USGS - Revised DRG Product Standard

USGS Digital Raster Graphics

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Revised DRG Product Standard

Approved May, 2001

U.S. Geological Survey (USGS) digital raster graphics (DRG) were produced under the original product standard from 1995 to 2001. A complete revision to the standard was proposed in 2000, completed in May 2001, and implemented starting in October 2001. This revision includes two significant changes:

Original coverage DRGs contain only 13 colors that model the line-drawing nature of a printed map. An additional color model was added to the standard to allow DRGs to be made with a color palette of up to 256 colors.
Original coverage DRGs have scan resolutions of 250 dots per inch (dpi). The revised standard allows scan resolutions anywhere in the range of 250 dpi to 1000 dpi.

In addition, many of the TIFF, GeoTIFF, and metadata characteristics of DRGs are stated more formally in the revised standard.

The revised standard is backwardly compatible with the original standard. All existing DRGs remain compliant with both standard versions. The USGS does not intend to systematically rescan all topographic maps at a higher resolution.

Copies of the new standard in PDF format can be retrieved from links at the bottom of this page.

Why changes were needed

Sample images

Change 1: Nonstandard color palette

Change 2: Scan resolutions up to 1,000 dpi

Impacts on existing DRG data

Impacts on existing applications

Implementing the revised standard

Copies of the revised standard

Why changes were needed

A normal USGS topographic map has at most six colors of ink. The white background is a seventh color, and up to six more colors are simulated with lithographic screens. The original DRG standard modeled this color scheme explicitly by requiring that all digital colors be mapped onto a standard palette of 13 colors. This model has several advantages for DRGs made from standard quadrangles, but it is not adequate for most other kinds of maps. It is very difficult to map additional colors onto the 13 standard DRG colors.

During the first phase of the DRG program (1995 to 1998), this was a problem for only a very small number of maps. Orthoimage maps, some bathymetric maps, and some Bureau of Land Management 100K maps could not be made to conform to the DRG standard. But the problem has become more serious as requirements to make DRGs of nonstandard maps have increased.

The need to make DRGs of U.S. Department of Agriculture Forest Service (FS) single-edition maps forced the issue. Single-edition maps print gray and red tints on top of other tint colors. This very rarely happens on standard USGS maps, where area-fill tints are almost always mutually exclusive. On a single-edition quadrangle, up to three tint colors can be printed over the same area, creating a large number of additional colors. Attempts to map these colors to the standard 13 DRG colors have not been successful. There is usually some loss of information and always significant degradation of image quality.

The single-edition maps are the most serious instance of the problem at this time because single edition maps actually replace standard topographic maps. But the problem is not unique to single-edition maps; it also occurs when making a DRG of any map other than a "normal" six-color USGS topographic quadrangle, including nearly all hydrologic and geologic maps.

Sample images

The following three images show the same area, approximately 1 square mile, of different DRGs of the same map. All three DRGs were made by scanning the published paper map. The sample area includes three overlapping tints: standard USGS woodland green, FS private inholdings gray, and FS boundary red.

1. DRG that conforms to the original product standard. In each area of tint overlap, only one color can be preserved. Since this one color must match a color in the standard DRG color palette, the information associated with all other colors is lost. The most obvious example is the loss of timber information under the gray inholding tint, but some other data, including a red road shield in the upper center of the image, are also lost. Note that combinations of tints can lead to "new" colors. For example, green + gray + red = brown. The image file of the full DRG is about 6.6 Mb.

2. Nonstandard colors at 250 dpi. All the information on the original map is preserved. Some data can be difficult to see in places where all three tints are present, but this is also true of the paper map from which the DRG was made. The increase in color resolution results in a clearer image even in areas of no color conflict, but it also increases the file size. The full DRG is about 14 Mb.

3. Nonstandard colors at 500 dpi. This image has approximately the same color characteristics as image 2, but the spatial resolution has been doubled. The results are a slightly sharper image and a much larger image file of 44 Mb.

Change 1: Nonstandard color palette

A raw scan of any map contains far more than 13 colors. Subtle color variations caused by ink bleed, overprinting, and paper irregularities are picked up by the scanner. A raw scan therefore has high color resolution, which gives it the "soft" appearance of a printed map.

The revised DRG standard will allow this type of image to be retained as the final product. This color model differs in two important ways from the original DRG color model:

It allows any number of colors up to 256.
With one important exception, this model does not require any standard red-green-blue (RGB) values for any entry in the color palette, or any standard ordering of color entries. That is, the data producer may use any colors in any order. The exception is the second color slot of the palette, which will be reserved for the map background color (usually, but not necessarily, white). This will allow the map background of any DRG, regardless of which color model is used, to be easily identified by GIS applications.

The original color model remains part of the standard. Data producers may use whichever model they or their customers deem appropriate. As a matter of production policy, the USGS intends to use the original color model whenever the source materials make it feasible to do so.

Change 2: Scan resolutions up to 1,000 dpi

As a separate issue, changing circumstances have made it desirable to allow higher scan resolutions in a standard DRG. Originally, 250 dpi was chosen as a reasonable compromise between image quality and file size. Several things have changed since the original standard was written:

In 1994, 10 Mb was still a very large file for most desktop computers. Today, files several times that size can be handled on most desktop systems.
The USGS now routinely makes DRGs from digital source materials with resolutions of 1000 dpi. The image must be downsampled to 250 dpi to make a standard DRG from these materials. This deliberate image degradation is of questionable value.

The revised standard allows any scan resolution between 250 and 1000 dpi for standard DRGs. As of October 1, 2001, most new USGS DRGs have scan resolutions of 500 dpi.

Impacts on existing DRG data

Existing DRGs are not affected by these changes. All original coverage DRGs will remain compliant with the revised standard.

Impacts on existing applications

The standardized palette of 13 colors was originally defined to aid geographic information system (GIS) applications. There are a variety of reasons why it is convenient for a GIS user or programmer to know in advance exactly what colors are in a DRG. For example, it is often useful to render the white background and green timber tint transparent so that other data, such as digital photographs, can be seen through the map. This is much easier if the palette positions and RGB values of white and green are known in advance. In a clean image, this technique permits crude feature separation by color. Applications that assume the current standard DRG color palette will not work on DRGs that use the new color model.

Reducing the color palette to 13 standard colors can be thought of as a preprocessing step for GIS applications. The cost is that some information is thrown away; the benefit is increased convenience for an important set of applications. For single-edition and other nonstandard maps, the cost in information loss can be very high.

GIS users of DRGs with nonstandard colors will therefore need to do their own color reduction, rather than have it done for them by the USGS. This is admittedly inconvenient. But it also allows a broader range of applications, because more information is preserved in the distributed product.

Implementing the revised standard

The USGS is currently making about 1,000 new DRGs per year. Roughly half of these are of revised maps, the rest are to correct errors in existing DRGs. This rate of production is expected to continue for the near future. The agency has no funding or plans to completely redo the product line at higher scan or color resolutions

At the beginning of fiscal year 2002 (October 1, 2001) the USGS began making DRGs to the revised standard. Most new DRGs have scan resolutions of 500 dpi. DRGs made from standard six-color maps continue to use the standard 13-color palette. But other maps, including Forest Service single-edition maps, have DRGs made with an expanded color palette.

The USGS implementation of the expanded color palette is more restricted than the standard requires. On every USGS DRG, an attempt is made to standardize the area-fill background colors: white, green, light red, light blue, light gray. The result is that most of the pixels are of standard colors, even though most of the colors are not standard. This compromise gives the DRG a reasonably consistent visual appearance and also allows GIS applications to easily render the background transparent. However, since most of the linework colors are not standardized, it is quite difficult to autovectorize the linework or separate features by color.

Sample DRG of a Forest Service single-edition quadrangle (self-extracting zip file, 13 Mb).

This DRG has a scan resolution of 500 dpi. White, green, light blue, light gray, and gray-green background colors are all standard in this image. About 82% of the pixels are of standard colors, even though the image contains more than 150 distinct colors. The image contains a moderate amount of color noise, but unlike many 13-color DRGs this noise does not obliterate the original map image. Most linework can be seen clearly, even if it cannot be automatically extracted through color recognition.

USGS data production is driven in part by availability of cooperative funding from other Government agencies. Projects with joint funding receive higher priority than projects funded by the USGS alone. Eliminating the backlog of single-edition maps will depend largely on availability of cooperative funding. For information about data production partnerships, see http://rockyweb.cr.usgs.gov/acis-bin/querypartner.cgi

Copies of the revised standard

Revised DRG standard in PDF format.

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