Digital Chart of the World Tiling Scheme
Individual Tiles | Tile Naming | DCW Subdirectories | Finding Tiles of Interest
Individual Tiles
Physically, the DCW data is broken up into tiles, each representing a 5 degree latitude by 5 degree longitude portion of the earth. [see image] These tiles must each be accessed individually for analysis or plotting. For instance, North Carolina has the bad luck of being split between four 5 degree tiles [see image]. There are 2094 tiles in all. Antarctica is an exception to the tiling scheme. It is represented by four 90 degree by 35 degree tiles. This was done because there the data in this region is very sparse.
Tile Naming
The globe is also logically, not physically, divided into 15 degree rows and columns. These are used in naming the tiles.
Each tile is named using a four character scheme; the first two characters being letters, the second two numerals. The two letters represent 15 degree columns and rows, respectively. That is, lettering starts at 180 degrees west and 90 degrees south with AA. All tiles between 180 degrees west and 165 degrees west start with the letter A. Those between 165 degrees west and 150 degrees west start with B, etc. The second letter is determined by longitude. The sourthern most tiles use A as the second letter, those between 45 degrees South and 30 degrees South use D. The next fifteen degree tile (30 to 15 degrees South) uses E as the second letter, etc. The letters "I" and "O" are not used.
Within these fifteen degree tiles the individual tiles are defined using the two numerals. The first numeral indicates the five degree column within the fifteen degree tile, the second the row number. Numbering starts at the lower left (Southwest) corner of the fifteen degree tiles. Thus, the numbers are arranged as below.
13 23 33
13 22 32
11 21 31
The tiles representing Antarctica are an exception to the naming scheme. As described before, they are 90 x 35 degree tiles. However, they take on the name of the 5 x 5 degree tile that would be at their Southwest corner.
To summarize, each physical five degree tile has a four digit name. The four digits are:
- A letter indicating the 15 degree tile column
- A letter indicating the 15 degree tile row
- A numeral indicating the 5 degree column within the 15 degree tile
- A numeral indicating the 5 degree row within the 15 degree tile
DCW Subdirectories
The tiles are split into four subdirectories under /ncsu/geodata/dcw. Each subdirectory contains tiles from 1/4 of the globe. [see image] They are split by longitude. There is also a utility directory, although it is not currently loaded in AFS. The subdirectories are:
- w180_w90: Contains tiles between 90 and 180 degrees West
Most of North America
- w90_w0: Contains tiles between 0 and 90 degrees West
Eastern US and South America
- e0_e90: Contains tiles between 0 and 90 degrees East
Africa, Europe, India, the former Western USSR
- e90_e180: Contains tiles between 90 and 180 degrees East
China, Japan, Australia, and the former Eastern USSR
- Utility: Contains overview data some of which can be very helpful in identifying tile locations.
Finding Your Tiles of Interest
There are several options for finding the name of the tiles for a specific geographic region. They are listed below in approximate order of desireability.
- Use one of the predefined maps for this purpose
- Use the maps describing the 15 degree tile names, estimate the appropriate cells within the 15 degree tiles.
- Load the [sample ArcView project] and browse the "Tile Name" views.
- Start a new ArcView project and load the libref and tileref coverages in the utility directory [this need more explination somewhere].
- Find the latitude and longitude of the your area of interest and do the calculations.
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