Hydrologic units map of Wyoming, modified from USGS fourth level units

The HUC dataset was compiled originally to provide the
National Water Quality Assessment (NAWQA) study units with
an intermediate- scale river basin boundary for extracting
other GIS data layers. The data can also be used for
illustration purposes at intermediate or small scales
(1:250,000 to 1:2 million).

This modified dataset was produced under contract with the
Wyoming Game and Fish Department for the purpose of wildlife
habitat prioritization in Wyoming.

The Geographic Information Retrieval and Analysis System
(GIRAS) was developed in the mid 70s to put into digital
form a number of data layers which were of interest to the
USGS. One of these data layers was the Hydrologic Units. The
map is based on the Hydrologic Unit Maps published by the
U.S. Geological Survey Office of Water Data Coordination,
together with the list descriptions and name of region,
subregion, accounting units, and cataloging unit. The
hydrologic units are encoded with an eight- digit number
that indicates the hydrologic region (first two digits),
hydrologic subregion (second two digits), accounting unit
(third two digits), and cataloging unit (fourth two digits).

The data produced by GIRAS was originally collected at a
scale of 1:250,000.

This data set is a modification for Wyoming of the original
United States HUC data produced by the USGS. The primary
modifications made in this data set by the Wyoming Water Resources
Center include 1) removal of hydrologic units that do not drain
within Wyoming and 2) modification of the hydrologic unit boundaries
to better encompass streams and lakes represented in the 1:100,000 scale
Digital Line Graph (DLG) of surficial hydrography. The HUC
boundaries have not been ""clipped"" to the Wyoming border, in order
to maintain the natural boundaries of the units.

This metadata has also been altered from original USGS
metadata to be more specific for Wyoming.

HUC数据集最初编制的目的，是为国家水质评估(National Water Quality Assessment, NAWQA)研究单元提供中等尺度的流域边界，用于提取其他地理信息系统(Geographic Information System, GIS)数据图层。该数据集也可用于中等或小比例尺（1:250,000至1:2,000,000）场景下的可视化展示。

本修改版数据集受怀俄明州鱼类和野生动物部门委托编制，用于怀俄明州的野生动物栖息地优先级划分工作。

地理信息检索与分析系统(Geographic Information Retrieval and Analysis System, GIRAS)于20世纪70年代中期开发，旨在将美国地质调查局(United States Geological Survey, USGS)关注的多个数据图层转换为数字格式。其中一类数据图层即为水文单元(Hydrologic Units)。该数据集基于美国地质调查局水数据协调办公室发布的《水文单元地图》，以及区域、亚区域、核算单元和编目单元的列表描述与名称。水文单元采用8位数字编码，其中前两位代表水文区域，接下来两位代表水文亚区域，再接下来两位代表核算单元，最后两位代表编目单元。

GIRAS生成的原始数据采集比例尺为1:250,000。

本数据集是美国地质调查局(USGS)原始美国本土HUC数据集针对怀俄明州的适配修改版本。怀俄明州水资源中心对本数据集进行的主要修改包括两项：一是移除所有未在怀俄明州境内汇流的水文单元；二是调整水文单元边界，以更好地覆盖1:100,000比例尺地表水文数字线图(Digital Line Graph, DLG)中收录的溪流与湖泊。为保留水文单元的自然边界，HUC边界未被‘裁剪’至怀俄明州州界范围内。

本元数据也已针对怀俄明州的使用需求进行调整，相较于美国地质调查局的原始元数据更为具体。