A tic is a registration or geographic control point for a coverage.
Tics allow coverage coordinates to be registered to a common coordinate system (universal transverse Mercator [UTM] meters, state plane feet, and so on) and, therefore, relate locations of features in a coverage to locations on the earth's surface. Tics are important for registering map sheets during digitizing and editing.
Most geographic data is compiled and automated from a series of map sheets or aerial photographs. Maps are mounted on a digitizer or scanner one sheet at a time and digitized. Aerial photos are mounted in pairs in a stereoplotter. Then the coordinates of all input sources are transformed into a single common coordinate system. Tics play an important role in map registration for these purposes.
Tics are used to register source material to the data capture device (for example, digitizer, stereoplotter) and transform coordinates from units, such as digitizer inches, into real-world coordinates, such as state plane feet or UTM meters.
Although each coverage contains tics, it is important to recognize that all the coverages in a GIS database should use the same tics. You need to establish tic locations using known, real-world locations that can be recorded for each map sheet prior to beginning coverage automation. These common tic locations should be given the same Tic-ID and recorded on every map sheet in your project to ensure that each coverage shares a common locational reference. Examples of suitable tics include corners of map sheets, intersections of roads, and control points located by survey markers.
Use the same tics recorded on each map sheet to generate a master tic file containing Tic-IDs and their known real-world x,y coordinates. Once a master tic file is created, it can be used to create tics for all data collected in the same area. Each coverage should contain at least four tics. For more information on establishing tics for your GIS database, see Map projections.
All tic information for a coverage is stored in the TIC file.
The tic.adf file contains the following items:
IDTIC | The User-ID for each tic |
XTIC | The tic's x-coordinate |
YTIC | The tic's y-coordinate |
Although the x,y coordinates for tics are available in the tic.adf file, editing these values does not change the coordinate system in which other features are stored.
You cannot edit TICs within ArcGIS Desktop. You need to use ArcInfo Workstation to modify coverage TICs.
Two other coverage files store essential information for coordinate management within a coverage: the coordinate definition file, prf.adf, holds the coverage's map projection information, and the tolerances file, tol.adf, holds a number of processing tolerances. One of the tolerances maintained in the tol.adf file is a tic match tolerance. Although these files are optional, using them will ensure proper coordinate management. For additional information, see Coverage tolerances and Map projections.
To learn about the tic definition for the attribute table, see Fields (items) in coverage attribute tables.