|USGS name||# quads||DNR#||name||compressed size|
In the tables above, o48123h1 refers to a 7.5' quad (at 1:24000 on paper)
lying in the block northwest
of 48°N,123°W. The quad is in row h (from the south) and column 1 (from
Quads beginning with k (There are only 4 in Washington.) are 15' wide. They are 1:25000 on paper.
Quads beginning with c cover the 1°×2° block of 128 small quads, and are 1:250,000 on paper.
Quads beginning with f cover a 30" by 60" block and are 1:100,000 on paper.
We have invented a designation of 'z' quads, which comprise all the 1:24000 quads in a one-degree block, mosaicked into a huge file.
The data file in the above example is named o48123h1.zip, and will unzip (sometimes automatically, depending on your browser) to o48123h1.tif and o48123h1.tfw. The latter is a short ASCII file which enables some software to know where (in UTM NAD27) the image lies on the earth.
The second column refers to the number of 7.5-minute quadrangles. An "o" file typically comprises one 7.5" quad, a "k" file two, and an "f" file 32 quads, though this varies at the edges.
As the Canadian border is actually slightly north of 49° across most of Washington, there is an over-edge quadrangle "Point Roberts OE N", covering the sliver of the US north of 49°. There is a special DEM file for this. However, there is no paper map by that name. The Point Roberts quad and the over-edge quad are on the same paper map sheet, hence in the same DRG file. There are also overedge quads that are mostly water, and a Dungeness quad that straddles two 7.5' quads. The 15'-wide "k" quads also are treated like this.
The DNR numbering scheme comprises a row number (starting with row 0 above 49°) and column number (starting with column 1 just east of 125°). Leading zeros on the row are dropped.
Files will unzip to about 2.5 times their zipped size. The unzipped .tif file will be in a non-proprietary "compressed" form. The largest files are those with water screen tints.