
# Map Projection Info Map Projection Type = Simple Cylindrical for Ceres Radio Science

## Description

In the simple cylindrical projection (Snyder, 1987), parallels of latitude and
meridians of longitude are straight lines that intersect one another at right
angles.

Images in this projection are centered on the equator. Map resolution is constant
throughout the image, thus increasing distortion away from the poles.

The transformation from latitude and longitude (LAT, LON) in degrees to LINE and
SAMPLE is given by the following equations.

SCALE = MAP_RESOLUTION

SAMPLE = SAMPLE_PROJECTION_OFFSET+SCALE*(LON-CENTER_LONGITUDE)+1

LINE = LINE_PROJECTION_OFFSET-SCALE*(LAT-CENTER_LATITUDE)+1

In the above definitions, integral values of LINE and SAMPLE correspond to the
center of a pixel, and the top left image pixel has LINE=1 and SAMPLE=1. LAT and
LON are the latitude and longitude of a given spot on the surface in degrees.

### LINE_PROJECTION_OFFSET
is the image line number minus one on which the map projection origin occurs.
The map projection origin is the intersection of the equator and the projection
longitude CENTER_LONGITUDE.

### SAMPLE_PROJECTION_OFFSET
is the sample number minus one on which the map projection origin occurs.

### CENTER_LATITUDE
is the value of the projection latitude in degrees, which is typically the latitude
that passes through the center of the projection.

### CENTER_LONGITUDE
is the value of the projection longitude in degrees, which is typically the
longitude that passes through the center of the projection.

### MAP_RESOLUTION
is the number of pixels per degree at the projection origin.

The following four keywords are defined in the PDS Data Dictionary, but their
usage in Gravity Recovery and Interior Laboratory (GRAIL) image products may be
non-standard.  In more conventional usage these keywords define the limits of
the area covered by the image or map.  In Radio Science products, their values
are the extremes at the pixel centers.

### EASTERNMOST_LONGITUDE
is the longitude of the easternmost pixel (center coordinates) in the map or image.

### WESTERNMOST_LONGITUDE
is the longitude of the westernmost pixel (center coordinates) in the map or image;
numerically, always less than EASTERNMOST_LONGITUDE.

### MINIMUM_LATITUDE
is the latitude of the southernmost pixel (center coordinates) in the map or image.

### MAXIMUM_LATITUDE
is the latitude of the northernmost pixel (center coordinates) in the map or image;
numerically always greater than MINIMUM_LATITUDE.

## References

PDS3 REFERENCE_KEY_ID = DAVIESETAL1995
The following reference entry might correspond to the PDS3 REFERENCE_KEY_ID listed 
above
Davies, M. E., Colvin, T. R., Oberst, J., Zeitler, W., Schuster, 
    P., Neukum, G., & McEwen, A. (1996). The control networks of the 
    Galilean satellites and implications for global shape. Icarus, 
    120(1), 20–32.

Snyder, John P., Map Projections -- A Working Manual,
    U. S. Geological Survey Professional Paper 1395, 383p., 1987.
