1. UNCLASSIFIED GRIDDING & BETA CORRECTION 11 May 1999 Meteorological Satellite Analyst SSgt Dave Mayer

2. UNCLASSIFIED Gridding Cont. -A grid is an array of points. -Grids are used to record regularly spaced samples over an area. -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. -A map projection is a one to one mapping of points on the curved surface to points on the plane. -Curved surface is the surface of the Earth and the plane is what we call a "map.

3. UNCLASSIFIED Gridding Cont. -Coordinates for points on the surface of the Earth are given in latitude and longitude. -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. -Map transformations are referred to as projections. -Most Maps are either conformal or equal-area.

4. UNCLASSIFIED Gridding Cont. 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. A grid cell is the area between grid coordinates. It might be useful to think of the grid as a map drawn on graph paper.

5. UNCLASSIFIED Gridding Cont. -Gridding is calculated by the receiver during or after the processing of an image. -Receiver uses the known (or expected) location of the satellite -Map projection options using TeraScan. Albers conic Mercatororthographic equal area cylindrical Mollweide rectangular equidistant azimuthal perspective stereographic Lambert azimuthal polarstereo UTM Lambert conic polyconic

6. UNCLASSIFIED GRIDDING ERRORS First rule of gridding: Do not trust the computer-generated overlay … it is entirely dependent on faulty ephemeris! Look for geography and compare with map overlay. Add or subtract as needed

7. UNCLASSIFIED Beta Correction -Final adjustment for center displacement -Due to distance of the storm from the subsatellite point or because of storm relative motion. -when upper features are a considerable distance from the subsatellite point, cyclone center appears farther away from the satellite. -Error is related to viewing angle of the satellite and the height of the cloud features.

8. UNCLASSIFIED Beta Correction Cont. -For average tropical cyclones center position should be adjusted -0.05deg latitude for storms 15-25deg from the SST. -0.1deg latitude For storms 30deg latitude or more from the SST. -Corrections are automatically determined by computer at some facilities.