Limitations_of_Data:

                                                                                
  These data were digitized at a scale of 1:250,000 with some portions          
 of coverage at 1:100,000- and 1:2 million scale. Limitations of                
 the data strictly revolve around this scale input. Any intention               
 of using it with better scale data (i.e. 1:24k hydrography) should be          
 carefully scrutinized first.                                                   
                                                                                

Procedures:

                                                                                
   The data was recieved as compressed giras tar files representing             
  either a 1:250k quad or a 1:100k quad.  Each file was named after its         
  respective quadrangle.  A coverage of 1:250k quadrangles was used             
  to divide the country up into four sections and get a list of names           
  for each section.  Using GIRASARC2, an aml designed to create an arc          
  info coverage out of a GIRAS file and a corresponding neat line coverage,     
  it was quickly discovered that many of the quad names were to long            
  for the program (i.e. sault_saint_marie) and a generic naming system for      
  files and coverages was incorporated.  In 1 of 10 cases, the name of          
  the quadrangle did not correspond with the name of the file.  These           
  problems were traced down and corrected (after all four sections were         
  converted there were many files left over...these wound up being all          
  the 1:100k quads which did not have similar names to the 1:250k               
  files).                                                                       
                                                                                
   After the files for a given section were all converted into arcinfo          
  format, a loop aml was run which copied a coverage and its neatline           
  cover into temporary storage (there was not enough room in info to            
  deal with a large number of files in one directory), attached to              
  that directory, ran a build on the coverage to create a line file,            
  and went into arcedit.  Once there, the outside polygon was selected          
  and deleted and the neatline coverage was copied in using get (in all         
  cases, the outline of the coverage quad never quite conformed to the          
  neatline which was why this step was needed) with intersectarcs on add.       
  Dangles were selected and extended 500 feet maximum. This took care of        
  all undershoots.  Overshoots were taken care of by CLEANing the coverage      
  into a seperate directory using a dangle length of 500 and tolerance of 2.    
  Both the neatline and huc coverages were deleted in the temporary space,      
  and the aml looped to the next coverage.                                      
                                                                                
   Another aml was then run which added an item to the .aat called outer,       
  went into info, and reselected lpoly# = 1 or rpoly# = 1 in the .aat and       
  calculated outer to = 1.  All coverages were checked for additional           
  dangles and then a MAPJOIN was run using NET as the feature option.           
  Finally, the MAPJOINed coverage was DISSOLVEd using the item HUC as           
  the dissolve item and NET as the feature option.                              
                                                                                
   The resulting coverage was then QC'ed for node and label errors and          
  remaining neat line arc problems (i.e. long neat lines still in the           
  coverage).  Many more problems arose in the western part of the country       
  than in the east.  Bordering HUC code disagreements between quads caused      
  a number of cases in which neatlines did not dissolve.  These were            
  corrected for the most part, however there are still several blatant          
  errors that need to be correced.  After all sections were complete,           
  the same procedure was done for the 1:100k quads, these were mapjoined        
  with the 1:250k quads and dissolve was then used.                             
                                                                                

Reviews_Applied:

                                                                                
   Peer review, 1/5/93, Doug Nebert, USGS-WRD, Reston:                          
                                                                                
   Data were reviewed for attribute accuracy against a 1:2million base through  
   random audit of polygon features.  Line attributes were verified by symbol-  
   ization on the screen.  Regions were shaded in to verify correct polygon     
   values for HUC at the Hydrologic Region level.  Documentation was updated.   
                                                                                
   Peer review, 10/18/93, Bill Battaglin, USGS-WRD, Lakewood, Co, memo to       
   Doug Nebert:                                                                 
   ----------------------------------------------------------------------       
   "I have completed a review of the 1:250,000 scale hydrologic units coverage  
   (HUC) and found the digital data and metadata to be of high quality.  I have 
   a few suggested improvements to the digital data and to the documentation.   
   Below is a summary of the methods I used to check feature accuracy in the    
   digital data base and the problems I found.                                  
                                                                                
   Digital Features:                                                            
                                                                                
   The line work for the HUC coverage was checked against the line work from:   
                                                                                
   (1) the 1:2,000,000 HUC coverage by plotting both data sets out on one large 
   graphic (about 1:3,000,000).  No major discrepancies were found except in    
   coastal areas where the 1:2,000,000 scale coverage had more detail than the  
   1:250,000 scale coverage.                                                    
                                                                                
   (2) line work from 1:24,000 scale digitized drainage basins in Colorado.  
   The match was generally good with departures generally less than 2500 meters.  
   The biggest departures were in Colorado and were as large as 4000 meters.                                            
                                                                                
   (3) line work from the 1:2,000,000 scale rivers coverage for the USA by      
   plotting both data sets out on one large graphic (about 1:3,000,000).  In    
   general the nesting of streams in HUC's was good and HUC boundaries inter-   
   sected steams at stream intersections.  In some places (NW Utah), 
   the streams coverage does not match the HUC  coverage that well, but this 
   could easily be because of the unusual nature of streams in these areas or 
   because of inaccuracies in the streams coverage.
                                                                                
   (4) line work from 1:100,000 scale streams from Colorado. The nesting of 
   streams in HUC's was very good.  Stream arc's for the most part did not 
   cross HUC arcs except at stream intersections.  The error (distance from 
   intersection to HUC line) between HUC lines and stream intersection was 
   less than 500 meters at all intersections checked(about 25).                                                                  
                                                                                
   Problems with Line work:                                                     
                                                                                
   (1) There was a very large number of very short arcs in the coverage (3211   
   Lt 1000 meters long and 1729 Lt. 100 meters long).  Most of these arcs were  
   internal (did not border on outside polygon) and coded as 250k edges(3)      
   (almost 3000) but some were 250k (2) lines and one was a 2m dlg (4). Arcs    
   with lengths of less than 100 meters (maybe even less than 1000 meters) are  
   difficult to deal with when editing subsets of the coverage, and they also   
   add to the overall size of the database.  I know many of these lines were    
   created in the process of edgematching the quads, but I think the informa-   
   tion content of these very short arcs is less valuable than the hassle and   
   overhead involved in keeping them in the coverage.                           
                                                                                
   (2) The edit distance for the coverage was set to a very small value.        
   This may have been required for earlier processing, however, it makes        
   the finished coverage difficult to work with.  I had to reset the edit       
   distance to a larger value when I wanted to select arcs in ARCEDIT           
   interactively.  This, of course, will be one of the things users will        
   want to do with the new HUC coverage.                                        
                                                                                
   Polygon labels/attributes:                                                   
                                                                                
   (1) Label point accuracy was checked by making a point cover of polygon      
   labels from the 1:2,000,000 HUC coverage and then doing an identify of       
   those points in the 1:250,000 scale HUC polygon.  This procedure looked      
   for both new or missing polygons, and was also used to check attribute       
   values.  I also dissolved both coverages by accounting unit and compared     
   the number and location of remaining polygons.                               
                                                                                
   Problems with labels/attributes:                                             
                                                                                
   (1) I discovered a total of 649 places where the HUC codes from the label    
   point of the 1:2,000,000 coverage did not match the HUC code for the         
   1:250,000 HUC polygon that it fell within. In other places the polygons 
   seemed to be the same but the HUC codes were different.  For example
   HUC 18020111 in the 1:2,000,000 coverage is coded as HUC 18020023 in the     
   1:250,000 coverage.  There were also internal polygon label differences 
   in Colorado (10090204 in 250k, 10180007 in 2m)
   
   (2) The dissolved 1:2,000,000 coverage contained 350 accounting unit         
   polygons while the dissolved 1:250,000 HUC coverage only contained 177.      
   There were large differences in the way the Accounting unit polygons         
   looked in Wyoming.                          
                                                                                
   Coverage Documentation:                                                      
                                                                                
   The coverage documentation was reviewed both editorially and for overall     
   completeness.  The documentation was editorially sound and did not need any  
   corrections.                                                                 
                                                                                
   Problems with the Documentation:                                             
                                                                                
   (1) The redefined items in the pat file were not defined in the data         
   dictionary portion of the documentation file.                                
                                                                                
   (2) The complete reference to the source material for the data is not in the 
   documentation file."                                                         
                                                                                
   Peer review, 11/10/93, Doug Nebert, USGS-WRD, Reston, memo to Paul Kapinos:  
   ---------------------------------------------------------------------------  
   "As you are aware, we have several digital versions of the hydrologic unit   
   a 1:250,000-scale version derived from National Mapping Division Geographic  
   Information Retrieval and Analysis System (GIRAS) data as part of their land 
   use mapping program of the 1970's and early 1980's.                          
                                                                                
   
   Peer review, 11/29/93, Paul Kapinos, USGS-WRD, memo to Doug Nebert:          
   -------------------------------------------------------------------          
   "The discrepancies in the hydrologic unit codes (and some boundaries)        
   in the State of California are due to the fact that the California           
   State Hydrologic Unit Map (HUM) was revised in 1978 but the 1:250,000-scale  
   digital data set was not.  The events that most likely occurred can be       
   summarized as follows:                                                       
                                                                                
     o The 1:500,000-scale HUM's were published by OWDC over a period of about  
       four years between 1974 and 1978.                                        
                                                                                
     o The National Mapping Division (NMD) overlaid the hydrologic unit         
       boundaries on their 1:250,000-scale land-use and land-cover map          
       series after each State HUM was completed, and later digitized these     
       boundaries and their respective codes.                                 
                                                                                                                                                             
     o Once all the HUM's were printed (including the 1978 revisions of         
       South Dakota), the 1980 1:2.5 million-scale United States wall map was 
       published using the up-to-date (1978) boundaries and codes.                                                               
                                                                                
    Based on the above summary, I would recommend using the boundaries          
    and codes from the 1:2.5 million-scale map and the 1:2,000,000 digital      
    data set.  Please be aware that other hydrologic unit boundaries and/or     
    codes may have been revised when individual State HUM's were reprinted      
    by OWDC.  I doubt if there has been any attempt to update any of the        
    digital data sets with these changes."                                      
                                                                                

Other_Related_Data_Sets:

                                                                                
  Any data set which has hydrologic unit codes as part of their data may be able
  to use this data.                                                             
                                                                                

References_Cited:

                                                                                
   USGS DIGITAL CARTOGRAPHIC DATA STANDARDS                                     
   LAND USE AND LAND COVER DIGITAL DATA                                         
                                                                                

Notes:

                                                                                
   U.S. Geological Survey and cooperating agencies until further review is      
   conducted and entered into this documentation file.                          

Other: