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11 May 1999 |
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Meteorological Satellite Analyst |
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SSgt Dave Mayer |
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-A grid is an array of points. |
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-Grids are used to record regularly spaced
samples over an area. |
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-When sampling over the surface of the earth a
grid is determined by a map projection, a sampling interval, an origin, and
the number of rows and columns. |
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-A map
projection is a one to one mapping of points on the curved surface to
points on the plane. |
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-Curved surface is the surface of the Earth and
the plane is what we call a "map. |
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-Coordinates for points on the surface of the
Earth are given in latitude and longitude. |
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-Coordinates in the map plane (x,y) are defined
in the usual manner with x horizontal and increasing to the right and y vertical and increasing upward. |
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-Map transformations are referred to as
projections. |
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-Most Maps are either conformal or equal-area. |
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A grid coordinate system is map plane with axes
parallel to the rows and columns of the grid and units equal to the
sampling interval. |
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A grid cell is the area between grid
coordinates. |
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It might be useful to think of the grid as a map
drawn on graph paper. |
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-Gridding is calculated by the receiver during
or after the processing of an image. |
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-Receiver uses the known (or expected) location
of the satellite |
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-Map projection options using TeraScan. |
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Albers conic
Mercator orthographic |
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equal area cylindrical Mollweide
rectangular |
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equidistant azimuthal perspective
stereographic |
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Lambert azimuthal polarstereo UTM |
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Lambert conic polyconic |
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First rule of gridding: Do not trust the computer-generated
overlay … it is entirely dependent on faulty ephemeris! |
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Look for geography and compare with map
overlay. Add or subtract as needed |
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-Final adjustment for center displacement |
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-Due to distance of the storm from the
subsatellite point or because of storm relative motion. |
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-when upper features are a considerable distance
from the subsatellite point, cyclone center appears farther away from the
satellite. |
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-Error is related to viewing angle of the
satellite and the height of the cloud features. |
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-For average tropical cyclones center position
should be adjusted |
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-0.05deg latitude for storms 15-25deg from the
SST. |
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-0.1deg latitude For storms 30deg latitude or
more from the SST. |
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-Corrections are automatically determined by
computer at some facilities. |
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Notes