How Identity works

This topic applies to ArcInfo only.

Description

Identity creates a new coverage by overlaying two sets of features. The output coverage contains all the input features and only those portions of identity coverage features that overlap the input coverage.

Input coverage features can be polygons, lines, or points. Identity Coverage features must be polygons. Output coverage features resulting from the overlay are of the same class as the input coverage features. Topology is built for the output coverage.

Feature attribute tables are updated. The feature attribute table for the output coverage contains items from both the input and identity coverage feature attribute tables. Items are merged using the old internal number of each feature. The following two tables list the items contained in the output coverage feature attribute table.

For polygon or point input coverages

Using the JOIN option (default)	Using the NO_JOIN option
AREA PERIMETER <out_cover> internal # <out_cover> User-ID <in_cover> internal # <in_cover> User-ID all subsequent <in_cover> PAT items... <intersect_cover> internal # <intersect_cover> User-ID all subsequent <intersect_cover> PAT items...	AREA PERIMETER <out_cover> internal # <out_cover> User-ID <in_cover> internal # <intersect_cover> internal #

For line input coverages

Using the JOIN option (default)	Using the NO_JOIN option
FNODE# TNODE# LPOLY# RPOLY# LENGTH <out_cover> internal # <out_cover> User-ID <in_cover> internal # <in_cover> User-ID all subsequent <in_cover> AAT items... <intersect_cover> internal # AREA PERIMETER <intersect_cover> User-ID all subsequent <intersect_cover> PAT items...	FNODE# TNODE# LPOLY# RPOLY# LENGTH <out_cover> internal # <out_cover> User-ID <in_cover> internal # <intersect_cover> internal #

Identity is one of several Overlay commands available. Two others are similar: Union and Intersect. They differ from Identity only in the features that remain in the output coverage. The attribute tables are updated for all three commands. Other Overlay commands are Clip, Erase, Split, and Update.

Example

POLY option

Only those portions of identity coverage polygons that overlap the input coverage are saved in the output coverage. All input coverage polygons are saved in the output coverage. Input coverage arcs are split at their intersections with polygons of the identity coverage. The resulting arcs are used to build polygons using a process similar to Build with the POLY option.

LINE option

Identity coverage arcs are used to split input coverages where they overlap. The LPOLY and RPOLY values for each resulting arc are updated to contain the internal numbers of the identity coverage polygons that are to each arc's left and right sides.

POINT option

All input coverage points are saved in the output coverage. The output coverage polygon feature class lists the identity coverage polygon within which each point falls.