CADRG (Compressed ARC Digitized Raster Graphics) Reader/Writer

In CADRG, the projection system is ARC. The ARC system, as described in MIL-A-89007, divides the surface of the earth ellipsoid into 18 latitudinal bands called zones. Zones 1-9 cover the Northern hemisphere and zones 10-18 (A through J, exclusive of I in CADRG) cover the Southern hemisphere. One zone in each hemisphere covers the polar areas. Each non-polar zone covers a part of the ellipsoid between two latitude limits and completely encircles the Earth. The datum for CADRG shall be WGS-84, as defined by DMA TM 8358.1. The WGS-84 coordinates for longitude and latitude in CADRG are signed values in the range -180 degrees <= longitude <= +180 degrees and -90 degrees <= latitude <= +90 degrees.

The CADRG dataset is composed of rectangular grids of frames of pixels for each zone. CADRG can be distributed in rectangular or non-rectangular areas, and with contiguous or non-contiguous coverage. Each frame is represented by a discrete file. The CADRG library is seamless; that is, the edges of contiguous source maps are indistinguishable, except by color variations that are due to the differences between the colors or patterns in original source graphics.

CADRG data files are arranged in a hierarchical folder/subfolder structure. All names and labels, and the format and structure of folders shall adhere to the conventions specified in MIL-STD-2411. The root folder contains a table of contents file (A.TOC), one or more folders of frame files, a legend folder, and one or more overview images. Currently, the legend folder and the overview images are not used in any cases.

The ARC system divides the surfaces of the earth ellipsoid into 18 latitudinal bands called zones. The CADRG database is composed of rectangular grids of frames of pixels for each zone. Each frame is represented by a discrete file, and the data files are arranged in a hierarchical folder/subfolder structure. The location and name of each file are recorded in a CADRG Table of Contents (TOC) file in the root folder.

The TOC file provides an overview of the data contents of the distribution media. It stores path names to each of the frame files. CADRG producers will choose the number of frame file folders in a given dataset and convention for assigning frame files to folders. Each frame file folder on a given dataset shall be uniquely named in a manner to be determined by an authorized producer. The producers may also assign nested frame file folders as needed to organize the frame files, using a variable hierarchy. The CADRG Writer in FME creates a unique frame file folder for each data series code, as described in the attribute section below. The frame files contains the tiled image and support data for the geographic frames on a CADRG dataset. The frame file naming convention shall be in accordance with MIL-STD-2411.

For each map or chart scale, which can be adjusted with the data series attribute, a constant latitudinal (row) and longitudinal (column) pixel interval shall exist in each zone.

Each frame shall comprise a rectangular array of 1536 by 1536 pixels (2,359,296 pixels). Each frame shall be tiled into grid of 6 by 6 subframes (36 subframes). Each subframe shall comprise a rectangular array of 256 by 256 output pixels (65,536 pixels). All frames and subframes within a zone shall abut in a mutually exclusive manner without any pixel overlap or pixel redundancy. The northern and southern boundaries of a zone generally will not fall exactly on the northern and southern boundaries of a frame or subframe. There shall be frame overlap between the zones.

The numbering convention for entities that are internal to the frame files shall conform to MIL-STD-2411. All index numbers shall start from 0. Rows and columns of subframes in a frame, pixels, and indices in frame file subentities shall be counted from 0. The origin for the subframe and pixel numbering within frames and subframes shall be from the upper left corner. Subframes and pixels shall be counted in row-major order from the origin. In addition, CADRG frames may be considered to form conceptual rows and columns within zones. This concept is used to define the naming convention of frames for various scales by using the scale and zone specific frame number. The rows and columns are numbered from 0. The origin for counting non-polar frame rows and columns in both the northern and southern hemispheres is the Southern-most latitude of the zone, and 180 degree west longitude, with columns counted in an easterly direction from the origin. The origin for counting polar frames is the lower-left corner of the polar zone, with rows and columns numbered from that origin.

For the non-polar zones, some visual distortion is present due to a stretch at the poleward latitude and shrink at the equatorward latitude in the East-West direction. There is no distortion along a selected parallel at the mid-latitude of each zone. Since an entire frame file of overlap is included between zones, there can be noticeable visual distortion in the overlap area for the very small scale maps (e.g., GNC, JNC). Distortion in the polar zones is less than 10% for most scales.

The CADRG format itself allows updates/replacements of frame files. However, the current versions of CADRG Reader and Writer in FME do not support CADRG updates/replacements.

During compression, the available colors in the source data shall be quantized from a palette of 16.7 million possible colors to a maximum 216 colors in the CADRG frame files. The quantized colors shall be defined in a LUT with 216 entries. A 217th entry is reserved for transparent pixels.

Additional color tables are provided wherein each of the 216 colors in the primary color table is mapped to a color in the reduced color table through use of a color conversion table. Standard CADRG data sets will include 216, 32, and 16 entry color tables. Each color entry contains 4 bytes. The first byte contains the red intensity level, the second byte contains the green intensity level, the third byte contains the blue intensity level, and the fourth byte contains a monochrome (grayscale) intensity level, which is an arithmetic combination of red, green and blue intensity levels. The equation used to calculate the monochrome intensity level is: 0.299 (Red) + 0.587 (Green) + 0.114 (Blue). The 216 or 217 entries in the color table contain RGB and monochrome values in the range 0 - 255. The entries in the color table represent RGBM values for pixels in the digitized map that represent RGBM values for pixels in the digitized map that represent digitized map data. Entry 216 is reserved for “transparent” pixels, for cases where the data is missing or not available at the given geographic location. The RGBM values in the CADRG color table for the 217th entry (index 216, for transparent pixels) are 0, 0, 0, 0.

Spatial compression shall be performed using a Vector Quantization (VQ) algorithm that employs a 4 x 4 compression kernel size with 4096 codebook entries.