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Elevation-Derived Hydrography READ Rules: Low-Confidence Area, Snow-Cover

A Low-Confidence Area, Snow-Cover is a reference polygon used by the 3DEP to define where it can be determined that there was snow on the ground at the time of collection. Light dusting of snow that does not alter the ground elevation more than about 0.8 inch (2 centimeters) is inconsequential and should not be delineated. Low-confidence areas may overlap other hydrographic features.

Attribute/Attribute Value

Each feature requires domain codes to be entered into the attribute table for the feature class (EDH Feature type description, associated geometry, and use classification table). See “Field Definitions and Domain Values for Attributes” section for more information on EDH code definitions.

Representation Rules

When delineating a feature, it must be created with the appropriate geometry, either point, line, or polygon, which is determined by the size of the feature or the length along different axes of the feature (table 12).

Special conditions: none.

Table 12. Low-confidence Area, Snow.

Kind of feature object                 


Shortest Axis

Longest Axis

0-dimensional (point)




1-dimensional (line)




2-dimensional (polygon)

greater than 0




Data Extraction

Capture Conditions

If snow is greater than approximately 2 centimeters deep, then capture.

Topology rules for adjacent features do not apply to low- confidence areas. For instance, they do not need to be snapped to adjacent features.

Attribute Information

FClass 9—Non-Hydrography Feature (elevation dataset limitation).

FCode 993—Low-confidence area, snow-cover.

EClass 9—Elevation dataset limitation.

Source Interpretation Guidelines


Imagery and a map depicting an low confidence area covered in snow.
An area in the Alaska Range (150.8094829°W 62.9773695°N ) shown as an example of a low-confidence area, snow-cover. Source data are from the National Hydrography Dataset (U.S. Geological Survey, 2020), which is used to provide examples of hydrographic feature types but may not have the same density and other characteristics of elevation-derived hydrography.