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underlay
To place a raster object so that it appears to be behind an existing object or image and will show around the perimeter and through any internal data holes in the object on top.
undershoot
An arc that does not extend far enough to intersect another arc.
Unicode
A character encoding scheme that is a super set of ASCII and Latin-1 with room for 65,536 characters (a 16-bit or 2-byte set). (Over 28,000 characters have been assigned so far.) The Unicode standard is still in committee; it is in the care of the International Organization for Standardization (ISO) and Unicode, Inc. (formerly the Unicode Consortium). Unicode itself is a subset of ISO 10646, and in terms of ISO 10646, Unicode is the "Basic Multilingual Plane." Unicode will include all characters needed for the written languages of the modern world.
unsupervised classification or automatic interpretation
The operation of a group of multispectral image interpretation functions (such as K-means) that statistically cluster cells into similar collections. When the classification procedures are complete, you identify and label the ground features or conditions that the clusters represent.
USDA
United States Department of Agriculture.
USDI
United States Department of the Interior.
USF&WS
United States Fish and Wildlife Service of the USDI.
USGS
United States Geological Survey.
UTM
Universal Transverse Mercator map projection.