Coverage tics
Release 9.3 Last modified November 25, 2008	Print all topics in : "Coverages"

What are tics?

A tic is a registration or geographic control point for a coverage.

Tics allow coverage coordinates to be registered to a common coordinate system (for example, 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.
From the Coverage Properties dialog box, you can add or update tics if you know their coordinates. You can also delete tics from the coverage.

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 processing tolerances" and "Map projections".

To learn about the tic definition for the attribute table, see About fields (items) in coverage attribute tables.

How to modify a coverage's tic points (ArcInfo and ArcEditor only)

Adding tics

1. In ArcCatalog, right-click the coverage to which you want to add a tic and click Properties.
   


2. Click the Tics and Extent tab.
   
3. Click Add.
   
4. Click in the X column and type the tic point's x-coordinate.
   
5. Click in the Y column and type the tic point's y-coordinate.
   
6. Click OK.
   

Updating tic coordinates

1. In ArcCatalog, right-click the coverage whose tics you want to modify and click Properties.
   


2. Click the Tics and Extent tab.
   
3. Click the ID of the tic whose coordinates you want to modify.
   
4. Click Update.
   
5. Click in the X column and type the tic point's x-coordinate.
   
6. Click in the Y column and type the tic point's y-coordinate.
   
7. Click OK.
   

Deleting tics

1. In ArcCatalog, right-click the coverage whose tics you want to delete and click Properties.
   
2. Click the Tics and Extent tab.
   
3. Click the ID of the tic you want to delete.
   
4. Click Delete.
   
5. Click OK.
   

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