**ReadMe2.doc -- information about NORM ED data**

**May 2007**

**Reference to *Science* Article**

The article "Roadless Space of the Conterminous United States" that
describes and discusses the NORM ED dataset (which defines the Euclidean
distance from any point to the nearest road) is in the May 4, 2007 issue
of *Science Magazine*. Anyone interested can access the article by
subscribing to the Science online magazine at
http://www.sciencemag.org/cgi/content/full/316/5825/736

Questions and inquiries may be directed to Raymond Watts at
<rwatts@usgs.gov>.

**Data Description**

The USGS is building datasets to study environmental effects of the U.S.
road network, one of the largest man-made constructions on earth. These
datasets collectively are called the National Overview Road Metrics
(NORM). This publication describes the first NORM dataset, NORM ED,
which quantifies straight-line or Euclidean Distance to the nearest
road.

The National Overview Road Metric Euclidean Distance (NORM ED) dataset
was developed to describe the extent and configuration of the spaces
between roads in the United States. The metric of NORM ED is Euclidean
distance, in units of meters, to the nearest road. The NORM ED value at
any point estimates the largest radius of a circle, centered at that
point, that contains no roads. Accuracy of the estimate is limited by
the accuracy of the source data, both in terms of roads depicted and
their positions.

NORM ED considers all roads to be equal, regardless of road surface,
width, and traffic volume. The dataset from which NORM ED was built
contains features ranging from interstate highways to jeep tracks,
although not necessarily with uniform detail or reliability in all
areas.

The full-resolution dataset provides distance-to-road (DTR) values on a
30-meter image, using an equal-area projection. NORM ED values can be
aggregated statistically---by averaging, for example---over areas of the
user's choice, and results are mathematically valid. NORM ED can
therefore be used to calculate comparable average DTR values for
counties, states, watersheds, ecoregions, or any other area of interest.

NORM ED is intended for use in analyses of land surface conditions at
multiple scales, particularly with application to the effects of roads
on ecology, watershed quality, wildlife habitat, and enjoyment of
roadless areas. It is closely related to other indicator variables, such
as road density.

NORM ED can be used to answer questions such as:

-   If the county builds a new road, how much will it diminish the area
    of open space that is more than ½ kilometer from a road?

-   What fraction of a particular watershed is within 100 meters of a
    road?

-   What point in the state is most remote from a road and precisely how
    remote is it?

When combined with other gridded datasets, NORM ED can answer questions
such as:

-   Are higher elevation areas more likely to be remote from roads than
    lower elevation areas?

-   Are the spaces in the road network larger in forests of the Eastern
    or Western United States?

## 

## Data structure of state flat files

The data are organized by state (or District of Columbia) in file
groups. The name of each file in a state's group begins with the state's
postal designation (NY = New York, etc.). Due to processing limitations,
California and Texas each had to be split into two groups of files.
These file groups start with CAN and CAS (California north and south),
and TXE and TXW (Texas east and west.) Each group contains four files: a
header file ending in .hdr; a world file ending in .blw; a projection
file ending in .aux; and a binary integer flat file ending in .bil.
Header files (.hdr) are ASCII text files that contain flat-file
descriptors such as the number of rows and columns, the number of bands
(one in this case), and the byte order. Information in the projection
(.aux) can also be found in the Projection & Resolution section of this
paper (pg. 5) or in the metadata files. The byte order parameter tells
software whether byte rearrangement is needed to convert between Intel
and Motorola standards for byte ordering. The world file (.blw) contains
the transformation parameters necessary to georeference the flat file
(i.e. to convert from row and column coordinates to real world
coordinates).

Each group of files (header file, world file, projection file, binary
integer flat file) along with the metadata file (.htm) and a read me
file (.doc) has been compressed into a single zip file using WinZip.

Once decompressed, the binary flat files are 16-bit integers stored by
rows in north-to-south order and within each row by cells in
west-to-east order. Together with the .hdr, the .aux, and .blw files,
these flat files can be used directly in ArcInfo or ArcGIS as images,
converted into grids (from ArcInfo command line IMAGEGRID, or through
ArcToolbox-Data Management-Copy Raster), or imported into many other
image analysis programs as Band Interleaved by Line (.bil) files.

Datasets can be downloaded from the U. S. Geological Survey at:

<http://rockyitr.cr.usgs.gov/trip/data/>

**Data Disclaimer**

TIGER 2000 and *Dynamap 1000* do not contain data that describes their
revision status either by individual feature or by area. Thus, the
source data for NORM ED represent conditions over a range of collection
and compilation dates and are not a true temporal snapshot of road
conditions of the United States. Nevertheless, this is the most complete
current dataset intended to portray all roads across the nation. It
fairly represents the overall status of documented roads in the U.S.
approximately for the year 1999, although detailed comparison of
*Dynamap 1000* data with 1990's aerial photographic images reveals
discrepancies, usually in the form of roads missing from *Dynamap 1000*,
in many areas. We have not made comparisons in enough places to make
quantitative statements about error rates or magnitudes. We note,
however, that comparison of DTR values between various data sources is
an effective method of comparison; small shifts in vector road positions
have modest effect, whereas addition or subtraction of roads causes
substantial DTR change. Because of the discrepancies in revision status
of these maps, they would not be valid data for use in an Environmental
Impact Statement.,

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