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Water: Georeferencing

Spatial Indeterminacy


In the 305(b) database that was created before the broad use of GIS (the Waterbody System),  Assessment Units were often defined as small watersheds (Figure 1). 

Water quality assessment data stored in the database often only applied to portions of these watershed assessment units.  In Table 1, Assessment Unit "WBA" is a total of 50 miles.  Thirty of these miles are Fully Supporting for the designated use of swimming, 4 miles are Not Assessed and 10 miles are Not Supporting.  Figure 1 defines the location of Assessment Unit "WBA", but this spatial information can not be used to display which 10 mile portion of this Assessment Unit is Not Supporting is designated use of swimming. 

The inability to map these 10 miles referred to in the database is a Spatial Indeterminacy problem.   It is easy to see how Spatial Indeterminacy in a database would prevent the display of the detailed Overall Use Assessment information in Figure 2 in a GIS where the Assessment Units are defined as watersheds as in Figure 1.


Spatial indeterminacy can be eliminated by entering water quality assessment data so that it applies to an entire assessment unit.  One strategy is to break the existing watershed assessment unit into smaller units that have homogeneous assessments.

Figure 1: Assessment Units in USGS Subbasin 05130204
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Figure 2: Use support rating for USGS Subbasin 05130204 
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Table 1: This sample database is showing assessment units with designated uses, which can be fully supporting or not supporting their designated uses depending on water quality.

Assessment Unit ID Size Designated Use Fully Supporting Not Assessed Not Supporting
WBA 50 miles Swimming 30 miles 4 miles 10 miles
WBA 50 miles Fishing 40 miles 10 miles 0 miles
WBB 39 miles Swimming 10 miles 10 miles 19 miles
WBB 39 miles Fishing 25 miles 10 miles 0 miles
WBB 39 miles Drinking Water 0 miles 20 miles 19 miles
WBC 25 miles Fishing 20 miles 0 miles 6 miles
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