are performed using
GCPs. Topographic maps of sufficient accuracy will therefore be
required to extract GCPs. The image is rectified according to the
projection of the map used. The location accuracy is 30 m (rms error)
for vertical viewing where the difference in elevation between the
highest and lowest points does not exceed 1250 m. For oblique viewing
at maximum angle (27 degrees), this accuracy can be met provided that
maximum elevation minus minimum elevation does not exceed 170 m. Note
that this level, like LEVEL 1B, does not consider distortion due to
terrain relief. The less pronouced the relief and the closer the
viewing angle to the vertical, the more accurate the product.

LEVEL 2 scenes are aligned with true North and images are resampled
accordingly. LEVEL 2 scenes can be processed in the form of a strip
along the satellite track comprising several perfectly matched scenes
(pixel to pixel). As an option and for the same price, LEVEL 2A and
2B scenes can be supplied with a 2 km x 2 km geographic grid in the
form of continuous lines or crosses at 2 km intervals.

LEVEL S scenes are rectified using control points for registration
with a reference scene. Registration accuracy is 0.5 pixels when both
scenes are recorded at the same viewing angle. This product is
intended primarily for multidate studies. LEVEL S1 scenes are
rectified with respect to LEVEL 1B reference scenes. LEVEL S2 scenes
are rectified with respect to LEVEL 2 reference scenes. The GEOSPOT
PRODUCTS including SPOT view basic, SPOT view plus and SPOT MAP are
delivered according to the GIS_Geospot digital format (for additional
technical information about the GIS_Geospot format, please contact :
tel [phone] or fax [phone], these new products include
orthoimage, layout in raster format,digital elevation model in raw
raster format, vector files in Arc/Info Export 6.0 and digital quick
look in TIFF format.

One way to access to the SPOT IMAGE Catalogue (DALI) is via the
Prototype International Directory using the 'LINK' facility.