Stream data for the SAA region SAA version 3.0 Version Date: 21 March 1996 ------------------------------------------------------------ Identification Information Data Layer Name: Streams Description: 1:100,000 EPA River Reach 3 stream coverage for the SAA region Keywords: stream, river, RF3 Citation: Richard A. Dulaney Lockheed Engineering and Sciences Company 1050 E. Flamingo Rd., Suite 126 Las Vegas, Nevada 89119 and Mason J. Hewitt III U. S. Environmental Protection Agency Environmental Monitoring Systems Laboratory Las Vegas, Nevada 89193-3478 Native Data Set Environment: Unix; Arc/Info 7.1 pathname: //hucs.e00.Z Scale: 1:2,000,000 File Format: The data layer is in one Arc/Info line coverage and is available in compressed Arc/Info export format. Use Restrictions: Data set must be cited when used. Access Restrictions: none ------------------------------------------------------------ Spatial Reference Information Datum: North American Datum 1983 (NAD83) Precision: single Projection: Albers Equal Area Units: meters Spheroid: WGS-84 1st Std Parallel: 34 00 00 2nd Std Parallel: 38 00 00 Central Meridian: -82 00 00 Origin: 33 00 00 False Northing: 0.0 False Easting: 0.0 Extent: West Bounding Coord.: East Bounding Coord.: North Bounding Coord.: South Bounding Coord.: Distance Resolution: Vertical Resolution: n/a ------------------------------------------------------------ Data Quality Information Thematic Accuracy: unknown Confidence: unknown Accuracy Method: unknown Horizontal Accuracy: unknown Confidence: unknown Accuracy Method: unknown Vertical Accuracy: n/a Logical Consistency: Completeness: ------------------------------------------------------------ Source Information Source Material: Organization: Date: Distance Resolution: unknown Contribution: Source Material: Organization: Date: Distance Resolution: unknown Contribution: Source Material: Organization: Date: Distance Resolution: unknown Contribution: ------------------------------------------------------------ Processing History Information Process Description: The RF3 file is based upon the USGS 1:100,000 scale hydrography DLGs. The data were acquired by EPA-OW on 240 tapes which contained 54,000 files. The first processing performed was to convert from UTM to latitude/longitude. This conversion was accomplished preserving the nearest 1/10,000 of a degree, which is well within the stated resolution of these data. The data were then collapsed into line records (trace files) without nodes, and the line records were concatenated. There were then 4 million line elements and 93 million latitude/longitude coordinates. Each line, or trace, retained a key record that can be directed back to the original DLG data tape if necessary. Traces were then assigned to USGS Cataloging Units (CU). Traces that crossed CU boundaries were assigned to both CUs. A CU is a geographic area representing all or part of a surface drainage basin, a combination of basins, or a distinct hydrologic feature. There are approximately 2150 CUs in the Nation. The USGS CU boundaries were developed at a scale of 1:2,000,000 (see Figure 1). They represent the "smallest element in the hierarchy of hydrologic units" (U.S. Geological Survey, 1982). CUs are not accurately correlated to topography and do not always correspond directly to true watersheds as they are more administrative in function. Horizon Systems Corporation, the prime contractor to EPA-OW, developed a software tool called PC Reach File (PCRF). PCRF is the program that performs the RF3 file construction. Production development of RF3 proceeds in discrete units corresponding to the CU. Because of the ability to run many of the steps in batch, a given analyst running PCRF may be working on as many as 25 CUs at a time. Building The RF3 Network. In order to update the RF2 file to RF3, the RF2 data, the DLG data and the CU boundaries are all downloaded to a PC running PCRF. There, an analyst identifies a starting point for the automated construction of the network topology. The starting point designated was the furthest reach downstream and the network was built up in the reverse direction of flow. Between reaches there may be gaps, usually along map sheet boundaries. Therefore, a search tolerance of 3/10,0000 of a degree or approximately 100 feet was specified in order to "bridge" these gaps. Edgematching takes place by the addition of segments in these gaps. Once this has been completed, the analyst will "replay" and supervise the network that was created. There may be some network discrepancies that are not able to be resolved by PCRF and that may cause a break in the processing. These are resolved by the analyst. Once the network is built, segment and milepoint numbers must be assigned along the network. The analyst begins this session by viewing both the RF2 and the processed DLG data together, color coded for differentiation. The analyst then will "tag" the RF2 segment endpoints to the corresponding DLG points. PCRF will then work upstream from the downstream end of the segment and allocate milepoints. The RF2 milepoints are retained in RF3, despite the actual length of the new segments. In other words, the milepoint figures will not reflect the true length of the RF3 reaches. For example, if an RF2 segment had ten reaches (differentiated by milepoints), the more detailed RF3 segment will also have ten milepoints, but the trace of these reaches will be entirely different. New milepoints are not being created because this would disrupt databases that index to the earlier Reach Files. The actual length of reaches as derived from the DLG trace is recorded in a field called SEGL. New segments and reaches are not an uncommon outcome of this conversion to the more highly resolved hydrography network. In this step, PCRF will assign new SEG/MI numbers to traces that appear in the 1:100,000 DLG that did not exist in the previous 1:500,000 Trace File.