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.

      Name                    Last modified      Size  Description
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      NORMED_meta_48.htm      11-May-2007 13:34   26K  
      NORMED_meta_AK.htm      11-May-2007 13:47   20K  
      NORMED_meta_HI.htm      11-May-2007 13:54   19K  

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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